Biogeochemical cycles of soil phosphorus in southern Alpine spodosols

Ž .Geoderma 91 1999 249–260

Biogeochemical cycles of soil phosphorus insouthern Alpine spodosols

M.A. Beck a,), H. Elsenbeer b,1

a Department of Crop and Soil EnÕironment Sciences, VPI and SU, Blacksburg, VA 24061, USAb Institute of Geography, 3012 Berne, Switzerland

Accepted 15 February 1999

Abstract

We applied the Hedley fractionation to three Spodosols from the southern Alps to describe theŽ .distribution with depth of biological, or organically-bound forms bicarb Po and NaOH Po and of

Ž .geochemical, or inorganically-bound forms resin Pi, bicarb Pi, NaOH Pi, HCl Pi, and residual Pof soil phosphorus. These Spodosols differed considerably in regard to the crystallinity of freealuminium, the presence of amorphous compounds, and their respective distributions with depth.In only one case did the depth function of biological and geochemical forms of soil phosphorusclearly reflect the depth trend in amorphous compounds. In two cases, either no trend or no cleartrend was obvious which suggests the presence of controls other than soil chemistry. Organically-bound forms of soil phosphorus are as high as 65% and 40% of total P in the topsoil and subsoil,respectively, and in all three soils, bicarb Po reaches a maximum of 80% of total available soilphosphorus at a depth of about 30 cm. We could not confirm previous claims of a stratification ofbiological and geochemical cycles in Spodosols, and we caution against an unwarranted relianceon soil taxonomy, instead of relevant soil chemical data, as a framework for the interpretation ofsoil phosphorus cycles in a pedogenetic context. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: podzolization; soil profiles; chemical P fractionation; vegetation

1. Introduction

Biological and geochemical processes control the cycling and availability ofŽ .phosphorus P in soils. These processes are either part of or an expression of

) Corresponding author. Fax: q1-540-231-7630; E-mail: [email protected] Present address: Department of Civil and Environmental Engineering, University of Cincin-

nati, Cincinnati, OH 45221, USA.

0016-7061r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0016-7061 99 00026-9

( )M.A. Beck, H. ElsenbeerrGeoderma 91 1999 249–260250

pedogenesis depending on whether one takes a long-term or short-term view,Ž .respectively. It has long been recognized 1 that different forms of soil P are

Ž .associated with biological and geochemical processes, and 2 that, therefore, theŽrelative proportions of these forms change in the course of pedogenesis Walker

.and Syers, 1976; Smeck, 1985 . To the extent that soil classification systems,Ž .such as the US Soil Taxonomy Soil Survey Staff, 1975 , reflect current

thinking about soil-forming processes, relative proportions of soil P forms mayŽ .be expected to differ among soil orders. Cross and Schlesinger 1995 , drawing

Ž . Ž .on previous work by Tiessen et al. 1984 and Sharpley et al. 1985 , examinedthis in detail and confirmed that the forms and availability of soil P vary withpedogenesis, as reflected by soil order.

In their analysis of the link between the biogeochemical cycle and the formsŽ .of soil P, and pedogenesis Cross and Schlesinger 1995 capitalized on the

Ž .Hedley fractionation Hedley et al., 1982 which clearly assigns operationally-defined soil P forms to the biological or geochemical cycle. They suggest that‘biological’ P encompasses all organic fractions, ‘geochemical’ P all inorganicfractions, and ‘labile’ or ‘plant-available’ P the sum of resin Pi, bicarb Pi and

Ž .bicarb Po. Considering only the surface soil 0–15 cm and a weatheringgradient defined by the ‘end-members’ Entisols and Oxisols, Cross and

Ž .Schlesinger 1995 demonstrated that the relative amount of ‘biological’ soil Pincreases with increased weathering at the expense of ‘geochemical’ soil P. Therelative proportion of ‘labile’ soil P, however, did not seem to depend on thedegree of weathering, as expressed in soil orders. For lack of relevant data,Spodosols could not be considered in this scheme.

Ž .Wood et al. 1984 provided a much less comprehensive comparison, but onethat included Spodosols, and that considered whole profiles. Instead of soil Pfractions as defined by the Hedley procedure, they used P retention data and

Žbiological and geochemical proxies plant roots and microbial biomass P, and.oxalate-extractable aluminium, respectively to infer biological and geochemical

soil P cycles, and to postulate a vertical stratification of these cycles in aSpodosol. An investigation of soil P fractions as defined by the Hedleyprocedure, of their depth distribution, and an interpretation of the corresponding

Ž .results in light of the unifying scheme proposed by Cross and Schlesinger 1995has so far been missing.

In this paper, we report the distribution of soil P fractions with depth for threeHaplorthods and discuss the results within the framework suggested by Cross

Ž .and Schlesinger 1995 .

2. Methods

Ž .The study area is located on the north-facing slope 50% of the OnsernoneŽ X X .valley in the southern Swiss Alps 46811 N, 8839 W at an elevation of 900 m

( )M.A. Beck, H. ElsenbeerrGeoderma 91 1999 249–260 251

above sea level. Mean January and July temperatures are y18C and 178C,respectively. Mean annual precipitation is 2200 mm, with higher monthly totalsin spring and fall than in winter and summer. Soil parent material is colluviumfrom, as well as, residual biotite gneiss. Three study sites were identified in

Ž . Ž .grassland S1 , and in beech Fagus sylÕatica forest stands with median standŽ . Ž .ages of 46 S2 and 108 years S3 ; stand ages were determined by den-

drochronology of all trees on S2 and S3. Land use before abandonment andforest regrowth was extensive grazing, mainly during spring and fall. Site S1 isnot grazed anymore, but cut occasionally.

At each site, three soil profiles were opened, and one selected for sampling byŽ .horizon. All sites were classified as Haplorthods; the grassland soil S1 ,

however, bordered a Haplumbrept. Soil samples were collected on September3–5, 1994, airdried, and ground to pass a 2 mm sieve. Organic C, oxalate and

Ž .dithionite–citrate–bicarbonate DCB extractable Al, Si and Fe were determinedŽ .as outlined in the Soil Survey Laboratory Methods Manual USDA, 1996 . The

Ž .Hedley et al. 1982 procedure was used to sequentially fractionate soil P intoŽ . Ž .inorganic Pi and organic Po fractions. Increasingly harsher treatments extract

P fractions that are increasingly less available to plants. The P fractions areextracted sequentially by anion exchange resin, followed by NaHCO , NaOH,3

HCl, and conc. H SO . Organic P is determined by difference, total PyPi.2 4Ž . Ž .Hedley et al. 1982 and Schoenau et al. 1989 give brief descriptions to the

interpretation of the extractable P pools. All samples were analyzed in triplicate.

3. Results

The podzolisation of profiles S1 to S3 is evident from the selected chemicalparameters in Table 1. The term podzolisation describes the mobilization and

Ž . Ž . Ž .redeposition of aluminium Al and iron Fe andror organic matter OMŽ .within a soil profile Buol et al., 1989; Browne, 1995 . This process is rarely

Ž .actually monitored Ugolini and Dahlgren, 1987 , but mostly inferred from thedepth profiles of these constituents, especially of their various extractable forms.

Ž .All three soils are equally acidic, but differ in their Al and silicon Sichemistry, and, related to that, in their anion exchange capacity. Oxalate extractsamorphous, including organically-bound Al, whereas dithionite–citrate–bi-

Ž .carbonate extracts in addition crystalline ‘free’ Al McKegue et al., 1971 . TheŽ .ratio Al rAl is known as the activity ratio Blume and Schwertmann, 1969o d

and may be interpreted as a measure of crystallinity and reactivity: the larger theratio, the lower the crystallinity. The ratio increases at all three sites with depth,but the increase, from similar values in the topsoil, is most pronounced at siteS3. Another indirect measure for crystallinity and reactivity is the differencebetween oxalate– and dithionite–citrate–bicarbonate-extractable Si. A differ-ence of Si ySi )0 is an indication of the presence of imogolite. In all threeo d


Table 1Selected chemical characteristics of the three studied soil profiles. OC: Organic carbon, Cp:Pyrophosphate-extractable carbon, Al , Si , Fe : Oxalate-extractable aluminium, silicon, and iron,o o o

respectively, Al , Si : Dithionite–citrate–bicarbonate-extractable aluminium and silicon, respec-d d

tively, AEC: Anion exchange capacity

Profile Depth, pH OC, Cp, Al rAl Si ySi AEC, Al q0.5o d o d oacm % % % mmolrkg Fe ,%o

S1 y11 4.12 9.2 2.5 1.16 0.06 y2.34 1.01Grassland 11–23 4.16 6.4 3.0 1.32 0.04 y1.85 1.49

23–35 4.31 4.5 2.9 1.48 0.13 y0.23 2.1535–51 4.49 3.3 2.4 1.43 0.13 1.76 1.6051–74 4.54 2.0 1.8 1.98 0.25 1.63 1.5374–114 4.61 2.3 1.7 1.05 0.35 y0.29 1.65

S2 Forest 0–7 3.78 17.9 5.1 0.90 0.02 y2.06 0.8246 years 7–17 3.79 11.2 3.7 1.10 0.02 0.40 1.18

17–25 4.14 6.9 2.7 1.14 0.03 2.58 1.6825–52 4.20 1.1 3.3 1.23 0.05 3.61 2.0752–80 4.82 3.4 2.2 1.90 0.47 8.16 2.2480–107 4.68 1.6 1.3 2.93 0.54 6.15 1.77

S3 Forest 0–6 3.93 12.9 3.3 1.12 0 1.70 0.86108 years 6–13 4.28 9.1 2.7 1.15 0.02 y0.89 1.07

13–27 4.69 5.4 1.8 1.27 0.05 0.56 1.5327–35 4.85 2.9 1.3 1.89 0.27 3.01 1.8835–47 4.25 2.4 0.9 2.83 0.48 8.38 2.4747–60 4.35 1.1 0.8 2.74 0.42 11.97 2.0360–83 4.31 0.7 0.5 5.42 0.64 11.43 1.8783–115 5.35 0.7 0.2 4.48 0.50 8.41 1.55

a Ž .This statistic is a criterium for spodic material )0.50 .

profiles, this difference increases with depth, but this increase is again mostpronounced at S3 where the presence of imogolite, as well as allophane, was

Ž .confirmed by electron microscopy Harsh et al., 1997 . We interpret anionŽ .exchange capacity AEC as a proxy parameter for the presence of reactive

amorphous substances indicative of the podzolisation process. It reaches thehighest measured values in the subsoil of S3. In as much as the magnitude ofAEC, of Si ySi , and of the Al activity ratio are indicative of the podzolisationo d

products imogolite andror allophane, our results are in line with the predictionsŽ .of Browne 1995 that their content should be highest in the bottom part of the

Bs horizon. They suggest that the extend of podzolisation differs among thethree Haplorthods.

The distribution of soil P fractions with depth for profiles S1–S3 is presentedŽ . Ž .Fig. 1 according to the framework proposed by Cross and Schlesinger 1995 .The relative distribution of the fractions with depth reflects those soil chemical

Ž .properties thought to control inorganic P fractions Table 1 . In particular, the


Fig. 1. Relative phosphorus fractions with depth for three soil profiles under different vegetation.

increase of Al rAl with depth, and in the subsoil from S1 to S3, is accompa-o dŽ 2 .nied by a corresponding increase in NaOH-Pi r s0.90 which indicates the

high affinity of amorphous Al for P. HCl-Pi increases with depth at all threesites due to the increasing influence of parent material. The labile Pi poolŽ .resinqbicarb P is highest in the surface horizon and decreases sharply with


Ždepth at all three sites to less than 5% total P. Total organic P bicarb Po and.NaOH-Po is at least 40% of total P in the top 30 cm, but its depth function

differs considerably from site to site. At S1, it reaches a peak of about 65% at adepth of 35 cm before decreasing. At S2, total Po is about 40% of total Pregardless of depth. At S3, total Po decreases steadily with depth. The difference

Ž .between biological and geochemical forms is most pronounced at S3 Fig. 2

Fig. 2. Biological and geochemical phosphorus.


where this difference increases steadily with depth, in accordance with the trendŽ .of the crystallinity ratio of Al Table 1 .

Ž .The contribution of bicarb Po to the available P pool resin Pqbicarb PŽ .follows a similar depth pattern at all three sites Fig. 3 , with a peak at a depth

of approximately 30 cm. The decrease with depth below that peak is mostpronounced at S3, whereas at S2, we observe an intriguing constancy with

Ž .depth, similar to the pattern of total Po Fig. 2 .

Ž .Fig. 3. Bicarbonate Po as a percentage of available P pools resin and bicarbonate extracts .

()

M.A

.Beck,H

.Elsenbeerr

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Table 2Ž .aMean soil P fractions by Hedley procedure for surface soils 0–15 cm

b c d dResin Pi BiPi BiPo NaOH Pi NaOH Po HCl Pi Resid P TP Bio P Geo P Avail. P BiPo as %y1

mg P g 12 15 48 38 149 11 161 435 196 238 76 63.5Std. error 1.6 0.9 4.6 2.4 16.7 2.0 34.0 53.5 18.0 36.5 6.2 1.2% of TP 2.8 3.5 11.1 8.8 34.2 2.5 37.0 – 45 55 17.4 –

a0–15 cm data was obtained as follows: S1s0–11 cmq1r3 11–23 cm; S2s0–7 cmq7–17 cm; S3s0–6 cmq6–13 cm.bBio Pssum of BiPo and NaOHPo.cGeo Pssum of all pools other than Po.dAvailable Pssum of resin Pi, BiPi and BiPo.


To facilitate a comparison of our results with the compilation in Cross andŽ .Schlesinger 1995 , we pooled the data for the 0–15 cm depth interval from our

Ž .three sites to arrive at ‘Spodosol’ characteristics Table 2 . For these Spodosols,biological P and geochemical P make up 45% and 55% of total P, respectively;residual P is 37% of total P; 17% of total P is available, 63% of which is in theform of bicarb Po.

4. Discussion

ŽWe extend the weathering gradient Entisols, Inceptisols, Aridisols, Vertisols,. Ž .Mollisols, Alfisols, Ultisols, Oxisols proposed by Cross and Schlesinger 1995

by including our Spodosol results from Table 2 in their figs. 4 and 5. With 45%Ž .of total P, the biological P from our Spodosols ns3 exceeds significantly the

Ž .35% reported for Oxisols ns1 which itself is within the error assigned toŽ .Ultisols ns5 . In accordance with this high proportion of biological P,

geochemical P in Spodosols is with 55% substantially lower than the 65%reported for one Oxisol. It follows that as long as such a weathering gradient isnot mistaken for a pedogenetic pathway, Spodosols could define one end-mem-ber, with Entisols being the other one. It is worthwhile to recall that this

Ž .comparison within the framework of Cross and Schlesinger 1995 is restrictedto the 0–15 cm soil layer. The Spodosol bicarb Po, as percentage of biologicalP, is with 63% in the same order of magnitude as the value for the Oxisol andsimilar to that of Ultisols.

A word of caution seems appropriate with regard to the position of Inceptisolsin this weathering gradient. As pointed out above, the soil at our site S1bordered a Haplumbrept. Had we classified it as such, its soil P fraction-depthpatterns would not have changed, and yet it would have ended up at the ‘wrong’end of the above weathering gradient. ‘Podzolised’ Inceptisols are quite com-mon, and they are likely to have soil P fraction distributions similar toSpodosols, as are the andic and related subgroups of Inceptisols. Order-level soiltaxonomy is not necessarily the most appropriate approach to define a weather-ing gradient relevant to soil P. Apart from that, the Inceptisol entry in the

Ž .sequence of Cross and Schlesinger 1995; figs. 3–5 must be treated withŽ .caution because the presumed source of information Schoenau et al., 1989

does, in fact, not include Inceptisols, but only Alfisols and Mollisols.The depth functions of biological and geochemical P, as well as bicarb Po , of

the three Haplorthods show rather distinctive differences. At S1, under grass-land, organically-bound P decreases only gradually with depth, with the indica-tion of a peak at about 30 cm depth. At S2, under active forest regrowth, Poshows no discernible trend with depth. At S3, under comparatively old regrowth,we observe a consistent decrease of Po with depth. The degree of podzolisationas expressed, for example, by the increasing Al activity ratio in the subsoil


Ž .Table 1 , explains the difference between S1 and S3, but not the peculiarpattern at S2. It is noteworthy that at this site soluble carbon levels, expressed as

Ž .pyrophosphate-extractable carbon Cp, see Table 1 tend to be higher in thesubsoil than those of S1 and S3, and to decrease less and more irregularly withdepth than those of S3. This might be a transient phenomenon, possibly ashort-lived response to the vegetation change that is also manifest in the depthdistribution of organically-bound P.

Organic carbon might also be indirectly involved in an apparently high ratioof inorganically- to organically-bound P. The difference in correlation of soil

Ž 2 . Ž 2 .organic C with bicarb Po r s0.93 and with NaOH Po r s0.49 attests tothe difference in the extracted chemical compounds, and the consequent lability

Žand possible mobility of the associated P forms Beck and Sanchez, 1994,.1996 . Dissolved organic carbon, presumably in the form of low-molecular-

Ž .weight organic acids Fox, 1995 , can compete with P for sorption sites on oxidesurfaces. The effectiveness of such a blocking of potential P sorption sites is a

Ž .moot point, however. Sibanda and Young 1986 , for example, found a strongcompetition of organic matter and P for sorption sites in tropical soils, whereas

Ž .Appelt et al. 1975 detected no such competition in an Andisol, and neither didŽ .Borggaard et al. 1990 in sandy Spodosols and podzolised Inceptisols. There

may, however, be simpler reasons for a less well defined horizonation of soil Pfractions with depth. A forest on a 50% slope is prone to occasional tree fall and

Župrooting which can result in considerable profile disturbance Schaetzl et al.,.1990 . Other agents of proisotropic processes that counteract horizonation are

burrowing animals which, in addition to moving subsoil material of the soilsurface, provide conduits for particulate organic matter from the forest floor toreach the subsoil. Hence, it is conceivable that entirely non-chemical processesaccount for the observed apparent uniformity of biological P distribution withdepth.

ŽIn as much as the vertical distribution of biological and geochemical P Fig.. Ž2 is indicative of corresponding subcycles Wood et al., 1984; Walbridge et al.,

.1991 , a stratification of these subcycles is not apparent in our three Spodosols,in contrast to the stratification proposed for a Spodosol at Hubbard BrookŽ .Wood et al., 1984 . This contrast may well reflect the different nature ofpedogenetic and surficial processes at the two sites, but it is more likely theresult of methodology. While we assigned operationally-defined soil P fractions

Ž .to these two subcycles, Wood et al. 1984 used proxy parameters thought tocontrol the distribution of soil P between these subcycles. Walbridge et al.Ž .1991 proceeded in a similar fashion and could not detect a stratification ineither an Inceptisol or Ultisol at Coweeta. Again, this may reflect a fact, or itmay be a procedural artifact. The use of the Hedley procedure avoids theseambiguities. Therefore, our results are not directly comparable, as there has notyet been, to the best of our knowledge any similar research on Spodosols. Ourdata do not support the notion of a vertical stratification of biological and


geochemical subcycles of soil P, but this conclusion should not be interpreted asa generalization for Spodosols.

5. Conclusions

The Hedley fractionation offers a direct approach to assess the relativeimportance of biological and geochemical processes in soil phosphorus cyclingin three Haplorthods. There was no evidence for a vertical stratification of theseprocesses, but there were distinct depth patterns of biological and geochemicalsoil phosphorus in these Spodosols. The existence of just one Spodosol-typepattern is therefore unlikely, and we caution against an unwarranted reliance onsoil taxonomy at the higher hierarchical levels as a framework for the pedoge-netic interpretation of soil phosphorus forms. In comparison with other soil

Ž .orders Cross and Schlesinger, 1995 , Spodosols seem to have the most organi-cally-bound phosphorus in the topsoil. Similar studies of a wider range ofSpodosols, as well as certain Inceptisols and Andisols, would be desirable toestablish whether a taxonomy-independent depth pattern of soil chemical proper-ties relevant for P cycling does exist.

Acknowledgements

This project was funded by the Swiss National Science Foundation, grant no.4031-33151, and by the WANDER foundation, grant no. 47r94, which alsoprovided support for MB. Barbara Hell and Randy Southard helped duringfieldwork under trying conditions. The data base for Table 1 was provided byRandy Southard, UC Davis, and Jim Harsh, WSU Pullman.

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Biogeochemical cycles of soil phosphorus in southern Alpine spodosols