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Soil Science Society of America Journal 65:352-358 (2001)
© 2001 Soil Science Society of America

DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Soil Organic Matter Pools and Carbon-13 Natural Abundances in Particle-Size Fractions of a Long-Term Agricultural Field Experiment Receiving Organic Amendments

Martin H. Gerzabeka, Georg Haberhauera and Holger Kirchmannb

a Department of Environmental Research, Austrian Research Centers Seibersdorf, A-2444 Seibersdorf, Austria
b Department of Soil Sciences, Swedish University of Agricultural Sciences, Box 7014, S-750 07 Uppsala, Sweden

Corresponding author (martin.gerzabek{at}arcs.ac.at)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The present study combined a physical fractionation procedure with the natural abundance of 13C ({delta}13C) to evaluate the effect of organic fertilizer applications, mineral fertilization, and fallow on changes in the organic C (Corg) associated with different particle-size fractions. The long-term agricultural field experiment was conducted since 1956 in Ultuna, Sweden, on a Eutric Cambisol. Organic C both in bulk soil samples and size fractions changed significantly since 1956. Fallow plots lost approximately one-third of their Corg from the topsoil layer (0–20 cm), whereas organic amendments based on an equivalent of 2000 kg C ha-1 yr-1 increased Corg up to twofold depending on the quality of the material applied (green manure < animal manure < sewage sludge < peat). Silt-sized particles increased in plots receiving sewage sludge or peat. Organic C in particle-size fractions responded significantly to treatments. Most Corg was found in the silt fraction. The relative contribution of the silt-sized particles to total Corg increased by 18% as Corg in bulk soil increased from 10.8 (fallow) to 32.0 (peat) g Corg kg-1 soil; the contribution of clay-sized particles decreased by a similar proportion. Mass balance calculations showed that the proportion of Corg originating from organic amendments decreased with particle size and that sand fractions were the most sensitive to the treatments. The natural abundance of 13C in bulk soil and size fractions increased significantly in the continuous fallow and was affected by organic amendments. The {delta}13C variations among size fractions were larger than among treatments and can be used as a fingerprint for differentiation. Our results suggest that silt-sized particles acted as medium-term sink for added Corg and that sand-sized fractions can be useful as sensitive indicators of changes in soil C status in response to land management.

Abbreviations: Corg, soil organic C • {delta}13C, natural abundance of 13C • PDB, Pee Dee Belemnite


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOIL ORGANIC C is the largest pool within the terrestrial C cycle. The annual C turnover through the terrestrial biosphere amounts to {approx}60 Gt (Schlesinger, 1997), which is around 9% of the atmospheric C pool (Esser, 1990). Agricultural management affects the accumulation of Corg by influencing the amount of plant residues returned to the soil (Campbell et al., 2000) and the rate at which the residues and organic matter decompose. Significant changes in Corg due to land management practices can only be observed after longer time periods. It would, therefore, be useful if Corg fractions could be identified that are more sensitive than bulk soil Corg to variations in the soil organic matter status.

The availability of Corg to microbial attack depends on its chemical composition, C/N ratio, humification state, and physical position within the soil matrix (Golchin et al., 1995). In natural soils the activity of microorganisms and plant roots and the formation of organo-mineral complexes lead to aggregation of soil particles, which influences soil C storage and dynamics and a range of other soil properties (Oades and Waters, 1991). Many authors (Christensen, 1986; Monrozier et al., 1991; Schulten et al., 1993; Desjardins et al., 1994; Guggenberger et al., 1994) found that Corg content increased with diminishing particle size, whereas the C/N ratio decreased. Jastrow et al. (1996) showed that organic matter recently introduced into soil is predominantly located in larger aggregates. Tisdall and Oades (1982) demonstrated that macroaggregates (<250 µm) can be destroyed by agricultural practices, whereas microaggregates cannot.

Turnover rates and the role of Corg in soil particle-size fractions have been studied by various authors (Tisdall and Oades, 1980; Christensen, 1986; Oades and Waters, 1991; Schulten et al., 1993; Beare et al., 1994; Buyanovsky et al., 1994; Cambardella and Elliott, 1994; Desjardins et al., 1994; Guggenberger et al., 1994; Nelson et al., 1994). It was shown that physical fractionation yields Corg pools of different functions and that dynamic processes of litter decomposition and introduction into more stable pools can be followed more easily, especially if isotopic methods are used in combination (Stemmer et al., 1999). Particle-size fractionations are therefore an interesting tool for research related with soil Corg dynamics. The dispersion method preceding the fractionation itself is a crucial part of the procedure (Christensen, 1992). Low-energy sonication is assumed to prevent the release of stable organic matter that is physically protected within microaggregates in natural sites (Stemmer et al., 1998).

Carbon-13 is a useful tracer for studying the decomposition and incorporation of organic material into more stable Corg (Andreux et al., 1989; Zaccheo et al., 1993; Desjardins et al., 1994; Garcia-Olivia et al., 1994; Piccolo et al., 1994; Golchin et al., 1995). The 13C content of Corg corresponds closely with the 13C content of the plant material and/or of the organic manure (Gerzabek et al., 1997) from which it originates. Microbial and chemical transformation during humification enriches 13C in silt and clay (Gregorich et al., 1994). Consequently, the introduction of organic manures whose 13C enrichment differs from that of the soil potentially enables the C derived from amendments to be traced into the existing Corg pool.

The objective of our study was to investigate Corg fractions most sensitive to long-term changes due to soil management and organic matter input. It should be investigated whether the most stable Corg pool, the clay-sized particles, or the silt-sized fraction act as a medium-term sink of Corg introduced through organic amendments of significantly different availability for microbial digestion. We combined a physical fractionation procedure following low-energy sonication (Stemmer et al., 1998) and the determination of natural abundances of 13C and 12C to evaluate the effect of organic fertilizer applications on soil organic matter using the long-term field experiment in Ultuna, Sweden. Basic data on the turnover of C and temporal changes of {delta}13C values in bulk soil (Gerzabek et al., 1997), N (Gerzabek et al., 1999), S (Kirchmann et al., 1996) and changes in physical soil properties (Gerzabek et al., 1995; Kirchmann and Gerzabek, 1999) have been reported previously.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site and Samples
The field experiment is in central Sweden near Uppsala (60°N, 17°E; elevation: 14 m above sea level), on a Eutric Cambisol with 37% clay and 41% silt. The parent material consists of postglacial clay with illite as the main clay mineral. The mean annual temperature is 5.5°C, and the mean annual precipitation is 660 mm. A complete documentation of the experiment and compilation of data can be found in Kirchmann et al. (1994). In 1956, the soil (0–20 cm depth) had 15 g kg-1 of Corg, 1.7 g kg-1 of N and a pH of 6.6. The area was used as arable land before starting the experiment, animal manure being applied as fertilizer. The experimental design consists of 14 treatments, laid out with four replicates in a randomized block design, the only difference between plots being the type of amendment. The individual plots (2 by 2 m) were separated by pressure-treated wooden frames. Seven of the treatments-fallow (continuous bare fallow), No N (plots did not receive N fertilizers), Ca(NO3)2 (80 kg N ha-1 yr-1), green manure, animal manure (well-decomposed), peat (sphagnum), and sewage sludge—were selected for the study. The organic amendments were selected to represent a wide range of microbial digestibility. The application of organic amendments, analyzed before use, was based on equal amounts of ash-free organic matter amounting to 2000 kg C ha-1 yr -1 on average. Organic matter was added by hand in fall in 1956, 1960, and 1963, and thereafter every second year. Tillage was performed by hand to a depth of 20 cm. All plots received annually in spring a dressing of 20 kg P ha-1 in the form of superphosphate and 35 to 38 kg K ha-1 in the form of KCl. Cereals, mainly barley (Hordeum vulgare L.), with some oat (Avena sativa L.) and spring wheat (Triticum aestivum L.), in total 70% within the experimental period; rape crops [fodder rape and oilseed rape (Brassica napus L.), mustard (Sinapis alba L.); in total 25%]; and fodder beet [Swedish turnip (Beta vulgaris L. var. crassa); 5%] were grown alternately in an irregular order. At harvest, the aboveground portion of the crop was completely removed. Topsoil samples (0–20 cm) were taken in October 1998, 1 yr after the organic matter addition in autumn 1997. Samples were deep frozen. Organic amendments of different years were characterized (Table 1). Note the large differences in C/N ratios.


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  		Table 1. Elemental and isotopic composition of organic amendments.{dagger}

 
Fractionation Procedure
The size fractionation was based on the method of Monrozier et al. (1991) and was accomplished with sieved samples (<=2 mm) as described in detail by Stemmer et al. (1998). To minimize destruction of labile particulate organic matter, the soil–water suspension was dispersed using low-energy sonication (0.2 kJ g-1 output energy) and then fractionated by a combination of wet sieving and repeated centrifuging (150 and 3900 g, based on a particle size of 2 and 0.1 µm, respectively). This successively yielded the size fractions >200 µm (coarse sand), 63 to 200 µm (fine sand), 2 to 63 µm (silt), 0.1 to 2 µm (clay), and <0.1 µm (fine clay), without addition of a flocculant. The fractions were freeze-dried and weighed. Fractionation was done in duplicate. This method has been successfully applied to both laboratory and field experiments (e.g., Stemmer et al., 1999). The main advantage of this gentle fractionation procedure is the fact that stable microaggregates are preserved and enzymatic parameters can be measured in the size separates (Stemmer et al., 1998).

Carbon and Isotope Analyses
Freeze-dried subsamples of the bulk soil and the fractions were powdered in an agate mill. As there was no inorganic C in the soil, Corg was equal to total C. Measurements were done in duplicate. Organic C was determined by dry combustion in an elemental analyzer (NA 1500, Carlo Erba, Milan, Italy). The released CO2 was separated from the other gases by gas chromatography and introduced via a gas-splitting interface into the isotope ratio mass spectrometer (MAT 251, Finnigan, Bremen, Germany) for measurement of isotopic abundances. The 13C results are expressed in the relative {delta} per thousand scale according to the equation {delta}13C{per thousand} = (Rsample/Rstandard - 1) x 103, where R = 13C/12C, and are related to the Pee Dee Belemnite (PDB). All results are expressed relative to the international V-PDB standard (Coplen, 1995). The analytical precision for Corg and {delta}13C is 0.02 g Corg kg-1 and 0.1{per thousand}, respectively.

Calculations and Statistical Treatment of Data
Analysis of variance, correlation analyses, and cluster analyses were accomplished using the program STATISTICA 5.0. The Corg balance in particle-size fractions was calculated using the continuous fallow treatment as a reference for native soil C and the No N or Ca(NO3)2 plots for estimating the root and stubble input. No N was used for peat plots and Ca(NO3)2 for all other treatments. This differentiation in reference plots was based on the yield levels. Due to the observed yield depression peat plots were better represented by the No N treatment. Root and stubble input was related to the mean yearly aboveground biomass yield levels (1987–1991) of the respective plots [No N: 3459 kg ha-1; Ca(NO3)2: 6965 kg ha-1; green manure: 6679 kg ha-1; animal manure: 6549 kg ha-1; sewage sludge: 8625 kg ha-1; peat: 3350 kg ha-1]. The following calculation was performed for all fractions: (i) remaining native Corg (fallow Corg) was subtracted from total Corg; (ii) after subtracting the yield adjusted root and stubble input (see Table 4) using the abovementioned reference plots the manure-derived Corg was obtained. The standard deviation ({sigma}) resulting from the balance calculation was estimated by Eq. [1]

(1)
where f is the ratio obtained from the mean yearly aboveground biomass yield levels of the respective plots divided by the mean yearly aboveground biomass of the reference plots [Ca(NO3)2, for green manure, animal manure, and sewage sludge; No N for peat].


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  		Table 4. Organic C origin in particle-size fractions estimated by the difference method

 
For estimating the manure-derived Corg in particle-size fractions, Eq. [2] was used

(2)
where {delta}s is the {delta}13C value ({per thousand}) of the particle-size fraction of manure-treated soil, {delta}Ref is the {delta}13C value ({per thousand}) of the particle-size fraction of the reference soil [Ca(NO3)2, No N for peat only], {delta}M is the {delta}13C value ({per thousand}) of the applied manure and CM is the C fraction originating from manure. A slightly overestimated uncertainty of CM, em was obtained using the following equation proposed by Puget et al. (1995)

(3)
where es, eRef, and eM are the standard deviations of the respective {delta} values and eorg is the standard deviation of the Corg measurement.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
pH and Particle-Size Fractions
Forty-two years of different treatments distinctly changed major characteristics of topsoil in the long-term experiment. The pH of bulk soil samples was 6.6 in 1956 and decreased distinctly in the peat and sewage sludge treated plots. The pH values measured were 6.3, 6.5, 6.9, 6.3, 6.7, 5.8, and 5.8 for fallow, No N, Ca(NO3)2, green manure, animal manure, sewage sludge, and peat, respectively. This effect has already been discussed in Kirchmann et al. (1996).

The particle-size fractionation procedure yielded mass recoveries between 96 and 100% (Table 2), typical for a combined wet sieving–centrifugation method. Coarse and medium sand were not affected by the treatments, whereas fine sand, silt, and clay exhibited clear responses. Treatments with high accumulation of Corg, like the sewage sludge and peat plots (Table 3), were lower in clay-sized and higher in silt-sized particles because of the larger portion of stable microaggregates in the silt-sized fraction. In contrast, those treatments that lost considerable amounts of the Corg previously present in 1956 (fallow, No N) showed considerably more clay-sized particles than the other plots. As the low-energy sonication methods used preserve stable microaggregates (Stemmer et al., 1998), this effect is probably due to destabilization or even decay of microaggregates in the silt-size class and consequently a transfer of small mineral particles into the clay fraction. The ability of low-energy sonication to disrupt unstable aggregates in the 2000- to 100-µm particle-size fraction while preserving stable microaggregates smaller than that was also recently proven by Roscoe et al. (2000).


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  		Table 2. Particle-size fractions in topsoil (0–20 cm) after 42 yr of different treatments

 

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  		Table 3. Response of Corg contents of bulk soil and particle-size fractions to different long-term treatments 42 yr after start of the field experiment.{dagger}

 
Organic Carbon in Bulk Soil Samples
Organic C concentrations in bulk soil samples from the upper 20 cm of the profile responded significantly to 42 yr of different treatments (Table 3). Compared with 1956 (15 g Corg kg-1 soil), the topsoil of the fallow treatment lost approximately one-third of its original Corg concentration. The Corg in plots receiving root and stubble input [No N, Ca(NO3)2] decreased less, whereas plots treated with organic manures exhibited higher Corg concentrations than at the beginning. The greatest increase in Corg concentration occurred in peat-treated plots, which had more than 30 g C kg-1 soil in bulk soil. The remaining ranking was: sewage sludge > animal manure > green manure > Ca(NO3)2 > No N > fallow. Due to the layout of the experiment, using equal amounts of Corg for each treatment, the Corg accumulation is a reflection of the quality of the organic input. Peat has a C/N = 70 and is therefore a poor quality substrate for microbial activity. Its addition resulted in a C/N ratio in bulk soil of 18.0 (Gerzabek et al., 1999). The C/N ratios of soil in other plots were distinctly lower [fallow: 8.9; No N: 8.9; Ca(NO3)2: 9.3; green manure: 9.6; animal manure: 10.7; sewage sludge: 9.7]. The organic matter in sewage sludge is subjected to an intensive microbial digestion during anaerobic and aerobic sewage treatment, leaving the sludge as a relatively recalcitrant residue. This explains the high stability of sewage-sludge-derived organic matter in the experiment. Animal manure, which is also well decomposed before application, contains organic matter of higher stability than green manure.

In a previous study, the C balance of the experiment was evaluated taking into account bulk density changes and Corg concentrations in soil samples of 12 yr from 1956 to 1994 (Gerzabek et al., 1997). From regression analysis, it was concluded that changes in Corg have been linear for the 37 yr since the start of the experiment, and that equilibrium between C input and output has therefore not been reached yet. Similar results were obtained in other long-term experiments (e.g., Jenkinson et al., 1994).

Organic Carbon in Particle-Size Fractions
Comparing the average C contribution of particle-size fractions to the total amount revealed the following ranking: silt > clay > fine clay > fine sand > coarse sand. The C partitioning between size fractions could not be explained solely by the particle-size mass distribution. Comparing average C and mass distributions between size fractions showed that coarse to medium and fine sand were depleted in Corg (C/mass ratios: 0.37 and 0.29), whereas silt, clay, and fine clay were enriched in C (C/mass ratios: 1.19, 1.11, 1.53). Several authors have confirmed that Corg concentrations increase with decreasing particle size (e.g., Christensen, 1986; Desjardins et al., 1994). Particle-size fractions differed distinctly in their response to the treatments (Table 3). Calculating the ratio between highest and lowest Corg contents, the largest variation occurred in the fine sand fraction (6.7), followed by coarse and medium sand (4.6), silt (3.9), fine clay (1.7), and clay (1.6). For bulk soil, the value was 3.0. Thus, sand and silt were clearly more affected by the treatments than the clay fractions.

According to our data, most of the Corg introduced into the Ap horizon was stored in the silt-sized fraction, and only little reached the clay fraction during the decomposition process. This was even clearer when calculating Corg partitioning on a percentage basis. Figure 1 shows statistically significant correlations between the bulk soil Corg content and the Corg present in size fractions. Organic C in silt increased from 52% in fallow plots to nearly 70% in peat plots, while Corg in clay-sized particles (0.1–2 µm) decreased from 36% (fallow) to 19% (peat). The C contribution of sand fractions tended to increase and that of fine clay to decrease with bulk soil Corg contents.



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  		Fig. 1. Portions of bulk soil organic C in particle-size fractions (in percentage; y) in relation to bulk soil organic C concentrations of the Ap horizon (0–20 cm; x). Linear regression functions: >200 µm: y = 0.042x + 0.651, R2 = 0.780**; 200–63 µm: y = 0.182x + 0.743, R2 = 0.841**; 63–2 µm: y = 0.733x + 46.2, R2 = 0.926***; 2–0.1 µm: y = -0.795x + 44.7, R2 = 0.991***; <0.1 µm: y = -0.193x + 8.503, R2 = 0.777**. *, **, and *** Significant at the 0.05, 0.01, and 0.001 probability levels

 
The above findings are supported by a simple balance calculation using the difference method (Table 4). Due to the number of arithmetical operations, standard deviations of the results were high. However, trends could be observed. Carbon originating from plant residues (roots and stubble) was not detectable in clay and fine clay fractions in plots with low plant biomass production (No N, peat). Native soil C was apparently the only C source for the clay fractions in these cases. Detectable contributions of plant residues to the clay fractions were present only in plots with high productivity [Ca(NO3)2, animal manure]. Significant contributions of plant residue C were evident in silt and fine sand. The proportions of Corg derived from organic amendments decreased from sand to silt and clay fractions. Organic C in the latter fraction consisted of approximately one-third amendment C (animal manure, peat), whereas in sand-sized particles most Corg originated from organic amendments.

Both silt and clay fractions contain major amounts of microbially derived organic material (Amelung et al., 1999). In our case the silt fraction clearly acted as a medium-term sink for the introduced Corg. This is probably connected with physically protected Corg (following the definition of Jenkinson and Rayner, 1977) involved within microaggregates. The transfer of Corg into the clay-sized fraction (mainly chemically protected Corg) by microbial action was slow. Thus, the significance of the clay-sized fraction for the medium-term sequestration of Corg at increasing Corg contents seems to be small. According to Buyanovsky et al. (1994) clay-sized particles contain Corg of high stability and slow turnover rates. It has been shown recently that clay-sized Corg is more aliphatic than that of bulk soil across many soils developed under different land use and climatic conditions (Mahieu et al., 1999). Nevertheless, physical protection may result in similarly low turnover rates as chemical stabilization of soil organic matter (Haider, 1999). The small amount of Corg present in sand-sized particles originating to a large extent from the organic amendments supports the common aggregate hierarchy theory (Oades and Waters, 1991). As macroaggregates are disrupted by the sonication procedure smaller particles might be transferred to smaller fractions and only larger young organic fragments remain in the coarse fractions.

Natural Abundance of Carbon-13 in Bulk Soil and Size Fractions
Prior to the experiment at Ultuna, the soil was under common agricultural practice, which included animal manure additions. Organic C had an initial {delta}13C value of -26.3{per thousand} in 1956 (Gerzabek et al., 1997), typical for soils cropped with C3 plants for long periods (Puget et al., 1995). The deviations from this original value after 42 yr (Table 5) were attributed to different factors: (i) the preferential volatilization of the lighter 12C isotope by microbial respiration increases {delta}13C values of residual Corg in fallow plots; (ii) exclusive C input through plant residues (mean {delta}13C value for shoots = -27.1 ± 0.9{per thousand}) induces more negative {delta}13C values than the fallow treatment; (iii) those organic amendments whose {delta}13C values differ from the stable fraction of original SOM plus plant input, represented by the No N and Ca(NO3)2 treatments (reference plots), might alter the 13C abundance. The deviation of the {delta}13C values from the reference was statistically significant for green manure, animal manure, and sewage sludge, and was consistent with the average {delta}13C values of these amendments (Tables 1 and 5).


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  		Table 5. Mean natural abundance ({per thousand}) values of topsoil samples (0–20 cm) after 42 yr of different treatments

 
Size fractions within treatments often showed larger variations of 13C abundances than bulk samples between treatments (Table 5). Peat plots exhibited the smallest differences between lowest and highest value (0.8), and green manure and No N plots the highest (2.0). The {delta}13C values generally increased from coarse to fine fractions. Note also that the fine clay fraction was isotopically very similar to the clay fraction. This indicates a close relation between these two fractions and high stability of the smallest size separate. Consequently, this very small fraction, obtained as supernatant from the last centrifugation step of the fractionation procedure, contained organic fractions older than just soluble organic matter or fragments derived from recently introduced organic matter from plant residues and manures.

Cluster analysis of all isotopic data was performed to investigate general similarities between treatments (graph not shown). A close relationship was obtained between animal manure and green manure treatments. Weaker similarities were observed for No N and Ca(NO3)2, and peat and sewage sludge. The isotopic pattern of fallow particle separates differed significantly from all other treatments.

We attempted to use differences in 13C abundances between size fractions of different treatments to estimate Corg originating from the respective organic amendment. The variability of basic measurements needed for the calculation (Eq. [2]) yielded large overall standard deviations for the estimates (Eq. [3]). Some of the estimates were consistent within errors with the balance calculation presented in Table 4. The contribution of peat Corg to Corg in coarse and fine sand was 60 ± 35 and 60 ± 31%. Sewage sludge Corg contributed 44 ± 24% to Corg in silt-sized particles, and animal manure Corg amounted to 43 ± 21% in the fine clay fraction. The limitations of the isotopic method in our case are due to (i) the small differences in {delta}13C between potential C sources and (ii) the possible isotopic fractionation of different Corg pools of the organic amendments and their preferential accumulation in size fractions. This restricted the application of the isotopic method in our investigation to a few cases only. Nevertheless, these isotope results emphasized in principle the applicability of the approach for organic amendments originating from C sources differing more significantly from soil C than in the present experiment.

Our results emphasized that the 13C natural abundance in particle-size fractions was influenced mainly by plant residue input, manure input, and the stability of the introduced manure C. Increasing {delta}13C values with decreasing particle size as observed in the present study were also reported by Baldesdent and Mariotti (1996) and Stemmer et al. (1999). The mechanisms involved are not fully clear yet. Explanations might include (i) the preferential loss of 12C during microbial digestion of Corg, overall losses being larger for very small organic particles that have been subject to repeated microbial attack, and (ii) the enrichment of organic compounds less available for microbial digestion (Roscoe et al., 2000). Already Benner et al. (1987) emphasized the distinct differences between major components of plant tissues with respect to stable C isotope composition. Their results showed a depletion of 13C in the most stable fraction, the lignins, which does not support the second explanation given above. Roscoe et al. (2000) did not observe the continuous enrichment of 13C with decreasing particle sizes. Only clay particles exhibited significantly more positive {delta}13C values than all coarser fractions, which showed no differences. This observation might be related to the soil type investigated. Roscoe et al. (2000) used a Dark Red Latosol from Cerrado, Brazil, for their study. Oades and Waters (1991) reported that an Oxisol did not show a hierarchical order of aggregates as a result of oxides rather than organic materials being the main stabilizing soil constituent. However, Roscoe et al. (2000) observed unstable macroaggregates in coarse fractions and stable microaggregates in the silt-sized fraction. Three soils investigated by Stemmer et al. (1999) originating from Denmark (Haplic Luvisol), Germany (Haplic Luvisol on loess), and Italy (Entisol) showed consistently increasing {delta}13C values with diminishing particle sizes with the exception of one coarse sand fraction. However, the range between largest and lowest values varied considerably, being 1.8, 1.3, and 0.7 for the Haplic Luvisol, Haplic Luvisol on loess, and Entisol, respectively. Of course, the soils used in the experiment of Stemmer et al. (1999) did not have the same history of agricultural land use nor a similar mineralogy. The Entisol was previously used as vineyard, whereas the other two soils were in crop production. Note that in the present study we observed a larger range of {delta}13C values in particle-size fractions within treatments than was reported for the three different soils discussed above. We might conclude that agricultural management, specifically use of mineral and organic fertilizers, could have a more significant impact on the natural abundance of stable C isotopes in physical soil fractions than different sites or soil types. Thus, investigation of {delta}13C values in particle-size separates might be used as a sort of fingerprint analysis for differentiating agricultural fertilizer management.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The physical fractionation method used in combination with low-energy sonication allows researchers to follow changes in soil structure due to organic amendments on a long-term basis. Organic C present in particle-size fractions clearly responded to 42 yr of different treatments. Organic C in the silt-sized fraction proved to be the main medium-term C sink. It increased with increasing bulk soil Corg contents, whereas the relative contribution of clay-sized C diminished. This behavior indicates that silt- and clay-sized C belong to Corg fractions of distinctly different behavior. However, no estimation of the long-term behavior of Corg sequestered in silt-sized particles could be derived from the present experiment. Organic C in sand-sized fractions showed the greatest response to treatments. Thus, we suggest that changes in Corg in the sand-sized fractions, a parameter that can be simply measured, may be a sensitive indicator of changes in soil C storage in response to land management practices. Measurement of natural isotopic abundances of stable C isotopes in particle-size separates provided fingerprints suitable for differentiation of the treatments.


   ACKNOWLEDGMENTS
 
We thank the Austrian Science Fund (Fonds zur Förderung der wissenschaftlichen Forschung) for funding this bilateral project. We are grateful to B. Temmel, K. Blochberger, and P. Herger for assisting with particle-size fractionations, isotopic measurements, and statistical analysis. The authors thank Pär Hillsström for the careful and responsible maintenance of the long-term experiment and Dr. M. Stachowitsch for linguistic help.

Received for publication January 24, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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D. Solomon, F. Fritzsche, J. Lehmann, M. Tekalign, and W. Zech
Soil Organic Matter Dynamics in the Subhumid Agroecosystems of the Ethiopian Highlands: Evidence From Natural 13C Abundance and Particle-Size Fractionation
Soil Sci. Soc. Am. J., May 1, 2002; 66(3): 969 - 978.
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