Rights / License: Research Collection In Copyright - Non ...23357/eth-23357-02.pdfSikuani) and alow-Psusceptiblemaizecultivar {Zea SA3) werecomparedwith aSwisscultivar {ZeamaysL. Corso)

Research Collection

Doctoral Thesis

Low-P tolerance of various maize cultivarsthe contribution of the root exudation

Author(s): Gaume, Alain

Publication Date: 2000

Permanent Link: https://doi.org/10.3929/ethz-a-003877645

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Diss. ETH Nr. 13529

Low-P tolerance of various maize cultivars :

the contribution of the root exudation

A dissertation submitted to the

SWISS FEDERAL INSTITUTE OF TECHNOLOGY

ZURICH

For the degree of

DOCTOR OF NATURAL SCIENCES

Presented by

ALAIN GAUME

Dipl. Ing. Agr. ETH-Zurich

Born March 03, 1970

Citizen of Epiquerez (JU)

Accepted on the recommendation of

Prof. Dr. E. Frossard, examiner

Prof. Dr. H. Sticher, co-examiner

Prof. Dr. A. Guckert, co-examiner

Prof. Dr. W. Horst, co-examiner

Zurich, 2000

Table of contents

LIST OF ABBREVIATIONS

ABSTRACT 1

RESUME 3

GENERAL INTRODUCTION 6

Importance of phosphorus (P) in crop production 6

P deficiency and availability : the problem 6

Strategies to alleviate P deficiency and low-P availability 7

Plant P efficiency 8

P nutrition: the contribution of root exudates 9

Objectives, hypothesizes and progress of this present research work 10

CHAPTER I: Phosphate acquisition by Zea mays L. in 13

sand-ferrihydrite-phosphate systems.

Keywords 13

Abstract 13

Introduction 14

Materials and Methods 15

Results and Discussion 20

Conclusion 48

Table of contents

CHAPTER II: Low-P tolerance of maize cultivars (Zea mays L.): 50

Significance of root growth, and organic acids

and acid phosphatase root exudation.

Preamble 50

Keywords 51

Abstract 51

Introduction 52



Conclusion 76

CHAPTER III: Aluminum resistance in two cultivars ofZea mays L.: 78

Root exudation of organic acids and influence of

phosphorus nutrition.

Preamble 78

Keywords 79

Abstract 79

Introduction 80



Conclusion 93

Table of contents

CHAPTER IV: Effect of some organic acids on P sorption, 94

desorption and exchangeability on a

synthetic ferrihydrite.

Preamble 94

Keywords 95

Abstract 95

Introduction 96



Conclusion 117

CHAPTER V: Effect of maize root mucilage on P 120

adsorption and exchangeability on

a synthetic ferrihydrite.

Preamble 120

Keywords 121

Abstract 121

Introduction 122



Table of

GENERAL CONCLUSION 143

P and Fe mobilization in soil: 144

the influence of soil mineral properties

Low-P tolerance of maize plants: 144

the contribution of some putative mechanisms

Root exudation 144

Root biomass, root length and other morphological traits 145

Efficiency of some organic root exudates, of the root system 146

and plant development in P mobilization

Tropical acid soils: 147

the Al and P dilemma

Fe acquisition by maize plants: 148

the contribution of some root exudates

The rhizosphere: 148

roots can affect mineral properties and P chemistry

Outlook 149

LITERATURE CITED 151

REMERCIEMENTS

CURRICULUM VITAE

List of abbreviations

ANOVA Analysis of variance

CEC Cation exchange capacity

CIAT International Center for Tropical Agriculture

CIMMYT International Maize and Wheat Improvement Center

Cp Phosphate concentration in solution

DNA Deoxyribonucleic acid

EAWAG Swiss Federal Institute for Environmental Science and Technology

E(t) Quantity of isotopically exchanged P at time t

ETH Swiss Federal Institute of Technology

Fed Dithionite-citrate-bicarbonate-extractable Fe

Fe0 Oxalate-extractable Fe

GA Galacturonic acid

ICRAF International Center for Research in Agroforestry

ITÖ Institute for Terrestrial Ecology

K Affinity constant of the compound for the ferrihydrite

MW Molecular weight

n Decrease with time of 33P activity remaining in the solution

NST NST90201 (S) CO-422-2-3-1-7 maize cultivar

PEP Phosphoerco/pyruvate

PEPC Phosphoefto/pyruvate carboxylase

PAE Phosphate acquisition efficiency

PGA Polygalacturonic acid

pNP p-nitrophenol

List of abbreviations

/;NPP p-nitrophenylphosphate

PUE Phosphate utilization efficiency

PZC Point of zero charge

Qa Amount of adsorbed compound

Qa max Maximal amount of adsorbed compound

R Introduced amount of radioactivity at time 0

r(t) Amount of radioactivity remaining in the solution after t minutes of

isotopic exchange

RM Root mucilage

RM dry Treatment root mucilage with dry ferrihydrite

RM wet Treatment root mucilage with ferrihydrite dispersed in deionized water

prior to mucilage adsorption

RNA Ribonucleic acid

SA3 SA3-C4HC (16x25)-2-4-9-7-B-B-B-B-l maize cultivar

SD Standard deviation

SE Standard error

SSA Specific surface area

Abstract -1

Phosphorus (P) deficiency is, with aluminum (Al) toxicity, one of the main limiting

factors for crop production in acid tropical soils. The strong P sorption on Fe and Al

oxides has been shown to be responsible for the low plant availability of both applied P

and soil P in these soils. Scientists have been able to identify maize genotypes that are

adapted to such unfavourable plant growth conditions. The major aims of this work were

to determine i) the influence of some iron oxide properties on plant P and Fe acquisition,

and ii) the contribution of root exudates in the low-P tolerance and Al resistance of some

maize cultivars. A low-P tolerant {Zea mays L. NST), a low-P, Al-resistant and acid soil

tolerant {Zea mays L. Sikuani) and a low-P susceptible maize cultivar {Zea mays L. SA3)

were compared with a Swiss cultivar {Zea mays L. Corso).

In a sand-ferrihydrite-P system, with Sikuani and Corso and two different ferrihydrites,

this work underlined the importance of some surface properties of soil minerals, such as

the specific surface area and the porosity, on P and Fe acquisition by maize. The effect of

the presence of roots on some properties of poorly crystallized minerals, such as

ferrihydrite, was demonstrated. The ferrihydrite and the P bound to the mineral

represented the only source of Fe and P for the plants. The exchangeability and the

acquisition of P by the plant decreased, while Fe acquisition increased with increasing

specific surface area and porosity of the ferrihydrite. Using the isotopic exchange kinetic

method, our results suggested that the main source of P for maize was the P isotopically

exchangeable in a week. After 12 weeks of plant growth, P and Fe acquisition by plants

were positively related to the higher root dry weight of Sikuani than of Corso. The

observed decrease of the specific surface area, of the porosity, and of the oxalate-

extractable Fe (Fe0), and the sorption of organic matter in the ferrihydrite adhering to the

roots were positively correlated with the decreased ferrihydrite P-sorption capacity during

plant growth. The presence of organic matter in the mineral adhering to the roots

demonstrated the release of organic compounds from maize roots.

Under hydroponic sterile conditions and after 18 days of plant growth, the contribution of

root exudation to the low-P tolerance of the four selected maize cultivars was confirmed.

Under P deficiency organic acid contents in roots, in phloem sap and in root exudates

increased. Differences between genotypes in the organic acids content of roots and

phloem were not related to their low-P tolerance. However, root exudation increased

Abstract - 2

more strongly for the cultivar NST, in particular, and Sikuani, than SA3 and Corso. There

was a difference between genotypes in the organic acid composition of phloem and root

contents, and root exudates. Trarcs-aconitic acid and malic acid were predominant. Root

acid phosphatase activity was higher in the cultivars tolerant to low-P conditions. The

release of protons from maize roots was low and was not related to the low-P tolerance of

maize cultivars. Under P deficiency root length, root dry weight, and anthocyanidin

coloration of leaves were higher in tolerant to low-P soils (NST and Sikuani), than in

susceptible cultivars (SA3 and Corso) and might contribute to the low-P tolerance of

maize plants. Our research demonstrated that the lower Al-related inhibition of root

growth in Sikuani, than in Corso was associated with a higher Al precipitation in the

presence of P in root tissues, higher contents and concomitantly higher exudation of

citric, malic and succinic acids, all known to complex Al, for Sikuani than for Corso.

The efficiency of some organic compounds, present in maize root exudates, to mobilize P

bound to ferrihydrite was studied. In competitive adsorption treatments with P on a

synthetic ferrihydrite, organic acids decreased P adsorption and increased P mobilization

and exchangeability of sorbed P. The effect of organic acids decreased in the order citric,

malic, trans-aconitic, succinic and formic acid and was higher when sorbed prior to P

addition, than the other way around. Nevertheless at the rate determined in our study, root

exudation of organic acids by maize might not contribute to a significant enhancement in

the P mobilization of P adsorbed on ferrihydrite in our conditions. Citric, malic and trans-

aconitic acids to some extent solubilized ferrihydrite. Similarly, the preliminary addition

of a high-molecular-weight root exudate, mucilage, collected on the nodal roots of Corso,

strongly decreased the subsequent P adsorption. This effect was mainly due to the

flocculation of ferrihydrite aggregates in the presence of mucilage, which limited the

transport of P from the solution to the adsorption sites. The mobilization by the root

mucilage of P sorbed on the ferrihydrite was low and the solubilization of ferrihydrite not

detected.

Résumé - 3

La déficience en phosphore (P) représente, avec la toxicité à l'aluminium (Al), un des

principaux facteurs limitant la production végétale dans les sols tropicaux acides. La forte

fixation de P sur les oxydes de fer et d'aluminium contenus dans ces sols a été démontrée

comme étant responsable de la faible disponibilité pour les plantes du P appliqué sous

forme d'engrais et du P du sol. Des génotypes de maïs adaptés à ces difficiles conditions

de croissance ont été identifiés. Les principaux buts de ce travail étaient de déterminer i)

l'influence de quelques propriétées des oxydes de fer pour l'acquisition de P et de Fe par

les plantes, et ii) la contribution des exsudats racinaires dans la tolérance de quelques

cultivars de maïs aux sols pauvres en P et à la toxicité à l'aluminium. Un génotype

tolérant aux sols pauvres en P {Zea mays L. NST), un génotype tolérant aux sols acides,

pauvres en P et présentant une haute teneur en Al {Zea mays L. Sikuani), et un génotype

non adapté aux sols pauvres en P {Zea mays L. SA3) furent comparés à un génotype

cultivé en Suisse {Zea mays L. Corso).

Dans des systèmes expérimentaux sable-ferrihydrite-P, avec Sikuani et Corso et deux

différentes ferrihydrites, ce travail souligne l'importance de quelques propriétés des

minéraux du sol, telles que la surface spécifique et la porosité, pour le prélèvement du P

et du Fe par le maïs. L'action racinaire sur certaines propriétés de minéraux faiblement

cristallisés, tels que la ferrihydrite, est démontrée. La ferrihydrite et le P lié à cette

dernière représentaient les seules sources en P et Fe pour la plante. L'échangeabilité et le

prélèvement du P par la plante diminue, alors que le prélèvement du Fe augmente avec

l'augmentation de la surface spécifique et de la porosité de la ferrihydrite. A l'aide de la

méthode des cinétiques d' échanges isotopiques, nos résultats suggèrent que la source

principale de P pour le maïs est le P isotopiquement échangeable en une semaine. Après

12 semaines de croisssance, le prélèvement de P et de Fe par la plante est positivement

corrélé à la matière sèche racinaire plus élevée chez Sikuani que Corso. La diminution de

la suface spécifique, de la porosité et de la fraction de Fe extractable par l'oxalate (Fe0),

et la présence de matière organique dans la fraction de ferrihydrite adhérante aux racines

sont positivement corrélés à la diminution de la capacité d'adsorption en P de la

ferrihydrite pendant la croissance de la plante. La présence de matière organique dans la

Résumé - 4

fraction de ferrihydrite adhérante aux racines démontre la libération de composés

organiques par les racines de maïs.

Dans des conditions hydroponiques stériles et après 18 jours de croissance, la

contribution de 1' exsudation racinaire à la tolérance aux sols pauvres en P des quatre

génotypes de maïs fut confirmée. Dans des conditions de déficience en P, la teneur en

acides organiques des racines, du phloème exsudé par la tige et des exsudats racinaires

augmentent. Les différences mesurées entre génotypes dans la teneur des racines et du

phloem en acides organiques ne sont pas liées à leur tolérance aux sols pauvres en P.

Cependant, l'exsudation racinaire de ces composés augmente pour les cultivars NST, en

particulier, et Sikuani plus fortement que pour SA3 et Corso. Des différences entre

génotypes existent dans la composition en acides organiques du phloème, des racines et

des exsudats racinaires. Les acides trans-aconiûque et malique sont prédominants.

L'activité racinaire en phosphatase est plus élevée dans les cultivars tolérants aux sols

pauvres en P. La libération par les racines de maïs de protons est faible et n'est pas

corrélée à la tolérance des cultivars aux sols pauvres en P. Lors de déficience en P, la

longueur racinaire, la matière sèche racinaire et la présence d'anthocyanidine dans les

feuilles sont plus élevés pour les cultivars tolérants (NST et Sikuani) que susceptibles

(SA3 et Corso) aux sols pauvres en P et pourraient contribuer à cette tolérance. Nos

recherches démontrent que la plus faible inhibition par Al de la croissance racinaire pour

Sikuani que Corso, est associée à une précipitation plus importante de l'aluminium dans

les tissues racinaires en présence de P, à une plus haute teneur et plus forte exsudation

des acides citrique, malique et succinique, connus comme pouvant complexé

l'aluminium, chez Sikuani que chez Corso.

L'efficacité de quelques composés organiques, présents dans les exsudats racinaires de

maïs, de mobiliser du P lié à la ferrihydrite, fut étudiée. Dans des expériences

d'adsorptions compétitives avec P sur une ferrihydrite synthétique, les acides organiques

testés diminuent l'adsorption du P, et augmentent la mobilisation et l'échangeabilité du P

adsorbé. L'influence des acides organiques diminue dans l'ordre citrique, malique, trans-

aconitique, succinique et formique et est plus importante lorsque leur asdorption se fait

Résumé - 5

avant celle du P, qu'après. Toutefois aux concentrations déterminées dans notre travail,

l'exsudation racinaire d'acides organiques par le maïs ne pourraient pas contribuer dans

une large mesure à la mobilisation de P adsorbé sur la ferrihydrite. La solubilisation à

différents degrés de la ferrihydrite par les acides citrique, malique et îra/w-aconitique est

démontrée. Dans des expériences d'adsorptions semblables, l'addition préliminaire d'un

exsudât racinaire de haut poids moléculaire, le mucilage, collecté sur les racines nodales

du cultivar Corso, diminue fortement 1'adsorption subséquente de P. Cet effet est

principalement dû à la flocculation des aggrégats de ferrihydrite en présence de mucilage.

Le transfert de P de la solution vers les sites d'adsorption étant ainsi limité. La

mobilisation par le mucilage du P lié à la ferrihydrite est faible. La solubilisation de la

ferrihydrite par le mucilage racinaire ne fut pas détectée.

General Introduction - 6

Importance of phosphorus (P) in crop production

Phosphorus, as phosphate, is an integral component of a number of important compounds

present in plant cells, such as the sugar-phosphates used in respiration and photosynthesis

and the phospholipids that make up plant membranes. It is also a component of

nucleotides used in plant energy metabolism and in the DNA and RNA molecules. P

requirement for optimal growth is in the range of 3-5 mg g" plant dry matter during the

vegetative stage of growth. P deficiency results in reduced plant growth, delayed crop

maturity, and a reduction in crop yield.

P deficiency and availability: the problem

Soil P deficiency is one of the most limiting factors affecting plant growth on a world¬

wide basis (Fairhurst et al., 1999). About 5.7 billion ha of soils do not contain sufficient

available P for optimum crop production (Sanchez and Salinas, 1981; World Bank, 1994;

Batjes, 1997). Von Uexkiill and Mutert (1995) estimate that 95% of the acid soils located

in tropical Africa, America, Asia, and the Pacific and Australia are deficient in P. In

many of these soils the extent of the P deficiency is so high that plant growth ceases once

the P stored in the seed has been exhausted. Phosphorus deficiency affects crop

production not only directly, but also soil fertility as a whole, rendering low-P soils prone

to degradation (Sanchez et al., 1997).

Soil P is derived from the weathering of the primary P minerals (mostly apatite).

Geochemical and biological processes of pedogenesis affect the total amount of P in soil

and its availability for plant (Walker and Syers, 1976). In strongly weathered soils there

tends to be less P, due to leaching and erosion. In the tropics and subtropics (Oxisols,

Ultisols and Alfisols), strong weathering is also correlated with an increase in the amount

of sesquioxides, which exhibit high P-sorption properties (Parfitt, 1978; Frossard et al.,

1995; Torrent, 1997), and a decrease in the proportion in primary calcium minerals

(Sanchez and Logan, 1992). While it is suggested (Tiessen et al., 1992) that in highly

weathered soils most of the plant-available P is derived from the mineralisation of

organic P forms, in less weathered soils plant-available P is derived predominantly from

inorganic P fractions. As a result of the inherent characteristics of soil parent material and


of weathering, the limited availability of P in tropical soils may be due to low content

and/or severe P sorption.

Strategies to alleviate P deficiency and low-P availability

The most obvious strategy to alleviate P deficiency is to add large quantities of phosphate

fertilizers to P-deficient soils, either as water soluble P fertilizer or as rock phosphate

(Roche et al., 1980; Von Uexkiill and Mutert, 1995; Sanchez et al., 1997). The alleviation

of soil P deficiency, with large applications of P fertilizer during vegetative crop growth,

has been an essential step in the development of large and sustainable yields of the crops

grown in low-P acid soils (Mutert and Sri Adiningsih, 1998). This strategy faces however

two problems: i) in tropical areas very little P fertilizers are usually available, either

because of the low purchasing power of the indigenous small-scale farmers or because of

the lack of infrastructure for distribution, and ii) phosphate resources are limited and not

renewable. For all these reasons the development of innovative local-scale farming

systems based on the use of renewable, and indigenous and economically available

source of P, such as recaptured urban residual P, waste recycled P, or animal and green

manure P is important. However, alternative P sources might not be able to cover the

severe P-deficiency conditions in the tropical acid soils and to sufficiently increase the

agricultural production in many developing countries.

Another possibility is to identify plant species that are efficient in term of P uptake, and

to include them in crop rotation in combination with relatively low levels of P inputs.

This can result in an increased accumulation of P in the biomass in the upper horizon,

leading in the long term to an increase in soil P fertility and to an increased production at

the agrosystem level. Successful examples of such strategies include the use of legumes

or other plants as green manure in pastures or cereal cropping systems (Friesen et al.,

1997; Gijsman et al., 1997; Rao et al., 1997; Sanchez et al., 1997; Oberson et al., 1999),

or the implementation of agroforestry systems (Hands et al., 1995; Sanchez et al., 1997).

The rotation of maize with Sesbania sesban used as green manure rather than continuous

maize cropping on P-deficient soil has been shown to increase maize yields (Maroko et

al., 1999). Such strategies are highly promising, but they encounter a resistance from


farmers' side, because for instance of the difficulty of managing legumes or because of

the long time needed for their installation (e.g. in the case of agroforestry systems).

Another strategy is to improve the P efficiency of crops and forage plants, i.e. either to

increase the acquisition of P by plants or to decrease the amount of P needed by plant to

obtain an optimum yield. This aim can be achieved either by breeding programs or by

genetic engineering. Breeding programs have been successful for instance in producing

maize cultivars tolerant to soil acidity (Granados et al., 1993; Pandey et al., 1994). Plant

biotechnology methods could be used to improve P-acquisition efficiency by identifying

genes responsible for the adaptation of given plants to low-P soils and to transfer them in

agricultural crops that exhibit a low P efficiency. But the use of P-efficient genotypes is

not an alternative to P fertilizer application. Improved plants will have to be included in

cropping systems in combination with relatively low levels of P inputs.

None of these research directions can be neglected in the search for an increased food

production through sustainable agriculture in the tropics. However, as suggested by Rao

et al. (1999a), to optimize the use of strategic P inputs and native soil P in P-limited soils,

research needs to focus on identifying genotypic differences and on understanding the

specific mechanisms involved in the acquisition of P from different P sources.

Plant P efficiency

Large differences in P efficiency exist between plant species and between cultivars within

species. Genotypes that can acquire and use scarce P resources more efficiently from

low-P soils could improve and stabilize agricultural production (Friesen et al., 1997; Rao

et al., 1999b). Genotypical differences in nutrient efficiency are related to differences in

efficiency of acquisition by the roots, or in utilization by the plant, or both (Sattelmacher

et al., 1994; Horst et al., 1996b; Rao et al., 1999a). Phosphate acquisition efficiency

(PAE) is defined as the total amount of P taken up by the plant, or as the amount of P

taken up per unit of root length. Phosphate utilization efficiency (PUE) is defined as the

dry matter production per unit nutrient in the dry matter (Marschner, 1995a).


Since plants take up their P by the roots as orthophosphate from the soil solution,

important parameters in determining PAE are: root length, diameter, branching pattern,

abundance and length of root hairs (Fredeen et al. 1989; Lynch et al. 1991; Mollier and

Pellerin, 1999). Other processes important for the PAE are the rate of orthophosphate

uptake from the solution (Km), the minimum concentration at which uptake occurs

(Cmin) (Waisel et al., 1996), the exudation of protons, of complexing or chelating

substances or of enzymes in the rhizosphere (Uren and Reisenauer, 1988; Raghothama,

1999; Jones and Farrar, 1999), and the symbiosis with mycorrhizal fungi (Wilcox, 1991;

Tarafdar and Marschner, 1994; Marschner, 1995a). The PUE on the other side includes,

besides P acquisition, P demand within the cell, P transport within the shoot and within

the root, compartmentation within the aerial parts (including the seeds) and within the

root, and shoot-root transport (Marschner, 1995a).

P nutrition: the contribution of root exudates

The zone of soil surrounding the root, the rhizosphere, is chemically, physically and

biologically different from that of the bulk soil. Typically the rhizosphere is characterized

by elevated microbial population and lower nutrient availability than the bulk soil

(Marschner, 1995a). The creation of the rhizosphere is driven by the release of a diverse

range of organic compounds from the root to the adjacent soil, fuelling microbial growth

and proliferation. The extent of this zone depends upon the quantity and diffusion

characteristics of the root exudates and soil properties such as water content, CEC, and

texture; typically the rhizosphere extends for 1 to 2 mm away from the root surface

(Waisel et al., 1996). Root exudates are known to contain both high (mucilage, proteins,

sloughed cells) and low (sugars, amino acids, organic acids) molecular weight

components (Uren and Reisenauer, 1988). Root exudates can facilitate the induction of

nodulation and the establishment of mycorrhizal association (Marschner, 1995a;

Kapulnik et al., 1993), they can contribute to metal detoxification (Horst et al., 1982;

Delhaize et al., 1993b), they can increase water availability (Guinel and McCully, 1986)

and root movement (Ray et al., 1988) or/and can attract root pathogens such as

nematodes (Cohn et al., 1996). There is now overwhelming evidence that some plants

directly modify the rhizosphere in order to gain access to previously unavailable reserves

General Introduction -10

of soil nutrients, e.g. Fe (Römheld, 1987) and P (Bar-Yosef, 1996b). While differences in

root morphological characteristics (growth and distribution, root diameter, and root hair

length) explain to a large extent the differences among cultivars in P acquisition

(Sattelmacher et al., 1994), there is still limited evidence that root exudates actually play

a significant role in plant P acquisition. This is mainly due to the poor experimental

techniques used for quantifying the rate of exudation from roots (Jones and Farrar, 1999).

Some root exudates can induce the release of orthophosphate to the soil solution and then

enhance P acquisition by plants. Root exudates relevant for plant P nutrition include

phosphatase enzymes, organic acids, and H+. Phosphatase can increase the hydrolysis of

soil organic P (Tarafdar and Jungk, 1987). Some organic acids, such as citric acid, malic

acid or oxalic acid, are known i) to compete with P for similar adsorption sites, whereby

organic acids directly replace P by ligand exchange (on crystalline Al- and Fe-oxides)

(Earl et al., 1979; Parfitt, 1979; Gerke, 1992; Hue, 1991; Staunton and Leprince, 1996),

and ii) to complex metal ions in the exchange matrix holding the P (Ca in calcium

phosphate or Fe3+ and Al3+ in Fe- and Al-oxides) (Zhang et al., 1985; Otani et al., 1996).

Protons released from plant roots can solubilize inorganic P (Grinsted et al., 1982;

Hedley et al., 1982; Moorby et al., 1988).

Objectives, hypotheses and progress of this present research work

The general objective of this research is to understand some processes governing P

acquisition efficiency in a model crop: maize, in order to increase on a long-term basis

plant yields in the tropical low-P acid soils. The specific aim of this present work is to

study the effect of P deficient conditions on the growth and the root exudation of various

maize cultivars known to be more or less tolerant to low-P conditions. And then to

determine under controlled conditions the efficiency of some selected root exudates on P

mobilization.

In collaboration with the International Maize and Wheat Improvement Center

(CIMMYT, Mexico) which has the world mandate for developing maize cultivars for

developing countries, some genotypes selected for different tolerance to low-P soils were

compared. We hypothesized that the tolerance of the cultivars tolerant to low-P


conditions might be due to a higher P-acquisition efficiency (PAE) through the root

exudation of organic acids or phosphatase enzymes, than cultivars susceptible to low-P

soils. Since proton release from roots and root architecture can also strongly affect the P

acquisition efficiency (Mollier and Pellerin, 1999; Bertrand, 1998) they will also be

considered.

The different sections of this work are organized as follows. First, in order to better

understand the ability of maize to mobilize the phosphate adsorbed on soil particles and

to investigate plant growth under phosphate stress conditions, a pot experiment will be

carried out (chapter I). We will use two synthetic ferrihydrites as a model for a mineral

highly reactive towards P. The acquisition of P by the plant from the rhizospheric and

non-rhizospheric portions of the soil will be determined. The efficiency of two maize

cultivars, with a priori different tolerance to low-P conditions, in acquiring P adsorbed on

the ferrihydrite will be studied. The effect of maize roots on the properties of the

ferrihydrite will complete this research.

In the second chapter of this research we will develop a system for measuring root

exudation under sterile conditions. We will then attempt to determine for four maize

cultivars, at a seedling growth stadium, if the selected differences in maize plant

tolerance to low-P conditions could be explained by the exudation of specific organic

compounds from roots and/or by different root growth. Our research will focus on the

release of organic acids, acid phosphatase enzymes, and protons.

In acid soils, crop production is often not only affected by P deficiency but also by the

presence of plant-toxic Al species. Some organic acids present in the roots and released

in the rhizosphere are known to complex Al (Delhaize et al., 1993b; Lan et al., 1995). In

chapter III we will determine if the selected resistance of a maize cultivar to acid soils

could be related to the synthesis and/or release of organic acids. We will also ascertain if

resistance to Al is related to the P nutritional status of the plant.


Organic acids can modify P sorption onto metallic oxides. The effect of some organic

acids released by maize roots on P sorption onto a ferrihydrite, and desorption, and on the

proportion of sorbed P remaining isotopically exchangeable will be determined (chapter

IV).

Seminal and nodal roots surfaces of maize, particularly apical zones, are covered by high-

molecular-weight mucilage, which consists mainly of polysaccharides (Cortez and Bill,

1982). In the last part of this research (chapter V) we will study the effect of mucilage on

P sorption and desorption on the surface of a ferrihydrite and on the proportion of sorbed

P remaining isotopically exchangeable.

Chapter I : P acquisition by maize in sand-ferrihydrite-P systems -13

Keywords: Iron acquisition, Iron oxide porosity, Isotopic exchange kinetic, Phosphorus

acquisition, Zea mays. L, Ferrihydrite.

Abstract

Iron oxide as a major compound in the sorption of phosphate in soils is responsible for

the low availability of both applied P and soil P and represents a main source of Fe for

plants.

This work underlined the importance of some surface properties of poorly crystallized

minerals, such as ferrihydrite, on the P and Fe acquisition by two maize cultivars, Corso

and Sikuani, in sand-ferrihydrite-phosphate systems. Sikuani was selected as tolerant to

low-P acid soils. Two ferrihydrites with different specific surface area (SSA = 172.9 and

317.9 m2/g) and porosity were compared. Using the isotopic exchange kinetic method we

defined the nature of the P taken up by the plant. The effect of the maize roots on the

properties of the ferrihydrite present in the rhizosphere was also determined.

P exchangeability and acquisition by the plant decreased with increasing SSA and

porosity of the ferrihydrite. In the ferrihydrite sample with a high SSA incubated without

plant the amount of isotopically exchangeable P decreased during the experiment, while

no change was observed for the ferrihydrite with the lower SSA. Fe acquisition by plant

increased with increasing SSA and porosity. P acquisition was higher in the fraction of

ferrihydrite adhering to the roots. Differences between Corso and Sikuani cultivars in P

and Fe mobilization was demonstrated. The observed larger root system of the cultivar

Sikuani than of Corso explained the higher acquisition of P and Fe by Sikuani than by

Corso after 12 weeks of plant growth. Our results suggested that the main sources of P

for both maize cultivars were the P in the solution and the P isotopically exchangeable

within one week.

The SSA, the porosity and the amount of oxalate-extractable Fe (Fe0) of the fraction of

ferrihydrite adhering to the roots decreased during plant growth, while no change to a

more crystallized form of iron oxide was observed. These modifications and the presence


of organic compounds in the iron oxide decreased ferrihydrite P-sorption capacity during

plant growth.

Introduction

Phosphorus (P) deficiency is one of the main limiting factor for crop production in acid

tropical soils (Granados et al., 1993). Aluminum and iron oxides and oxyhydroxides

(named afterwards oxides) are the main minerals governing P sorption in acid soils (He et

al., 1992; Parfitt, 1978; Barrow, 1985; Torrent et al., 1990, 1992). The reaction of P with

metallic oxides involve a rapid, strong ligand exchange with reactive hydroxyl groups

located on the oxide surface. The degree of crystallinity or porosity of iron oxides also

strongly affects the extent of phosphate sorption (Willett et al., 1988).

The strong P sorption on metallic oxides has been shown to be responsible for the low

plant availability of both applied P and soil P in acid soils (He et al. 1991; Parfitt 1978;

Menon et al. 1995). Laboratory and greenhouse studies indicate that P sorbed onto

metallic oxides is potentially available to plants although it can be only very slowly

released (Parfitt 1979; Soltan et al., 1993; He et al. 1994). This observation is in

agreement with the long-term residual effect of P fertilizers determined in variable-

charge soils (Barrow 1985). Most of these studies have been done, however, with well

crystallized minerals, such as goethite and hematite. A lack of information exists on the

effect of poorly crystallized iron oxides, such as ferrihydrite, on the P nutrition of plants.

Because of its high specific surface area, ferrihydrite can profoundly influence soil

properties, even if present in low concentrations (Childs, 1992). Ferrihydrite can

represent most of the specific surface area and P adsorption-sites in soils (Childs, 1992).

The solubility of Fe in soils is largely controlled by Fe oxides (Lindsay, 1991). The low

solubility of these minerals is the main cause of Fe deficiency in plants.

To cope with low P or Fe availability in soils plants have evolved various strategies:

increase their nutrient acquisition efficiency (total amount of nutrient taken up by the

plant, or amount of nutrient taken up per unit of root length) and/or increase their nutrient

use efficiency (dry matter production per unit nutrient in the dry matter) (Marschner and

Römheld, 1994). Large differences in P and Fe acquisition-efficiency or use-efficiency


exist between different species and also between genotypes of a single species

(Marschner 1995a; Sattelmacher et al., 1994; Horst et al., 1996a). More specifically, there

is now ample evidence that some plants can profoundly modify the physico-chemical and

biological properties of their rhizosphere in order to take up P and Fe present in sparingly

soluble minerals (Marschner, 1995 a).

The aim of this work was to assess the importance of some physicochemical properties

(specific surface area, porosity, amount of P adsorbed) of a poorly crystallized iron oxide,

ferrihydrite, on the P and Fe acquisition by maize. Two maize cultivars with a priori

different tolerance to soils with a low P-availability were considered. In a first step the

uptake of P and Fe by maize from P-ferrihydrite complex was studied in a pot

experiment. Then the changes in isotopically exchangeable P remaining on the oxide

after different periods of plant growth was investigated in order to obtain some

information on the nature of the P taken up by the plant. And in the last part the changes

in ferrihydrite properties (specific surface area, porosity, oxalate-extractable Fe) upon

plant growth was studied. All along this work the fraction of ferrihydrite adhering to the

roots was distinguished from the fraction of ferrihydrite non-adhering to the roots so as to

check if the presence of maize roots has had an effect on ferrihydrite properties and

mobilization of P and Fe.

Materials and Methods

Substrate.

The substrate used in this work was a sand-ferrihydrite mixture with in a weightweight

ratio of 100:1. Quartz sand (0.7-1.2 mm and 5-8 mm) was first washed into a 1.5 M HCl

solution for 72 h, thoroughly rinsed in distilled water and dried at room temperature. Two

2-line ferrihydrites, called hereafter ferrihydrite I and II, were used after having been

ground (labor planet mill type Pulverisette 5 by Fritsch) into particles with a diameter

between 20 and 200 jim. Ferrihydrite I was purchased from Aldrich (N° 37,125-4), while

ferrihydrite II was synthesised as described by Schwertmann and Cornell (1991). The

preparation of ferrihydrite II is presented in the chapters JV and V. The properties of both

ferrihydrites are presented in the Table 1.


Table 1. Ferrihydrite properties. Fe0 : oxalate-extractable Fe (Schwertmann, 1964); Fej :

dithionite-citrate-bicarbonate- extractable Fe (Mehra and Jackson, 1960). Fe0 / Fe^: ratio

oxalate-extractable Fe / dithionite-citrate-bicarbonate-extractable Fe.

Ferrihydrite I Ferrihydrite II

2 -1

Specific surface area (SSA) (m g ) 172.9 317.9

Porosity (%) 44.8 96.3

Pore diameter (nm) 5.9 2.7

Single particles diameter (nm) 10.0 5.0

Point of zero of charge (PZC) 7.3 7.6

Fe0 (mg g" ) 435.4 487.7

Fe0 / Fed 1.0 1.0

Preparation ofsand-ferrihydrite-P substrates.

Increasing amounts of P in the form of KH2P04 were mixed to 17.5 g of ferrihydrite in

300 ml of 50% Hoagland nutrient solution (Hoagland and Arnon, 1938), without P and

Fe. The four following concentrations of P were added to the mixture: Pi= 6.6 10~3, P2=

24.8 10"3, P3= 33.0 10"3 and P4= 66.0 10"3 tig P m"2 ferrihydrite. These concentrations

represented a total amount of 20.0, 75.0, 100.0 and 200.0 mg P per pot within the

ferrihydrite I treatment, and a total amount of 36.8, 137.9, 183.9 and 367.7 mg P pro pot

within the ferrihydrite II. The mixture was shaken for 36 h on an end-over-end shaker and

the P concentration remaining in the solution was determined (Table 2). The ferrihydrite -

P-Hoagland suspension was then mixed with 1.45 kg of quartz sand (0.7-1.2 mm) and

poured into 2 1 PVC cylindrical pots (10.5 cm diameter, 22.5 cm height). At the bottom of

each pot 300 g of the 5-8 mm diameter sand were placed in order to prevent anaerobiosis.


Table 2. P in solution (300 ml) and adsorbed after P adsorption on 17.5 g ferrihydrite

(mg P l"1 and |ig P nT2 ferrihydrite). P1= 6.6 u,g P m~2 ; P2= 24.8 u,g P m"2 ; P3= 33.0 |ig

P rn2

; P4= 66.0 (ig P m"2; n.d. : not detected; mean value ± SE; n = 6.

P P in solution (mg PI") P adsorbed (jig P m"2 ferrihydrite)

Ferrihydrite I Ferrihydrite II Ferrihydrite I Ferrihydrite II

Pi 0.02 ±0.002 n.d. 6.60 ±0.093 6.60 ±0.031

p2 0.21 ±0.018 0.13 ±0.010 24.78 ±0.217 24.79 ±0.189

p3 0.84 ±0.024 0.69 ±0.021 32.92 ±0.234 32.96 ±0.218

P4 87.64 ±0.834 24.13 ±0.616 57.32 ±0.615 64.70 ±0.583

Vegetal material

Two maize cultivars, Zea mays L. Corso and Zea mays L. Sikuani, were compared. Corso

is a Swiss silo maize with fast growth during early development. ICA V-110 Sikuani

(called hereafter Sikuani) is an open pollinated variety developed in Colombia by

recombining selected acid soil-tolerant lines derived from Population SA3 (Friesen et al.,

1997). It yields 4.0 t grain/ha under normal soil conditions and 2.1 t grain/ha under acid

soil conditions (pH = 4.2 and 60% Al saturation).

Kernels were selected based on their weight (between 0.28-0.30 g) in order to limit the

variability between the plants. The seeds were surface sterilized using 18 M H2SO4 for

30 s. After rinsing, they were dipped into 95° alcohol for 5 min, subsequently washed

with sterile water and dipped into 10% P-free H2O2 for 30 min, and finally rinsed

thoroughly with sterile water.


Plant growth

Three pre-germinated seeds were transplanted per pot. At the three leaves vegetative

growth stage, the pots were thinned to one plant per pot. Controls were carried out with

the same treatments but without plant. During the entire experiment the pots were kept

near the water holding capacity by weighing once and watering once or twice a day with

the modified Hoagland and Arnon nutrient solution (1938). This nutrient solution

consisted of MgS04 (1 mM); Ca(N03)2 (2.5 mM); K2S04 (1.25 mM); H3B03 (0.01 mM);

MnS04 (0.001 mM); ZnS04 (0.001 mM); CuS04 (0.0005 mM); Na2Mo04 (0.0005 mM).

The pH was adjusted to 5.5. The pots were placed in a growth chamber at 25/18 °C,

75/90% relative humidity, 16/8 h day/night regime and a light intensity of 250 |imol

quanta m" sec" . The position of the pots was changed within and between the treatments

twice a week.

Sampling and analysis

The sampling periods were fixed at 3, 6, 9 and 12 weeks plant growth. At each sampling

date four pots of each P concentrations with plants, and three control treatment pots

without plants, were selected. The plants were cut at 3 to 5 cm above substrate level in

order to simplify the following work steps. After weighting, the pot content was

transferred with care into a 5 1 Pyrex Becher. This receptacle was slowly shaken and

some milliliters of the substrate suspension were collected and filtered (0.025 urn). After

determining the pH, the solution was analyzed for total P (Bowman, 1989) and inorganic

P (John, 1970). Roots were shaken to separate the sand-ferrihydrite fraction non-adhering

to the roots. To collect the fraction of substrate adhering to the roots, roots were briefly

rinsed with a few ml of deionised water. The ferrihydrite (20-200 |im) was properly

separated from the sand (0.7-1.2 and 5-8 mm) in both sand-ferrihydrite fractions with

distilled water using a sieve of 0.3 mm diameter mesh and afterwards air-dried.

Ferrihydrite was analyzed for total P (HC104 digestion method; Dick and Tabatabai,

1977; Cade-Menun and Lavkulich 1997), isotopically exchangeable P (Fardeau et al.,

1985), inorganic C (determination of Ca and Mg carbonates; El Mahi et al., 1987), total C

(CNS-Analyser Carlo Erba ANA 1500), organic C (total C-inorganic C), porosity,

specific surface area (SSA), and oxalate-extractable Fe (Fe0) (amorphous iron extraction;

Schwertmann, 1964).


The different plant parts, roots, leaves, shoot and spikes were thoroughly rinsed with

deionized water and fresh weight was recorded. Dry weight was determined after drying

in an oven (8 h, 80°C). Dry plant material was cut and ground in an agate ball mill

(Schwingmühle type MM-2 by Retsch) and analyzed for total Fe (ICP, Varian Liberty

220) and total P by spectrophotometry (Kontron Uvikon 800), according to John (1970).

Because of the impossibility to totally remove iron oxide adhering at the root surface, Fe

content was only determined in the aerial parts of the plants.

Isotopic exchange kinetic method

A 1/100 ratio ferrihydrite/water suspension is shaken for 24 h on an end-over-end shaker.

One ml of a P solution containing (2 10 Bq) was added to the 99 ml of suspension at

time zero and shaken. At time t = 1, 10, 40, 100, 1440, 4320 and 10080 min, 2 ml of the

suspension were removed with a plastic syringe and the solution immediately separated

from the solid phase by filtration at 0.2 \\m. Phosphate ions concentration in solution (Cp)

and pH were determined after 100 min and 10080 min. The introduced amount of

radioactivity at time 0 (R, Bq) and the amount of radioactivity remaining in the solution

after t minutes of isotopic exchange (r(t), Bq) were measured. The quantity of

isotopically exchanged P at time t, E(t) {\ig P m"2 ferrihydrite) was calculated assuming

31 33that (i) P and P have the same fate in the system and (ii) whatever the time t, the

specific activity of the phosphate in the soil solution is identical to that of the isotopically

exchangeable phosphate in the whole system (Equation [1]).

r(t)/(ACP) = R/E(t) [1]

The factor A represents the soil / solution ratio of 1 g of ferrihydrite in 100 ml of water so

that (ACp) is equivalent to the water-soluble P content of the ferrihydrite expressed in \ag

P m"2 ferrihydrite.

Therefore,

E(t) = ACPR/r(t) [2]

Chapter I : P acquisition by maize in sand-ferrihydrite-P systems - 20

The amount of P isotopically exchangeable within 1 min, E i mjn, is an estimate of the

pool of the free phosphate ions. This is the sum of the phosphate ions in soil solution plus

the phosphate ions in the solid soil phase that instantaneously exchange with phosphate

ions in the solution (Salcedo et al. 1991). In this work the amount of P isotopically

exchangeable within a week (E i week) was calculated as follows.

E i week = ACPR/r( 10080) [3]

The amount of P non isotopically exchangeable within a week (E > i week) was calculated

as the difference between the total amount of P sorbed onto the ferrihydrite and E i week.

Statistical analysis

Standard errors calculation was performed using Excel software (Microsoft office 97).

Analysis of variance was carried out using Statgraphics statistical software (one-way

ANOVA, Multiple Range Test).

Results and Discussion

Plant growth, P and Fe uptake

Plant growth

Effect of P supply. The total amount of P brought to the system strongly affected plant

growth (Table 3). The biomass increased with increasing P supply, while the roots/aerial

parts ratio decreased. Excepted for the highest P supply (P4= 66.010"3 fig P m"2

ferrihydrite), P deficiency symptoms were observed on leaves and were characterized by

anthocyanidin formation: red-purple interveinal chlorosis on length of leaves and by deep

purple tints on the limb of the leaves. At the lowest P supply treatment (treatment Pi)

plant growth stopped after 4-5 weeks.

Effect of the ferrihydrite. Ferrihydrite properties affected roots and aerial parts biomass

production, and roots/aerial parts ratio (Table 3). These three parameters were higher for

ferrihydrite I than ferrihydrite II treatments except for the lowest P supply.


Table 3. Influence of ferrihydrite properties and P nutrition on roots and aerial parts dry weight

after 12 weeks plant growth. P1= 6.6 (ig P m"2 ; P2= 24.8 |ig P m"2 ; P3= 33.0 |ig P m"2 ; P4= 66.0

(lg P m"2; Aerial parts: leaves + shoot + spikes.

Cultivar P Dry weight (g) Roots / Aerial parts

Ferrihydrite I Ferrihydrite II Ferrihydrite

Roots Aerial parts Roots Aerial parts I II

Corso Pi 0.62ab

1.83aa 0.51ab 1.37ba 0.34 a 0.37 a

P2 4.16ab 14.25aa

1.10bb

5.51ba

0.29a

0.20b

P3 9.92ab

34.16aa 3.57bb

17.25ba

0.29a 0.21b

P4 17.1ab

76.15aa 8.75bb 58.01 ba 0.23 a 0.15b

Sikuani Pi 1.13aa 1.64aa 0.94aa 1.28ba 0.69a 0.73a

P2 7.98aa ll.llab 2.52ba 5.73ba 0.72 a 0.44 b

P3 16.56aa 29.78ab

6.03ba 13.33bb 0.56a 0.45 b

P4 28.34aa 71.04ab 15.13ba 52.32bb

0.40a 0.29b

Dry weight: within the same row and the same parameter (roots, aerial parts, roots/aerial parts

ratio) mean values followed by the same first letter are not statistically different at a

probability level a = 0.05. Within the same column and the same P treatment mean values

followed by the same second letter are not statistically different at a probability level a = 0.05.

n = 6.

Ratio roots/aerial parts: within the same row mean values followed by the same letter are not

statistically different at a probability level a = 0.05. n = 6.


Effect ofthe maize cultivar. Differences between the two cultivars were observed in terms

of total biomass production and root-aerial part ratio. Aerial parts production was higher

for the cultivar Corso while total biomass and roots production, and the ratio root/aerial

part were higher for the cultivar Sikuani. The larger root system for Sikuani than for

Corso might contribute to the better adaptation of Sikuani to low P availability, through a

larger occupation of soil. The vegetative development of the cultivar Sikuani was much

slower than that of the cultivar Corso (Figure 1). After 12 weeks of growth no spikes

were present for Sikuani, treatment P4, while for Corso these vegetative organs already

represented 44% of the dry weight of the aerial parts in the ferrihydrite II treatment. This

phenomenon has to be seen as a specific variety property.

P acquisition and P concentration ofplants.

Effect of P supply. P content and P concentration of plants increased with increasing P

supply (Table 4; Table 5 A-D). For the four distinct levels of P supply, P concentration in

the plant tissues was below the critical value (3 mg P g dry weight) reported for maize

(Reuter et Robinson, 1997).

Effect of the ferrihydrite. P concentration and uptake in plant were higher for ferrihydrite

I than for ferrihydrite II treatments although the absolute quantities of P added per pot

were lower in the treatment ferrihydrite I than in the treatment ferrihdrite II (Table 4;

Table 5 A-D). These results suggest that ferrihydrite properties, porosity and specific

surface area, significantly affect P acquisition by both maize cultivars.

Effect of the maize cultivar. Differences between both cultivars were noticed. After 12

weeks of plant growth P concentration in the whole plant and P acquisition was higher

for Sikuani than for Corso (Table 4; Table 5 A-D). However, during the first 9 weeks of

plant growth, P acquisition from ferrihydrite was higher for cultivar Corso than for

cultivar Sikuani, in spite of the higher root development observed for Sikuani (Table 3).

The faster vegetative development of Corso might also explain the higher P acquisition

during the first weeks of plant growth. On the other hand the lower P requirement of

Sikuani in the initial growth period, related to slower plant development, might partly

explain its adaptation to low-P soils.


Plant growth (weeks)

Figure 1. Biomass production for two maize cultivars (Corso and Sikuani) grown in the

presence of a phosphated ferrihydrite as the sole source of P and Fe. Treatment ferrihydrite JJ,

P input 66 tag m"2. Mean value; n = 4 ; • : roots; A : stalk (shoot + leaves); : ears (spikes);

: male flowering time; open symbols: Corso; full symbols: Sikuani.


Table 4. Influence of ferrihydrite properties and P nutrition on P

-2concentration in plant after 12 weeks plant growth. P]= 6.6 flgPm" ; P2=

24.8 (ig P m"2 ; P3= 33.0 jag P m"2 ; P4= 66.0 |ig P m"2 ; mean value ± SE;

n = 4.

Cultivar P P concentration in total plant (mg P g" DW)


Corso Pi 0.32 ± 0.006 0.39 ± 0.006

P2 0.30 ± 0.006 0.29 ± 0.008

P3 0.54 ± 0.020 0.39 ±0.012

P4 0.73 ± 0.025 0.68 ± 0.028

Sikuani Pi 0.30 ± 0.006 0.35 ± 0.006

P2 0.35 ±0.010 0.37 ±0.017

P3 0.57 ± 0.027 0.48 ± 0.025

P4 0.84 ± 0.029 0.78 ± 0.023


Table 5 A. Ferrihydrite I: total and isotopically exchangeable P, P in solution and P mobilized by Zea mays L.

-2. -2Corso. Pi= 6.6 u.g P m"z ; P2= 24.8 u,g P m"z ; P3= 33.0 tig P m ; P4= 66.0 ug P m . n.d.: not detected.

P Plant P P P P P P

growth remaining on in mobilized exchangeable non-exchangeable exchangeable

ferrihydrite solution by plant in 1 week (E(t)) in 1 week (E(t)) in 1 week (E(t))

(weeks) (ig P m"2) (% P total)

p2 3 24.2aA

0.007aA

0.6c

9.5aA

14.7aA

39.4aA

6 23.2b

0.001aA

1.3b

7.9b

15.2a

34.3b

9 21.7c

n.d. 1.7a

7.2c

14.5a

33.2b

12 21.5cB

n.d. 1.8a

6.7cB

17.8aA

31.3cB

Control 3 24.7A

0.015A

- 9.9A

14.8A

40.1A

Control 12 24.4A

0.008A

- 8.9A

15.5A

36.5A

p3 3 30.9aA

0.037aB

1.3c

15.8aA

15.1aA

51.1aA

6 27.9b

0.021b

3.9b

12.0b

15.9a

42.9b

9 24.4c

0.008"

7.4a

9.0c

15.5a

36.6c

12 23.6cB

0.001cB

7.8a

7.8cB

15.8aA

33.1cB

Control 3 32.5A

0.076A

- 16.9A

15.6A

52.0A

Control 12

3

31.8A

0.045A

- 16.6A

15.2A

52.2A

P4 56.9aA

0.215aB

5.4c

35.9aA 21.0aA 63.1 aA

6 48.3b

0.023b 14.8" 28.4 b 19.8 a 58.9 b

9 40.9c

0.007c

22.5a

21.3c

19.6a

52.0c

12 40.3cB

0.005cB

22.6a

19.6cB

20.7aA

48.7cB

Control 3 57.8A

7.524A

- 36.5A

21.0A

63.1A

Control 12 55.5A

5.973A

- 36.7A

18.8A

66.1A

Within the same P treatment and the same column, mean values followed by the same small letter are not

statistically different at a probability level a = 0.05 ; Within the same P treatment, the same column and the

same sampling time, mean values followed by the same capital letter are not statistically different at a

probability level a = 0.05


Table 5 B. Ferrihydrite I: total and isotopically exchangeable P, P in solution and P mobilized by Zea mays L.

-2Sikuani. Pi= 6.6 (Xg P m"z ; P2= 24.8 (ig P m'z ; P3= 33.0 Lig P m"z ; P4= 66.0 Lig P m . n.d.: not detected.

Plant P P P P P P



(weeks) (LigPrn"2) (% P total)

p2 3 24.5aA

0.009aA

0.5b

9.8aA

14.7aA

40.2aA

6 23.6a

0.002a

0.8b

8.3b

15.3a

35.2b

9 21.8" n.d. 1.6 a 7.6 b 14.2 a 35.2 b

12 21.1bB

n.d. 2.2a

6.2cB

14.9aA

29.3cB

Control 3 24.7A

0.015A

- 9.9A

14.8A

40.1A

Control 12 24.4B

0.008A

- 8.9A

15.5A

14.6aA

36.5A

p3 3 31.4aA

0.045aB

1.0d

16.8aA

53.4aA

6 29.4b

0.030b

2.9c

12.9b

16.5a

44.0b

9 24.8c

0.011c

6.6b

9.5c

15.3a

38.5c

12 22.6dB 0.001cB 8.8 a 6.5 dB 16.1aA 28.6 dB

Control 3 32.5A

0.076A

- 16.9A

15.6A

52.0A

Control 12 31.8A

0.045A

- 16.6A

15.2A

52.2A

p4 3 57.0aA

0.306aB

4.1d 35.9aA 21.1aA 63.0 aA

6 53.6" 0.107 b 9.9 c 32.2 b 21.4 a 60.1 b

9 40.5c

0.006c

23.2b

20.6c

19.9a

50.9c

12 36.7dB

n.d. 27.4a

14.9dB

21.8aA

40.6dB

Control 3 57.8A

7.524A

- 36.5A 21.0A 63.1 A

Control 12 55.5A

5.973A

- 36.7A

18.8A

66.1A

Explanation for the small and capital letters following the mean values see Table 5 A.


Table 5 C. Ferrihydrite II: total and isotopically exchangeable P, P in solution and P mobilized by Zea mays L.

-2 -2Corso. Pi= 6.6 fig P m ; P2= 24.8 ixg P m"z ; P3= 33.0 tig P m"z ; P4= 66.0 (ig P m . n.d.: not detected

Plant P P P P P P


f h A tsolution by plant in 1 week (E(t)) in 1 week (E(t)) in 1 week (E(t))

(weeks) (jig P m"2) {% P total)

p2 3 24.1aA

0.005aA

0.2a

7.5aA

16.7bA

30.9aA

6 24.1a

n.d. 0.3a

1.3b

22.8a

5.4b

9 23.8a

n.d. 0.3a

1.1b

22.7a

4.6b

12 23.7aA

n.d. 0.3a

0.9bA

22.8aA

3.9bA

Control 3 24.5A

0.012A

- 7.6A

16.9A 31.0A

Control 12

3

24.3A

0.003A

- 1.7A

22.6A

6.2A

p3 31.7aA

0.029aB

0.4c

14.3aA

17.4bA

45.1aA

6 31.0a 0.008b 0.9" 12.5 a 18.5" 40.3 a

9 30.4a

n.d. 1.4a

7.9b

22.5a

25.9b

12 30.3aA

n.d. 1.5a

6.9bA

23.3aA

22.9bA

Control 3 32.0A

0.091A

- 14.2A

17.8A

44.4A

Control 12 31.5A

0.062A

- 8.6A

22.9A

27.3A

p4 3 63.7aA

0.128aB

1.6d

36.2aA

27.5cA

56.8aA

6 60.0b

0.012b

5.2c

27.0b

33.0b

45.0b

9 57.0c

n.d. 7.4b

16.7c

40.3a

29.3c

12 56.1cB

n.d. 8.1a

13.7dB

42.4aA

24.5dB

Control 3 64.4A

3.866A

- 36.7A

27.7A

23.6B

Control 12 63.0A

0.340A

- 20.3A

42.7A

32.2A

Explanation for the small and capital letters following mean values see Table 5 A.


Table 5 D. Ferrihydrite II: total and isotopically exchangeable P, P in solution and P mobilized by Zea mays L.

Sikuani. Pj= 6.6 jig P m~z ; P2= 24.8 jig P rnz

; P3= 33.0 \Lg P m"z ; P4= 66.0 ug P m". n.d.: not detected.

P Plant P P P P P P



(weeks) (ug P ml) {% P total)

P2 3 24.1aA 0.007aA 0.2b 8.2aA 15.9bA 33.9aA

6 24.0a n.d. 0.3b 1.6" 22.5a 6.6"

9 23.8a n.d. 0.3b 1.3b 22.5a 5.4b

12 23.6aA n.d. 0.6a 0.9 bB 22.7aA 3.6bA

Control 3 24.5A 0.012A - 7.6A 16.9A 31.0A

Control 12 24.3A

31.8aA

0.003A

- 1.7A 22.6 A 6.2 A

P3 3 0.034aB 0.3c 14.4aA 17.4bA 45.3 aA

6 31.2a 0.010b 0.8 b 12.8a 18.4b 40.9"

9 30.5a n.d. 1.4a 8.1" 22.5a 26.5c

12 30.1aA n.d. 1.7a 6.3 bA 23.8 aA 21.0dA

Control 3 32.0A 0.091 A - 14.2 A 17.8 A 44.4 A

Control 12 31.5A 0.062 A - 8.6 A 22.9 A 27.3 A

P4 3 64.1aA 0.301aB 0.8 d 34.9 aA 29.1 dA 54.5aA

6 60.4b

0.048bB

3.9C 24.9b

35.5c 41.2b

9 56.4c n.d. 7.4b 17.5c 38.9b 31.Ie

12 54.6cB n.d. 9.5a 11.6dB 43.0aA 22.5 dB

Control 3 64.4A 3.866A - 36.7 A 27.7 A 23.6B

Control 12 63.0A

0.340A

- 20.3A

42.7A

32.2A

Explanation for the small and capital letters following mean values see Table 5 A.


Fe acquisition and concentration by plants.

Effect of P supply. Symptoms of Fe deficiency were visible within each P treatments.

Except for the highest P supply (P4= 66.0T0"3 (j,g P m"2 ferrihydrite), P deficiency

symptoms strongly dominated Fe deficiency symptoms on the leaves. Fe deficiency

symptoms were observed as bright yellow interveinal chlorosis (striping) on entire length

of new leaves. Fe concentration in the aerial parts increased with increasing P supply up

to a maximum and then decreased (Figure 2). Since the total amount of Fe in the aerial

parts continuously increased with increasing P supply (Figure 3), the lower Fe

concentration observed at the highest rate of P application was related to higher increase

in dry matter.

Effect of the ferrihydrite. The Fe content of maize was higher with the ferrihydrite I than

with the ferrihydrite II (Figure 3). The larger root system within the ferrihydrite I than the

ferrihydrite II treatments might account for this difference. However, the ratio of Fe

content in the aerial parts to the root biomass was lower for ferrihydrite I than ferrihydrite

II treatments (Figure 4). This suggests that maize roots were more efficient in acquiring

Fe from the ferrihydrite II than from ferrihydrite I. This can be explained by the highest

porosity and specific surface of the ferrihydrite II.

Effect of the maize cultivar. Fe concentration and content in the aerial parts was higher in

Sikuani than in Corso (Figure 2 and Figure 3). As reported by Brown (1967) Fe uptake in

maize is genetically controlled. The higher Fe content in the aerial parts of Sikuani than

of Corso might be explained by the larger root system for Sikuani than for Corso. Since

the ratio Fe content in the aerial parts/root biomass was lower for Sikuani than for Corso

(Figure 4), the higher content in the aerial parts in Sikuani than in Corso did not appear to

be related to a more efficient Fe acquisition mechanism in Sikuani. The difference

between both cultivars could also be due to a higher efficiency in Fe transport from roots

to shoot in Sikuani.


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0.24 -

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0.16 -

0.12

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0 20 40 60 0 20 40 60

-2

P treatment (jig P m )

Figure 2. Relation between the P inputs and the Fe concentration in the aerial parts for two

maize cultivars (Corso and Sikuani) grown for 12 weeks in the presence of a phosphated

ferrihydrite as the sole source of P and Fe. Pi= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P

m"2 ; P4= 66.0 fig P m"2 ; mean value ± SE ; n = 4 ; O : Corso; À : Sikuani.


14

12

10 -

ft

a ft

es s

ft

2 -

0

Ferrihydrite I 4 Ferrihydrite II

n i i i

0 20 40 60 0 20 40 60

-2

P treatment (|Lig P m )

Figure 3. Relation between the P inputs and the Fe uptake in the aerial parts for two maize

cultivars (Corso and Sikuani) grown for 12 weeks in the presence of a phosphated ferrihydrite

as the sole source of P and Fe. P1= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P m"2 ; P4=

66.0 fig P m~2 ; mean value ± SE ; n = 4 ; O : Corso; A : Sikuani.


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Changes in some physico chemical properties of the mineral substrate

pH value in solution

Plant growth resulted in an increase in pH values (Figure 5). This was related to the

uptake of nitrate as the sole source of nitrogen during plant growth which led to the

release of HCO3" or OH" into the solution to maintain pH equilibrium in plant tissue

(Marschner, 1995b; Nye, 1986). Increase of pH values were higher for Sikuani than for

Corso. These increases were positively correlated for both cultivars to their root biomass

production. In the treatments incubated without plant the higher pH values for

ferrihydrite II than for ferrihydrite I, observed in particular for P4, reflected the higher

quantity of sorbed P on ferrihydrite II and was related to a higher release of OH group

from the surface of the oxide.

Evolution ofP in the substrate during plant growth.

Inorganic P in the solution. The concentration of inorganic P (Pi) in the solution

decreased very rapidly with plant growth, due to plant P uptake (Table 5 A-D). As shown

by the control treatment, Pi sorption onto the ferrihydrite during the experiment was

relatively low for ferrihydrite I, but high for ferrihydrite II treatments. The high specific

surface and porosity and therefore the higher number of sorption sites on ferrihydrite II

explain this observation. This in turn is coherent with the lower P acquisition and total

biomass production of both maize cultivars with the ferrihydrite II than with the

ferrihydrite I (Table 3). No significant differences were measured between total P and Pi

in the solution (data not shown).

P isotopically exchangeable on the ferrihydrite. The amount of P isotopically

exchangeable within one week (E 1 week) remained constant in the presence of ferrihydrite

I without plants during the 12 weeks of incubation while it decreased in the presence of

both maize cultivars (Table 5A and B, Figure 6). In the presence of the ferrihydrite I, the

amount of P taken up by maize during the entire growth period is totally accounted for by


the changes in P remaining in the solution and in E i week (Figure 7, Table 5 A-B). The

pool of P non-exchangeable within one week (E >j week) did not change during the 12

weeks of the plant growth. This suggests that in the presence of ferrihydrite I the main

sources of P for both maize cultivars were the P in the solution and the P isotopically

exchangeable within a week.

Because of its higher specific surface and porosity, E i wee)j decreased steadily in the

presence of ferrihydrite II without plant during the 12 week of incubation while E >i week

increased (Table 5 C-D). In the presence of maize, E i week decreased more rapidly than

in the samples incubated with the ferrihydrite II without plant (Tab 5 C-D). As for the

experiment with the ferrihydrite I, a tight relation was observed between the quantity of P

remaining in the solution plus E i week and the quantity of P taken up by the plant of the

one side in the cultivated treatments, and the quantity of P remaining in the solution plus

E i week in the incubated sample on the other side. This suggests that also in the presence

of ferrihydrite II the main sources of P for both maize cultivars were the P in the solution

and the P isotopically exchangeable within a week.

The transfer of P ions into the pool of P non exchangeable in one week in the presence of

the ferrihydrite II explained also the lower P acquisition and total biomass production by

both maize cultivars grown in the presence of this mineral than when grown in the

presence of the ferrihydrite I. Differences between both cultivars in term of P mobilized

by plants and P exchangeable within one week (Table 5 A-D) were well related with the

biomass production of both cultivars (Table 3).


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7.4

7.2

Ferrihydrite I Ferrihydrite II 4

20 40 60 20 40 60

-2

P treatments (jug P m )

Figure 5. Effect of plant growth on the pH value in the solution for two maize cultivars (Corso

and Sikuani) grown for 12 weeks in the presence of a phosphated ferrihydrite as the sole source of

P and Fe P1= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P m"2 ; P4= 66.0 fig P m2

; mean

value ± SE ; n = 4 ; O : Corso 12 weeks; A : Sikuani 12 weeks; D : Control t = 0; : control 12

weeks.


Isotopic exchange time (days)

Figure 6. Changes in the isotopically exchangeable P content of the ferrihydrite I and II

after 3, 6, 9 and 12 weeks of plant growth (maize cultivar Corso) as compared to the

isotopically exchangeable P content of the same substrate incubated without plant for 0

and 12 weeks. Treatment P4 = 66.0 fig P m"2 ; mean value ± SE ; n = 4 ; O: 3 weeks

growth; A: 6 weeks growth; V: 9 weeks growth; O: 12 weeks growth; D: control t =

0; : control 12 weeks.


a

+

C

to

s

vi

Ph r*

m

40

30

+ S? &H 20I WD

10

0

Ferrihydrite I

0

r =0.999

y = 0.92x + 1.28

Ferrihydrite II

P, ir r = 0.978

y = 0.91x + 1.22

20 40

Control: P

0 20 40

+ Pi.e. (t = 1 week) solution

(|Lig P m"2)

Figure 7. Relation between the amount of P isotopically exchangeable within a week (Eiweek) and the P

remaining in the solution in the substrate incubated without plant (axis X), and the amount of P taken

up by the plant plus the amount of P remaining in the solution and E i week after plant growth (axis Y).

Pi= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P nf2 ; P4= 66.0 fig P m"2 ; mean value ± SE ; n =

4.#: Corso; A: Sikuani; open symbols: 3 weeks ; full symbols: 12 weeks.


Evolution ofP sorbed on ferrihydrite fractions adhering and non-adhering to the roots.

The total P content of the ferrihydrite of the substrate adhering to the roots was always

lower than in the fraction of ferrihydrite non-adhering to the roots (Table 6 A-B). The

concentration of P ions in solution during the isotopic exchange kinetic (Cp), the ratio

rl/R and the amount of P exchangeable within 1 min (E i mjn) were also affected by the

adherence of the ferrihydrite to the roots. In the beginning, the plant took up P adsorbed

on oxides particles in the vicinity of the roots. After 3 weeks of plant growth, within both

ferrihydrite treatments the Cp, E \ ^n and rl/R were lower in the fraction of ferrihydrite

adhering to the roots than in the one non-adhering to the roots. A lower ratio rl/R is

related to a higher P sorption capacity (Fardeau, 1993). At the end of the experiment, the

values of the Cp and E i mjn were still lower, but the ratio rl/R higher, in the fraction of

ferrihydrite adhering to the roots than in the non-adhering fraction. According to Fardeau

(1993) the stronger decrease of water soluble P in the fraction of ferrihydrite adhering to

the roots should have resulted in an decrease of the ratio rl/R, i.e. to an increase of the P

sorption capacity, rather than in an increase in rl/R which denotes a decrease in the P

sorption capacity. This points out to significant modifications in the P chemistry in the

root vicinity after 12 weeks of plant growth. These could be due to a change in the oxide

sorption properties related either to changes in the ferrihydrite porosity following root

colonisation, and/or to the adsorption of organic compounds at its surface following root

exudation. Indeed phosphate and organic compounds can compete for the same

adsorption sites on the iron oxides (Staunton and Leprince, 1996).


Table 6 A. Adhering- non-adhering ferrihydrite I to the roots by Zea mays L Corso and Sikuani. ; P2= 24.8 fig P

m"2 ; P3= 33.0 fig P m"2 ; P4= 66.0 fig P nf2; Cp: P concentration in solution within the isotopic exchange

kinetic; R: the introduced amount of radioactivity at time 0; rl: the amount of radioactivity remaining in the

solution after 1 minute of isotopic exchange; E \ ^^ amount of P adsorbed on the oxide exchangeable within 1

min; Adh.: ferrihydrite adhering to the roots; Non-adh.: ferrihydrite non-adhering to the roots.

P Cultivar Sampling P total Cp in solution rl/R E i mm

(weeks) OigPrn2) (mgPl"1) (figPrn"2)

Adh. Non-adh. Adh. Non-adh. Adh. Non-adh. Adh. Non-adh.

P2 Corso 3 23.988aa 24.247aa 0.023ba 0.031aa 0.117bb 0.142aa 0.114aa 0.126aa

12 20.321bb 21.809ab 0.002ab 0.002ab 0.236aa 0.110bb 0.005bb 0.011ab

Sikuani 3 24.213aa

24.602aa 0.028ba 0.039aa 0.138bb 0.161aa 0.117ba 0.140aa

12 19.922bb 21.746ab 0.003ab 0.002ab 0.209aa 0.092bb 0.008ab 0.013 ab

Control 12 - 24.418 - 0.042 - 0.233 - 0.104

p3 Corso 3 30.207aa 30.968aa 0.050ba 0.064aa 0.212bb 0.256aa 0.136aa 0.145aa

12 23.033bb 25.791 ab 0.006bb 0.012ab 0.350aa 0.213bb 0.010bb 0.033ab

Sikuani 3 30.970aa 31.632aa 0.066aa 0.071aa 0.227bb 0.282aa 0.168aa 0.146aa

12 22.418bb 24.012ab 0.003ab 0.006ab 0.329aa 0.190bb 0.005bb 0.018ab

Control 12 - 31.241 - 0.098 - 0.471 - 0.120

p4 Corso 3 56.110ba 57.268aa 0.306ba 0.421aa 0.435bb 0.480aa 0.389ba 0.507aa

12 39.788bb 44.180ab 0.020bb 0.030ab 0.479aa 0.269bb 0.024bb 0.065ab

Sikuani 3 58.247ba 59.732aa 0.455ba 0.619aa 0.467ba 0.512aa 0.564ba 0.699aa

12 35.328bb 40.690ab 0.010bb 0.019ab 0.483 aa 0.240bb 0.012bb 0.046ab

Control 12 - 55.526 - 0.663 - 0.609 - 0.630

Explanation for the letters following the mean values see Table 6 B.


Table 6 B. Adhering- non-adhering ferrihydrite II to the roots by Zea mays L Corso and Sikuani.

Explanation on determined parameters (Cp; rl/R; E i,^; Adh.; Non-adh.) see Table 5 A.

P Cultivar Sampling P total Cp in solution rl/R E i min

(weeks) (fig P m"2) (mgPF1) (fig P rn 2)

Adh. Non-adh. Adh. Non-adh. Adh. Non-adh. Adh. Non-adh.

P2 Corso 3 24.069aa 24.141aa 0.002aa 0.005aa 0.021bb 0.034aa 0.030ba 0.046aa

12 22.637bb 23.827aa 0.003aa 0.003aa 0.064aa 0.028ba 0.015bb 0.034ab

Sikuani 3 24.000aa 24.068aa 0.003aa

0.009aa 0.019bb 0.034aa 0.050ba 0.083aa

12 22.314bb 23.763aa 0.002aa 0.002aa 0.063 aa 0.020bb 0.010bb 0.031 ab

Control 12 - 24.307 - 0.010 - 0.029 - 0.108

P3 Corso 3 31.420aa 31.762aa 0.012ba 0.027aa 0.040bb 0.065aa 0.094ba 0.131aa

12 29.072bb 30.560ab 0.011 aa 0.013ab 0.179aa 0.050bb 0.019bb 0.082ab

Sikuani 3 31.456aa 31.852aa 0.014ba 0.038aa 0.045bb 0.073 aa 0.098ba 0.164aa

12 28.803bb 30.504ab 0.008aa 0.011 ab 0.155aa 0.036bb 0.016bb 0.096ab

Control 12 - 31.565 - 0.040 - 0.079 - 0.159

P4 Corso 3 61.367ba 63.848aa 0.162ba 0.295aa 0.220bb 0.251aa 0.232ba 0.370aa

12 54.633bb 58.550ab 0.064bb 0.080ab 0.413aa 0.181 bb 0.049bb 0.139ab

Sikuani 3 61.780ba 64.153aa 0.201 ba 0.317aa 0.235bb 0.288aa 0.269ba 0.346aa

12 53.213bb 56.837ab 0.027bb 0.046ab 0.382aa 0.150bb 0.022bb 0.096ab

Control 12 - 63.042 - 0.616 - 0.424 - 0.457

Within the same P treatment, the same cultivar, the same sampling time and the same parameter (P total, Cp in

solution, rl/R and E \ m,n) mean values followed by the same first letter are not statistically different at a

probability level a = 0.05; Within the same P treatment, the same cultivar, the same parameter and the sameferrihydrite fraction mean values followed by the same second letter are not statistically different at a probabilitylevel a = 0.05; n = 4


Modification of the oxalate-extractable Fe (Fe0) content offerrihydrite. The amount of

oxalate-extractable Fe (Fe0) decreased in the ferrihydrite I and II during plant growth,

while Fe0 remained constant in the incubation experiments without plants (Figure 8).

After 12 weeks of plant growth this decrease was higher for ferrihydrite II than for

ferrihydrite I, and higher by Sikuani than by Corso. Fe0 decreased much more in the

fraction adhering to the roots than in non-adhering fraction. Fe0 decrease was stronger at

high P supply and related high plant and root development. Dithionite-citrate-

bicarbonate-extractable Fe (Fed) (Mehra and Jackson, 1960) remained constant for both

ferrihydrites during the entire experiment. The Feo/Fed value decreased during plant

growth.

The decrease of oxalate-extractable Fe (Fe0) might result from the crystallisation of the

ferrihydrite during plant growth, and/or from a mobilisation of Fe0 by the plant. A

positive correlation existed between the uptake of Fe in the aerial parts and the decrease

of oxalate-extractable Fe (Fe0) (Figure 9). However, and even without any determination

of the Fe content in roots, the difference between the decrease of Fe0 and the uptake of P

by the plant is too high to be only explained by a plant mobilization.

Modification of the specific surface area (SSA) and of the porosity of the ferrihydrites.

With the presence of plants the SSA of both ferrihydrites decreased during the entire

experiment, while for the control treatments the SSA remained constant (Figure 10). The

decrease of the SSA of the fraction of ferrihydrite adhering to the roots was much higher

than that of the fraction of ferrihydrite non-adhering to the roots. Furthermore, increasing

concentration of P in the system and related higher plant and root development also

enhanced this decrease.

The decrease of the specific surface area of both ferrihydrites during plant growth was

closely related to the decrease of the porosity of the ferrihydrite aggregates (Figure 11).

The decrease of the porosity was higher for Sikuani than for Corso, and was higher for

the fraction of ferrihydrite adhering to the roots.


The decrease in SSA and porosity could be related to solubilization/precipitation cycles

due to varying redox conditions. Although maize is classified as a strategy II plant in Fe

acquisition (Römheld and Marschner, 1986), which means that maize roots have a low

reducing capacity, saturation with water, the presence of organic exudates and iron

reducing micro-organisms can result in the apparition of reducing conditions in the

vicinity of the roots (de Willigen and van Noordjwijk, 1984; Uren, 1984; Uren and

Reisenauer 1988; Munch and Ottow, 1982; Fischer, 1988). Adsorption of high molecular

weight compounds root exudates at the surface or/and in the aggregates of ferrihydrite

might also reduce the porosity and the specific surface area.

Iron oxide XRD determination. At the end of the experiment for both 2-lines ferrihydrites

adhering and non-adhering to the roots, no Fe-(hydr)oxide other than ferrihydrite was

detected (data not shown). However it is important to notice that the sensitivity of the

XRD method is relatively low. As shown by Schwertmann and Murad (1983) storage of

ferrihydrite in aqueous suspension and pH changes can result in the formation of goethite

and hematite. Nevertheless the presence of phosphate retards the transformation of

ferrihydrite into crystalline products (Paige et al., 1997). Observed reduction in porosity,

SSA and Fe0 could be explained by the apparition of a more crystallized form of iron

oxide such as hematite or goethite.

Modification of the organic matter content of ferrihydrite during plant growth.

The content of organic matter in ferrihydrite increased during plant growth when

compared to the control treatment without plant (Table 7). The sorption of organic

compounds was higher on ferrihydrite II than on ferrihydrite I. The concentration of

organic C was higher in the fraction of ferrihydrite adhering to the roots than in the non-

adhering fraction. The concentration of organic C in the fraction of ferrihydrite both

adhering and non-adhering to the roots was higher for Sikuani than for Corso. These

results support the suggested influence of adsorbed organic compounds on the observed

(i) decrease of the ferrihydrite porosity on the fraction adhering to the roots, and (ii)

decreased P sorption capacity on the fraction of ferrihydrite adhering to the roots.


S

to

q? 2.0

2.0 -Pl P2P3 p4

Pl P2P, p41.5 -

Sikuani

Ferrihydrite I

1.0 -

0.5 -

on

Sikuani TÄ

Ferrihydrite II

1.5

1.0 -

0.5 -

0.0

pi P2P,

Corso

Ferrihydrite I

P, P2 F

Corso

Ferrihydrite II

0 20 40 60 0 20 40 60

P treatment ([ig P m )

Figure 8. Influence of the presence of plant on the oxalate-extractable Fe (Fe0). Pi= 6.6

fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P m"2 ; P4= 66.0 fig P m"2 ; mean value ± SE ; n

= A.%: ferrihydrite adhering to the roots; : ferrihydrite non-adhering to the roots; open

symbols: 3 weeks; full symbols: 12 weeks; : Control 12 weeks.


4.0 -


DX

c*

173 -

172

<

in

^ 111Ua

1 "O'«

=3t/2

S 169

ftCZ3

168

Ferrihydrite I Ferrihydrite II- 320

310

- 300

- 290

280

270

WD

fS

es

u

*-

20 40 60 20 40 60

P treatment (j^g P m )

Figure 10. Modification of the specific surface area (SSA) after 12 weeks plant growth. Pi= 6.6

fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0 fig P m"2 ; P4= 66.0 fig P m"2. mean value ± SE ; n = 3.

• : Corso; A: Sikuani; B : Control; full symbols: ferrihydrite fraction non-adhering to roots;

open symbols: ferrihydrite fraction adhering to roots.


45.2

44.8

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44.0 -

43.6

Ferrihydrite -. FerrihydriteII

- 94.0

97.0

91.0 £©U

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88.0

i n ' n n i ' i ' i '

167 169 171 173 270 290 310 330

Specific surface area (SSA) (m g" )

85.0

Figure 11. Change of the ferrihydrite porosity related to the specific surface area

(SSA) after 12 weeks plant growth. P1= 6.6 fig P m"2 ; P2= 24.8 fig P m"2 ; P3= 33.0

fig P m"2 ; P4= 66.0 fig P m"2; mean value ± SE ; n = 3. On fig 10 : ?u P2, P3, P4 for

A; •: Corso; A: Sikuani; : Control; full symbols: ferrihydrite fraction non-

adhering to roots; open symbols: ferrihydrite fraction adhering to roots.


Table 7. Influence of 12 weeks plant growth and P supply on organic matter in ferrihydrite for

Zea mays L. Corso and Sikuani. P2= 24.8 fig P nf2 ; P3= 33.0 fig P m"2 ; P4= 66.0 |ig P m"2; Adh

oxide adhering to the roots; Non-adh.: oxide non-adhering to the roots, mean value ± SE; n = 4.

Cultivar Organic matter content on ferrihydrite (fig C m"2)


Adh. Non-adh. Adh. Non-adh.

p2 Corso 6.43 ±0.342 1.39 ±0.220 7.52 ± 0.577 1.10 ±0.211

Sikuani 12.26 ±0.708 1.68 ±0.244 9.59 ±0.612 1.45 ±0.

Documents

Rights / License: Research Collection In Copyright - Non ...23357/eth-23357-02.pdfSikuani) and alow-Psusceptiblemaizecultivar {Zea SA3) werecomparedwith aSwisscultivar {ZeamaysL. Corso)