Plant and Soil 71,477-486 (1983). A-54 �9 1983 Martinus Nijhoff/Dr W. Junk Publishers, The Hague. Printed in the Netherlands.

Root growth and litter decomposition in a coffee plantation under shade trees

G. CUENCA, J. A R A N G U R E N * and R. HERRERA Centro de Ecologia, Instituto Venezolano de lnvestigaciones Cientificas (IVIC), Apartado 1827, Caracas 1010 A, Venezuela

Key words Coffee Decomposit ion rate Nutrient cycle Root distribution Root productivity VA Mycorrhiza

Abstract In a non-fertilized coffee plantation under shade trees the root biomass was excavated to estimate its distribution in the soil profile. One third of total fine (less than I mm) roots was found in the first 10cm of soil; the cumulative total to 30cm reached 73%. A highly variable and transient amoun t of fine roots colonized the litter layer. Root production both in the litter and in the first 7.5 cm of mineral soil was estimated from sequential samplings and was 10 g m - 2 yr - ~ and 660 g m - 2 y r - 1

respectively. The decomposition rate of weighed averages of litter fractions in the coffee plantation, calculated as the ratio of litter fall rate to the amount found in the soil was k = 4.8. Shade tree leaves, the major component of litter descomposed slower than coffee leaves and these slower than flowers and fruits. Litter bag experiments showed considerable slower rates when mesh was 0.03 m m than 0.5 mm. Nitrogen and phosphorous showed increases in concentrations as decomposition progressed while potassium, calcium and magnes ium followed a decrease in concentration that paralleled that of dry weight loss. In comparing the decomposition rate for litter with or without coffee roots growing in the bags, a tendency to show faster decomposition rates was found for the treatment with roots. These differences were however, only significant for one month for shade tree leaves litter. Nitrogen amounts remaining in shade tree leaves litter was lower in the treatment with roots than without roots. Potass ium concentration in roots was positively correlated with potassium concentration in

decomposing leaf litter where roots were growing. These results suggest that while roots growing attached to decomposing litter had little or no effect in speeding the decomposition process, the superficial roots seem to play an important role in absorbing very efficiently the mineralized nutrients from litter. The anatomical study of roots showed that the plantation is intensely infected with V-A mycorrhiza. External mycorrhizal hyphae did not to play a role in at tachment of roots to decomposing litter while root hairs were found to grow in profusion on root surfaces oriented toward litter.

Introduction

Litter decomposition and the concomitant liberation of mineral nutrients is important to nutrient cycling in natural vegetation and crops. Soil litter acts as a transition phase between living biomass and the soil. In tropical forests the amounts of nutrients returned to the soil via litter are c a . three times higher than that for temperate forests 2~

In coffee plantations under shade-trees litterfall and decomposition rates are similar to tropical rainforests 3.22. Nutrient return to the soil follows the

* Present address: Depar tamento de Biologia y Qulmica, Instituto Universitario Pedag6gico de Caracas, Avenida Pa6z, El Paraiso, Caracas.

477

478 CUENCA, A R A N G U R E N AND HERRERA

decomposition of litter with K, Ca and Mg being released at approximately the same rate as organic matter is lost while N and P are retained longer and sometimes accumulated 1,22.

The coffee root system is superficial. Additionally, fine roots occur in the litter layer. The role of the root system in recycling nitrogen from both shade trees and coffee has been assessed by Aranguren e t al. 2 Coffee roots have vesicular arbuscular mycorrhizas (VAM) 13 and there has been interest in assessing their role in nutrient cycling 7. Gadgil and Gadgil 8 found that ectomycorrhiza in Pinus roots inhibit litter decomposition, an effect due to either antibiotic effects on soil bacteria or to enhanced ability to obtain mineral nutrients, by the symbiotic fungi, which do not depend directly on litter as an energy source. In contrast, in tropical rain forests it has been suggested that mycorrhizal hyphae may help decompose litter 23. New results both from field 12 and laboratory 16 seem to support these findings.

In the present work we have tried to estimate the relative growth rate of fine coffee roots, in the litter layer and in the mineral soil of an unfertilised coffee plantation under shade trees. The objectives of this study were, 1) to estimate root biomass distribution, 2) the seasonal growth of fine roots as related to physical environmental parameters, 3) to assess the rate of litter decomposition, the release of mineral nutrients from litter and 4) to try to establish the role of VAM mycorrhiza in these processes.

Materials and methods

Experimental area The area was situated at 1400m altitude in Miranda State, Venezuela. Annual precipitation is

1200m with a dry period in February-March. Mean annual temperature is 19~ with little seasonal variation. The soils are acid, derived from well weathered mica-schists, and with pronounced slopes i 1.

Coffee arabica var. Mundo Nuevo was planted 25 years ago and the plantation has been managed with minimal inputs and no fertilizers. The shade trees Erythrina sp. and Inga spp. form a canopy at ca. 20 m; a second shade-tree s tratum at 15 m is formed by Heliocarpus americanus, Clethra sp. and

Ficus sp. Towards the edges of the plantation banana plants are abundant.

Coffee root distribution and growth The root biomass of 4 coffee plants was excavated at increasing distances from the trunks 2, roots

separated from soil and divided into two diameter classes using 1 m m as the limit between them. Samples were taken for analyses. Roots present in 50 0.25 m 2 quadrats pushed into the litter were separated and subsampled for analyses. The seasonal variation in fine root biomass was followed both in the litter layer and in the mineral soil; in the former by extracting the roots that penetrated 50 plastic mesh bags which contained litter as explained below. The changes in root biomass in the upper 7.5 cm of mineral soil were estimated from monthly samplings for a year with 13 cutting cylinders 7.56 cm in diameter pushed into the soil and subsequently extracted. The roots were then separated by

sieving and flotation. Subsamples were taken for analyses.

R O O T G R O W T H AND LITTER D E C O M P O S I T I O N OF C O F F E E 479

Litter decomposition Litter decomposit ion was estimated with two sets of plastic mesh bags containing litter as

described by Aranguren et al. 2 Decomposit ion rates estimated as dry weight loss from 4 bags collected monthly from each treatment, were compared for coffee leaves, shade-tree leaves, twigs and flower and fruit fractions. Samples were taken from each of these for analyses.

Root 9rowth and litter decomposition The effect of coffee roots infected with vesicular-arbuscular mycorrhiza (VAM) was assessed by

placing on the plantation floor 100 2 mm mesh bags each containing 40 g shade-tree leaves and 8 g coffee leaves. Every week the bags were checked and 50 were lifted and any roots found growing into them severed. The remaining bags were left in place. Once a month 4 bags from each treatment were carefully taken to the laboratory and their contents sorted into leaf litter fractions and roots. Samples were taken from each fraction for analyses.

Chemical analyses

All samples were dried at 80~ weighed and ground in a steel mill prior to chemical analyses. Appropriate aliquots were digested in a mixture of HCIO 4 and H2SO 4 in the presence of vanadium pentoxide. N and P were analyzed colorimetrically following the method of a Technicon II autoanalizer L~ and K, Ca and .Mg by flame atomic absorption spectrometry.

0 -10

10-20 E

20-30

~ 3 0 - 5 0 0

Roots > 1 m m

m .

O 1OO 2 6 0 3OO 4OO 5 6 0

L i t t e r

0-10

10-20 E

20-30

~ 3 0 - 5 0

- - ]

I Roots < 1 m m

i i

0 lb 2'0 3o 40 go

Dry weight roots, g / r r~

Fig. 1. Root biomass (dry weight) distribution in the soil profile and in the litter. Upper diagram refers to roots > 1 mm in diameter while lower diagram refers to those < 1 m m in diameter. The roots in the 30-40 cm and 40-50 cm intervals were pooled.

480 CUENCA, A R A N G U R E N AND HERRERA

Anatomical studies

The interface between growing fine coffee roots and the leaf litter in close contact with them was studied on samples from the plantation floor fixed on collection in FAA, dehydrated in alcohol and embedded in parafin. 12 lain sections were stained with either safranin fast green or anilin blue.

Results and discussion

Fig. 1 shows coffee root distribution from the litter layer to 50 cm depth, for the fractions above and below 1 mm diameter. While the larger fraction showed a

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Fig. 2.

AIM=jI IAISIOI.t!IjI!IMLA (A) Monthly precipitation along the year of study. (B) Seasonal variation in living root

biomass (expressed as dry matter) in litter bags which contained decomposing litter. (C) Seasonal trend in both living and dead root biomass in mineral soil to 7.5 cm depth.

ROOT G R O W T H AND LITTER D E C O M P O S I T I O N OF C O F F E E 481

steady increase in biomass with depth, the finer fraction showed a curve with maxima at 0-10cm.

Seasonal variation in the amount of root in the litter bags on the plantation floor (Fig. 2B) and the first 7.5 cm of mineral soil (Fig. 2C) showed that the fine roots, which penetrated the litter bags were affected by water regime. When the litter bags were placed on the plantation floor there was a flush of root growth. As decomposition progressed the growth peaks which coincided with periods of high pluviosty, became progressively smaller. The effect of the presence of litter on root growth outside the mineral soil has been documented for tropical rainforests in Amazonia 14. In order to calculate the increase in root biomass in the litter and mineral soil compartment we used the method of adding the positive changes in root biomass sampled according to Persson is. These calculations yielded 1 0 g m - E y r -1 for the roots in the litter layer and 6 6 1 g m - E y r 1 for the mineral soil fraction, although the inherent high variability of root growth does not permit a finer resolution. In the study of root biomass in the mineral soil living and dead roots were not differentiated so calculated values may overestimate production. In the litter layer it represents only living roots. Values for this compartment are lower than that reported for a boreal forest in Sweden 19 and an order of magnitude lower those reported for a tropical rainforest in Amazonas 14.

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60-

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Time (month)

Fig. 3. Amount of remaining litter (dry weight) as a function of time in a coffee plantation. At the beginning of January 112 g of dry mixed litter was placed in each litter bag. Solid circles represent averages of 4 bags 30 microns mesh; open circles represent averages of 4 bags 0.5 mm mesh.

482 CUENCA, ARANGUREN AND HERRERA

Litter decomposition rate The decomposition rate for the weighed averages of litter fractions in the coffee

plantation calculated as the ratio of litterfall to the amount of litter on the soil 17 is k = 4.8. This is comparable to the highest estimates for Colombian coffee plantations 22. The fraction which decomposed fastest was flowers and fruits (k = 20), coffee leaves decomposed with k = 10 while the major component of litter in the plantation, shade-tree leaves decomposed with k - - 4 . With the litterbag method rates of decomposition were lower for bags of 0.5 mm mesh and much slower with 30 p.m mesh bags. In both cases slower decomposition rates occurred between January and March, the drier period. The process then accelerated but slowed again towards the end of the year. Although the litter bag method introduces changes in the decomposition process which thus gives a dubious estimate of the decomposition 24, it gives a relative estimate of effects of micro and macroorganisms. The soil fauna have an important role in dismembering the litter fractions prior to decomposition by microorganisms (Fig. 3). In the 0.5 cm mesh litter bags Trachelifus sp., Parajulus sp., ants, slugs and snails and abundant earthworms were present.

Nutrient concentrations, measured in different litter fractions over a period of l l months, showed N and P increasing in concentration as decomposition

Table 1. Mineral nutrient concentration (~o dry weight) in litter fractions contained in 0.5 mm litter bags at different intervals after placing them in a coffee plantation

Litter fraction N P K Ca Mg

t I t 2 t~ t 2 t~ t 2 t l t2 t l t2

Shade-tree leaves* 1.08 2.53 0.08 0.17 1.10 0.22 1.56 1.25 0.19 0.17 Twigs* 0.81 1.11 0.07 0.11 0.31 0.07 1.13 0.38 0.16 0.10

Coffee leaves** 1.35 2.55 0.10 0.17 0.99 0.26 1.28 1.08 0.33 0.42 Flowers andfruits** 0.99 1.56 0.10 0.16 2.01 1.00 0.97 0.33 0.32 0.19

* F r a c t i o n s t x = 1 m o n t h , t2 = 11 months. ** Fractions t~ = 1 month, t z = 7 months.

Table 2. Effect of VAM-coffee roots on the decomposition rate of litter in comparison with litter which decomposed in the absence of roots (control) expressed as percentage of initial dry weight

decomposed

Time Total litter Shade-tree litter Coffee litter

(months) Roots Control Roots Control Roots Control

6 67.7 64.5 62.4 59.3 93.7 90.2

12 71.0 69.2 67.9* 63.9* 100.0 100.0

* Differences statistically significant at 2.5~.

ROOT GROWTH AND LITTER DECOM POSITION OF COFFEE 483

Table 3, Amounts of mineral nutrients in remaining litter in litter bags experiments both in the presence of roots (Roots) and when roots were removed from litter bags (Control)

Nutrient Shade-tree leaves Coffee leaves (mg)

6 months 12 months 6 months

Roots Control Roots Control Roots Control

Phosphorus 16.9 18.2 12.7 13.1 0.58 0.88 Nitrogen 453.1 471.3 250.7* 375.8* 15.2 32.3 Calcium 170.1 240.3 162.5 261.7 10.6 17.2 Potassium 36.6 29.4 29.6 31.6 1.13 1.99 Magnesium 30.4 33.7 28.4 28.9 1.31 1.92

* Differences statistically significant at 5~o.

progresses (Table 1). K, Ca and Mg decreased in concentra t ion parallel to the loss of dry weight. In coffee leaves, litter Mg showed a small increase in concentrat ion; a fact previously reported but without explanation 4' 6,15.

Effect of VA M roots In the t reatment designed to exclude the superficial coffee roots from the litter

bags, it was found that the frequency of lifting the litter bags and cutt ing of all roots that had grown towards them, was very effective. On the average 10.07 g dry weight roots m - 2 yr ~ grew into the litter bags not lifted while in treated bags only 0.32 g m - 2 yr 1. When compar ing the rate of decomposi t ion of both litter fractions with or without roots variances were inhomogeneous and so non- parametr ic statistics were used to compare the weight losses mon th to month. The Mann-Whi tney U test applied 21 has a power-efficiency of 95.5~o (Table 2).

Al though there was a tendency for faster decomposi t ion rates in the presence of roots, these differences were generally not statistically significant. For shade-tree leaves the values were statistically different in April 1981. Gadgil and Gadgil 9 found differences of 50~o or more despite decomposi t ion of pine needles in New Zealand proceeding much slower. V A M coffee roots appear to have no effect of the decomposi t ion rate of litter.

Table 3 shows the results of the same experiments expressed in terms of nutrients remaining in litter after given intervals. The only significant differences found were those for the amoun t of ni trogen in shade-tree leaves after a year in the litter bags. When the roots that had penetrated the litter bags were analyzed for nutrients it was found however that their concentra t ion of potassium was a significantly correlat ion with the potassium concentra t ion of the litter in which they were growing (Fig. 4). These results suggest that while roots in decomposing litter have no effect on the rate of the process, they very efficiently absorb the mineralized nutrients from that source in an otherwise nutrient poor soil.

484 CUENCA, ARANGUREN AND HERRERA

E

o

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4 0

30

2C

10

o

o o

o o

o o

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I I I I 1 2 3 4 5 6

K in litter, mg/g

Fig. 4. Correlation between K concentration in litter and in roots that had penetrated the litterbags (correlation significant to 59'o).

Fig. 5. VA-mycorriza arbuscles in section of a coffee root.

ROOT GROWTH AND LITTER DECOMPOSITION OF COFFEE 485

T h e a n a t o m i c a l s t u d y of t he coffee r o o t s c o l l e c t e d f r o m the field s h o w e d the

p l a n t a t i o n was i n t e n s e l y i n fec t ed w i t h V A m y c o r r h i z a 5. Fig. 5 s h o w s t he V A M of

a coffee r o o t . T h e a d h e s i o n o f r o o t s to the su r f ace of d e c o m p o s i n g l eaves is q u i t e

s t r o n g a n d r e s i s t ed t he f i x a t i o n a n d s e c t i o n i n g t r e a t m e n t s . W e o b s e r v e d t h a t t h e

a t t a c h m e n t o c c u r s t h r o u g h r o o t h a i r s t h a t we re f o u n d to g r o w in m o r e p r o f u s i o n

o n t he r o o t s ide t o w a r d s t he lea f sur face . N o e x t e r n a l h y p h a e we re o b s e r v e d to

i n t e r v e n e in t he c o n t a c t b e t w e e n the t w o s t r u c t u r e s as is t he case in A m a z o n i a n

r a i n fo re s t x2, w h e r e b o t h e c t o m y c o r r h i z a a n d V A m y c o r r h i z a h a v e b e e n f o u n d .

Acknowledgements The authors wish to thank Dr V. Garcia for his collaboration and suggestions in relation to the anatomical study of roots, to Dr T. St. John for fruitful discussion during the experimental period and to E. Orellana, G. Escalante L. Martin and S. Flores for laboratory assistance. One of us (J.A.) received partial funding from the Venezuelan CONICIT. The junior author wishes to express his gratitude to the Department of Ecology and Environmental Sciences at the Swedish Agricultural University, Uppsala for the use of their facilities during a sabbatical leave.

References

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486 R O O T G R O W T H A N D LITTER D E C O M P O S I T I O N O F C O F F E E

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