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Soil Science Society of America Journal 64:1467-1473 (2000)
© 2000 Soil Science Society of America

DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Carbon and Nitrogen in the Enriched Labile Fraction along a Climosequence of Zonal Steppe Soils in Russia

A. Rodionova, W. Amelunga, I. Urusevskajab and W. Zecha

a Inst. of Soil Science and Soil Geography, Univ. of Bayreuth, D-95440 Bayreuth, Germany
b Inst. of Soil Science, Moscow State Univ., 119899 Moscow, Russia

wulf.amelung{at}uni-bayreuth.de


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Breaking aggregates by plowing resulted in the decomposition of formerly physically protected soil organic matter (SOM), including the enriched labile fraction (ELF), but it was unknown to what extent such effects were controlled by climate. To investigate this question, aggregate-size fractions were obtained from each of five native and adjacent long-term cultivated topsoils across a climosequence in the Russian steppe. After ultrasonic dispersion of small macro- (250–2000 µm) and microaggregates (53–250 µm) at 100 and 500 J mL-1 (only for so-called stable macroaggregates) and a particle-size fractionation, density fractions <1.85 g cm-3, 1.85 to 2.07 g cm-3, 2.07 to 2.22 g cm-3 (= ELF for the small macroaggregates) and >2.22 g cm-3 were obtained from the fine silt–sized particles. In all fractions, C and N contents were determined. The stable aggregates were found only at native sites, were almost free of ELF, and showed their C maximum in the two lightest fractions. Cultivation reduced the C and N contents in all aggregates. In small macroaggregates, C losses occurred primarily as ELF, whereas microaggregates lost C in the fine silt density range of 1.85 to 2.22 g cm-3. The C partition among the fine silt density fractions was not related to climate. Losses in ELF were also not related to climate, suggesting that ELF represents a C pool that is site-specifically influenced by cultivation. The C losses from fractions <2.07 g cm-3, however, increased as the climate became dryer and warmer, suggesting that they reveal interactive effects of climate and land use on physically stabilized SOM.

Abbreviations: ELF, enriched labile fraction • SOM, soil organic matter


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
ORGANIC MATTER IN SOIL can be divided into pools of different SOM release and turnover time (van Veen et al., 1984; Parton et al., 1987). The partitioning of organic C and N among such pools depends on several factors, such as cultivation history (Tiessen and Stewart, 1983; Dalal and Mayer, 1986; Balesdent et al., 1988), fertilization (Christensen, 1988) or climate (Bird et al., 1996; Trumbore et al., 1996; Amelung et al., 1998). With increasing cultivation intensity, but also as soil temperature increases, SOM of the less stable pools is decomposed, as indicated by decreasing portions of sand-sized SOM (Bird et al., 1996; Christensen, 1996; Amelung et al., 1998) or light fraction C (Christensen, 1992; Trumbore et al., 1996). However, parts of the SOM are not accessible for microbial degradation, because they are physically protected from microbial attack in soil aggregates (Elliott, 1986; Gupta and Germida, 1988; Amelung and Zech, 1996). It is unclear to what extent such physical stabilization processes may control SOM dynamics at different climates.

Tisdall and Oades (1982) described in their model of aggregate hierarchy four aggregate-size classes, differing in organic and inorganic bonding agents. According to Tisdall and Oades (1982), transient SOM is situated at the surface of microaggregates and in the space between aggregates, protecting the inner structure of the macroaggregates (Elliott, 1986; Gupta and Germida, 1988). This SOM becomes accessible for microbial decomposition after aggregate disruption by cultivation practices (Elliott, 1986). To describe those processes, Cambardella and Elliott (1994) developed a fractionation sequence for the identification of physically protected C and N in soils. By combining a density fractionation with an aggregate-size fractionation for cultivated Mollisols in Nebraska (USA), the authors obtained an ELF that had a density of 2.07 to 2.22 g cm-3 and was recovered from the fine silt fraction (2–20 µm) after weak but complete ultrasonic dispersion of the small macroaggregates (22.5 J mL-1 for the soils of Cambardella and Elliott, 1994). According to Cambardella and Elliott (1994), ELF originated from inside macroaggregates, and its N content and potential N mineralization rate exceeded those of the other fractions; yet, it is not known whether the ELF concept can be applied to other soils than those investigated by Cambardella and Elliott (1994). Furthermore, there are no data about ELF at native sites.

Lein (1940) conducted similar investigations with Luvisols and Chernozems of the "Strelitz Steppe" (UNESCO-Natural Park close to Kursk, Russia). Based on Tjurin (1937), he divided soil samples into seven density fractions (1.7–2.76 g cm-3; using a bromoform–cumene mixture) and analyzed them chemically and microscopically. One of the investigated fractions had a density similar to ELF (2.0–2.25 g cm-3; Lein, 1940); it consisted mainly of a mixture of organic substances and clay. After theoretical essays (Han, 1946; Antipov-Karataev et al., 1948; Han, 1969), additional experimental investigations were conducted with Kastanozems and Solonetzes in the area of Arschan-Selmen (e.g., Kononova, 1963; Diakonova, 1972; Titova, 1969, 1976; Travnikova and Titova, 1978). In these studies, density fractions were obtained from bulk soil (<1 mm) and clay particles (<1 µm), and characterized by humic acid fractionation, infrared microscopy, x-ray diffractometry, and elementary analysis, to explain the colloidal behavior of SOM in Solonetzes and associated Kastanozems. Shaymukhametov et al. (1984), Shaymukhametov and Titova (1984), and Titova and Kogut (1991) used bromoform–ethanol mixtures for density fractionation. They found that increases in SOM as a result of increasing periods of fertilization (5–60 yr) were reflected by increasing proportions of fractions with low (<2.0 g cm-3) and intermediate density (2.0–2.2 g cm-3).

In summary, we know little about the relevance of the ELF concept for describing land-use and climate influences on SOM dynamics for different soil types. The aim of this study was, therefore, to investigate for zonal soils of Russia the impact of (i) long-term cultivation, and (ii) climate on C and N in ELF and associated fractions.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Soils
To identify climate effects on the ELF in the Russian steppe, five representative sites of similar parent material were selected from the study of Rodionov et al. (1998). The native soils comprised Greyzems (Argiudolls) under deciduous forest, Phaeozems (Hapludolls) under meadow steppe, and Kastanozems in the dry steppe (Argiustolls) and semi-desert (Calciustolls, Soil Survey Staff, 1998; Table 1) . All sites were level to gently sloping (<2%). We took undisturbed composite samples (0–10 cm) from five subsites of each a native field and an adjacent long-term (>50 yr), conventionally tilled plot. The air-dried samples were sieved to 2.8 mm (Cambardella and Elliott, 1994).


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  		Table 1 Texture, pH and organic C contents of the studied soils (0–10 cm)

 
The cultivated soils had never received any significant organic fertilization. We do not have detailed information about mineral fertilization; it has probably been used for the Kastanozem sites. Illites and montmorillonites dominated the clay fraction of all profiles, the texture ranged from sandy to clay loam. Organic surface horizons were missing at all sites. Mean annual temperature at the sites ranged from 4.1 to 8.0°C, mean annual precipitation ranged from 325 to 712 mm. For more information concerning cultivation history, site properties, and sampling procedure see Rodionov et al. (1998). The Calcic Chernozem of the site Novo-Annenskij from the study of Rodionov et al. (1998) was not considered in this investigation, because we could not exclude the possibility that this site received organic fertilization in the last decades.

In addition to the five native and cultivated sites, we included the native topsoil of a Dark Grey Forest Soil (Dep. of USSR Agriculture, 1977) in the study (Table 1). According to FAO (1997) this soil is also classified as Haplic Greyzem, not reflecting its special soil properties. The Dark Grey Forest Soil is rich in SOM (Table 1), and covers about 30% of the Greyzem area in the Russian Federation (Simakova et al., 1996).

Aggregate- and Particle-Size Fractionation
An aggregate-size fractionation by means of wet-sieving according to Cambardella and Elliott (1994) yielded the following four fractions: silt- and clay-sized particles (<53 µm), microaggregates (53–250 µm), small (250–2000 µm), and large macroaggregates (>2000 µm). For subsequent isolation of the ELF, Cambardella and Elliott (1994) stated that small macroaggregates have to be destroyed completely by ultrasonic treatment. Our preliminary experiment showed that small macroaggregates of different sites responded differently to ultrasonic dispersion, and that they were not dispersed at an energy input of 22.5 J mL-1 used by Cambardella and Elliott (1994). Dispersion of these aggregates was achieved at an input of 60 to 500 J mL-1 ultrasonic energy. As Morra et al. (1991) claimed that an ultrasonic energy input >100 J mL-1 may alter SOM structure, we used 100 J mL-1 to disperse the small macroaggregates. Energy output was calibrated according to North (1976). Afterwards we divided the microaggregates (53–250 µm) and the small macroaggregates (250–2000 µm) into the following size fractions: clay (<2 µm), fine silt (2–20 µm), coarse silt (20–53 µm), and, if present, into micro- (53–250 µm) and small macroaggregates (250–2000 µm) again, using centrifugation (for the isolation of clay-sized material) or sedimentation (for larger particles). At individual samples from native sites (Dark Greyzem, Haplic Phaeozem, Haplic Kastanozem; Sites 1, 4, and 8) some of small macroaggregates were not completely dispersed at energy input levels of 100 J mL-1. The remaining small macroaggregates after the 100 J mL-1 treatment were, therefore, dispersed again with 500 J mL-1 ultrasonic energy input. This energy corresponded to that used for recovering the sand fraction >250 µm in the course of a particle-size separation for the studied soils (Rodionov et al., 1998). Small macroaggregates which survived the first ultrasonic treatment of 100 J mL-1 were designated stable and those disintegrating during the 100 J mL-1 treatment were designated friable.

Density Fractionation of the Fine Silt Fraction (2–20 µm)
To obtain the ELF, we separated the aggregates of fine silt size (2–20 µm) according to Cambardella and Elliott (1994) into the density fractions <1.85 g cm-3 (plant debris), 1.85 to 2.07 g cm-3 (less homogeneous tissue fragments, single fungal filaments, casts, organo–mineral complexes), 2.07 to 2.22 g cm-3 (clay–organic matter complexes, ELF), and >2.22 g cm-3 (primary particles of low C content). Sodium polytungstate (Na6H2W12O40·H2O; Sometu, Berlin) was used to adjust the density of fractionation solution. The fractionation was conducted with 5 g sample weight and 50 mL density solution. All density fractions were decanted over a Büchner funnel equipped with a glass-fiber filter GF-8 (Schleicher & Schuell, Germany), and filtered with the help of a vacuum pump. The residues on the filters were washed with distilled water into a 500-mL centrifuge beaker and centrifuged. Washing was repeated twice with about 1 L water to withdraw as much salt as possible from the fractions.

Chemical Analyses
Total C and N were determined with an Elementar Vario EL Analyzer (Elementar Analysensysteme GmbH, Hanau, Germany) in the bulk soils, the aggregate-size, particle-size, and aggregate-density fractions. The samples were free of carbonates (Rodionov et al., 1998).

All analyses and fractionation steps were performed in duplicate.

Statistical Analyses
After an analysis of variance with the software package Statistica 5.1 for Windows (StatSoft, 1995; ANOVA/MANOVA subroutines), comparison of means was carried out by contrast analysis. It should be noted that only systematic differences between fractions or between paired native and cultivated sites were tested. Significant differences may thus occur irrespectively of the standard deviation in tables and figures, representing the variation of the measured parameter across the climosequence. The statistical results presented in Table 4 need to be carefully interpreted, because the amount of sites was smaller than the amount of fractions used in MANOVA.


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  		Table 4 Average C distribution among fine silt–sized density fractions of stable and friable small macroaggregates (250–2000 µm) of the Haplic Greyzem (Profile 1), Haplic Phaeozem (Profile 4), and Haplic Kastanozem (Profile 8)

 

   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Converting native grassland to farmland resulted in a decline of the SOM content (Dalal and Mayer, 1986; Elliott, 1986; Tiessen and Stewart, 1988). In the Russian soils under study, C and N loss occurred from all aggregates larger than 53 µm (Table 2) . This might imply that net aggregate stability could have decreased also. The loss of aggregates >2000 µm supported this hypothesis (P < 0.001), though sample variability did not allow further evidence through a gain in any of the smaller aggregate fractions (Table 2).


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  		Table 2 Mean C and N contents as well as average proportion of aggregates in Russian steppe soils after the first aggregate-size fractionation in water without ultrasonic dispersion. For paired sites only; the standard deviation was calculated across all sites and is given in parenthesis

 
Carbon and Nitrogen in Aggregate Density Fractions
The C stocks of the small macroaggregates tended to increase with increasing density of the fraction (P > 0.05); between one- and two-thirds of the C in the small macroaggregates was recovered in the fraction >2.22 g cm-3 (Table 3) . The C distribution among density fractions of fine silt particles from the microaggregates revealed a picture similar to that of the macroaggregates. Upon cultivation the percentile proportion of C in the heaviest density fractions increased in the fine silt particles of both aggregate fractions (Table 3). In the small macroaggregates, this was accompanied by a decrease in the ELF (2.07–2.22 g cm-3), whereas in microaggregates, the C proportions in the fraction 1.85 to 2.07 g cm-3 also tended to decline (Table 3). Our results suggest the following processes to occur:


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  		Table 3 Distribution of C and N among density fractions of fine silt particles (paired samples only) obtained from small macro- (250–2000 µm) and microaggregates (53–250 µm). The standard deviation was calculated across all sites and is given in parenthesis

 
(i) Usually, the lightest fraction has the fastest turnover time (Dalal and Mayer, 1986: Trumbore et al., 1996). Nevertheless, although mineralization losses had occurred in the cultivated sites, there was no preferential loss of C from the lightest relative to the other fractions. Apparently, mineralized light-fraction material of aggregates at the cultivated sites had been replaced by crop residues.

(ii) The C losses upon cultivation occurred mainly from the fraction 2.07 to 2.22 g cm-3 and, less clearly, from the fraction 1.85 to 2.22 g cm-3 of the fine silt–sized aggregate particles (Table 3). This supports previous findings for N (Cambardella and Elliott, 1994), that especially the density fraction 2.07 to 2.22 g cm-3 was a sensitive measure to indicate an impact of land use on the SOM in aggregates. While this density fraction seems to be stable in the native soil, it became available for microbial degradation in the course of cultivation.

(iii) A variable C partition among density fractions of the fine silt fractions suggested that the density structure of aggregates was no constant state property but depended on soil type and management.

When discussing the C distribution among density fractions of fine silt from aggregates it should be recalled that these fractions contain only a minor part of bulk C; the fraction 2.07 to 2.22 g cm-3 represented 4 to 8% of the total C in the native soil, for instance (Table 3, bottom). Hence, mineralization of C from these fractions explains about a fifth of the total C losses through cultivation.

According to Cambardella and Elliott (1994), the ELF is characterized by an enhancement of N enrichment and of potential N mineralization rates rather than by its C content. However, we could not find a special N enrichment in the ELF (Table 3). The C/N ratio decreased with increasing density of the fine silt particles for both the large micro- and small macroaggregates (Fig. 1) , indicating that there was an increased degree of SOM alteration with increasing particle density (Christensen, 1992). As a result, the heavier fractions contained higher proportions of total N, exceeding those of C by as much as 10% on the average (Table 3). A slight decrease in C/N ratios of the light and heavy density fractions upon cultivation suggested that long-term cropping affected not only the relative quality of SOM at primary particles (Christensen, 1996; Rodionov et al., 1999) but also of that within the aggregates.



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  		Fig. 1 Average C/N ratio in density fractions of fine silt–sized particles obtained from small macro- and microaggregates of native Russian steppe soils

 
Stable and Friable Aggregates
As mentioned earlier, stable macroaggregates were not dispersed at 100 J mL-1 ultrasonic energy. These stable aggregates were found only in samples from the native Haplic Greyzem (Dark Grey Forest Soil according to the Dep. of USSR Agriculture, 1977, profile 1; Table 1), Haplic Phaeozem, and Haplic Kastanozem, but never in samples from cultivated sites. According to Abrukova (1971), this corresponds to the rheologic properties of aggregates of Chernozems, suggesting that the breakdown of stable aggregates might be responsible for the risk of erosion at cultivated Chernozems.

Particle-size fractionation of the stable small macroaggregates revealed that they contained 300 g kg-1 clay on average. The amount of clay thus exceeded that of the friable small macroaggregates by 100 g kg-1 aggregate (data not shown). On average, 40% of the organic matter in the stable small macroaggregates was found in the clay fraction, compared to about 20% in that of the friable ones. It seems therefore reasonable to speculate that the higher stability of the stable aggregates might be favored by their higher content in organo–mineral complexes <2 µm.

In search for a second, stable ELF, a density fractionation was also conducted for fine silt particles obtained from the stable small macroaggregates. It revealed a significantly different C partition among the fine, silt-sized density separates relative to that of the respective friable aggregates. The fine silt of the stable aggregates contained more C in the light fraction and less C in the ELF density range than that of the friable ones (Table 4). Apparently, ELF is a function of aggregate stability. In turn, enrichment of light-fraction C might reflect a more effective protection against decay in stable compared to friable aggregates. Dalal and Mayer (1986) found that light-fraction SOM can be rapidly mineralized, and Tiessen et al. (1984) suggested that especially labile organic matter can be stabilized in aggregates.

Influence of the Climate on the Aggregate-size and Aggregate-density Fractions
The percentage of large macroaggregates decreased in the sequence Haplic Greyzem (290 g kg-1) > Haplic Phaeozem, Haplic Kastanozem (190 g kg-1) > Calcic Kastanozem (96 g kg-1) > Gypsic Kastanozem (28 g kg-1). Also at cultivated sites, large macroaggregates decreased with decreasing ratio of mean annual precipitation to potential evaporation . According to Tisdall and Oades (1982) and Waters and Oades (1991), the large macroaggregates consist of small macroaggregates. The ELF has been previously defined to exist only in small macroaggregates (Cambardella and Elliott, 1994). The proportion of small macroaggregates did not change systematically with either the P/E ratio, the C or clay content of the sites (P > 0.05 for cultivated and native sites, respectively). Also the relative distribution of C among density fractions of the small macroaggregates was not related to the climatic regime, neither for the native nor for the cultivated sites (Table 3). We could also not find any zonal trends for the C and N in density fractions of the microaggregates. The following discussion thus only refers to the small macroaggregates

Cultivation affected the C distribution among the fine silt-density separates of the small macroaggregates (see above). We therefore wanted to know whether climate might influence the degree of such land-use effects on C and N in aggregate-density fractions. The decrease of the ELF following cultivation did not relate to climate indices again (Table 3), confirming that ELF dynamics was not related to climate. In contrast, the light fractions tended to lose less C upon cultivation (i.e., the difference in C proportions between cultivated and native sites increased) as the climate became cooler and moister, that is, as the P/E ratio increased (Fig. 2) . This trend also existed for the 1.85 to 2.07 g cm-3 density fraction. Probably, the SOM of the two lighter fractions was more rapidly mineralized upon cultivation at higher temperatures (Trumbore et al., 1996), or, respectively, less light SOM was supplied by crop residues as the climate became drier (Sala et al., 1988). It seems also reasonable to assume that lower degradation intensities at the Northern site of the Haplic Greyzem favored the accumulation of debris of the densely growing fine roots of the wheat and oat plants in the two lighter fractions of the fine silt particles. A deeper root system of the deciduous forest at the Greyzem sites might provide as much root debris to the light fractions of the aggregates as the cultivated plants.



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  		Fig. 2 Cultivation effect on the C proportion (in % of respective fine silt–aggregate C) in the <1.85 and 1.85 to 2.07 g cm-3 density fractions obtained from fine silt–sized particles of small macroaggregates as related to the precipitation/evaporation ratio P/E

 
The lower the P/E ratio was, the higher were the N losses from the density fraction <1.85 g cm-3 of the fine silt particles of the small macroaggregates (Figure 3) . Thus, an increasing loss of soil C from the light density fractions (Figure 2) was accompanied by an increasing loss of soil N upon cultivation.



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  		Fig. 3 Cultivation effect on the N proportion (in % of respective fine silt–aggregate N) in the <1.85 g cm-3 density fraction obtained from fine silt–sized particles of small macroaggregates as related to the precipitation/evaporation ratio P/E

 

   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
In this study, we attempted to apply the method of Cambardella and Elliott (1994) for aggregate-density fractionation and ELF isolation to soils different to those in Nebraska, USA. The results showed that a transfer of the fractionation procedure to other sites is not easily possible, even for the pedogenetically similar grassland soils of the Russian Steppe. Higher ultrasonic energies were required, for example, for dispersion of macroaggregates from the Russian sites, and it remains to be investigated how far these higher ultrasonic energy inputs induced an ELF disruption.

The two-step dispersion of macroaggregates into stable and friable secondary particles employed here enabled us to assume that especially labile organic components of the fine silt fraction contributed to the stabilization of the aggregates. Therefore, the procedure developed by Cambardella and Elliott (1994) was a sensitive method that allowed us to relate the different physical stability of aggregates to fractions with different chemical properties.

The degradation of C of fine silt particles from small macroaggregates or microaggregates amounted to 40% of the total C loss upon cultivation. The mineralization of C took place mainly in the density fractions 2.07 to 2.22 g cm-3, and, in microaggregates, also in the fraction 1.85 to 2.07 g cm-3. The ELF of the small macroaggregates and also the respective density fraction of fine silt particles of the microaggregates can thus be considered as pools that indicate the influence of cultivation independently of the climatic regime. In contrast, the changes due to cultivation of the density fractions <2.07 g cm-3 correlated with the P/E ratio of the sites. Thus, the aggregate-density fractionation was a useful tool for decoding interactive effects of climate and land use on SOM.Department of USSR Agriculture 1977; 1997; Soil Survey Staff 1998


   ACKNOWLEDGMENTS
 
We are grateful to E.T. Elliott and J. Six for the advice during sample fractionation, to Matthias Sumann, Germany, and N.A. Maleschin, Kursk for their support during sampling, and to the anonymous reviewers for their help in improving the original manuscript. The project was funded by the German Science Foundation (DFG Ze 154/26-2,3).

Received for publication January 7, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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