Dynamics of13C-labeled mustard litter (Sinapis alba) in particle-size and aggregate fractions in an...

$: Dynamics of13C-labeled mustard litter (Sinapis alba) in particle-size and aggregate fractions in an agricultural cropland with high- and low-yield areas$
Dynamics of 13C-labeled mustard litter (Sinapis alba) in particle-size andaggregate fractions in an agricultural cropland with high- and low-yieldareasAngelika Kölbl1*, Margit von Lützow1, Cornelia Rumpel2, Jean Charles Munch3, and Ingrid Kögel-Knabner1

1 Lehrstuhl für Bodenkunde, Department für Ökologie, Wissenschaftszentrum Weihenstephan für Ernährung, Landnutzungund Umwelt, Technische Universität München, 85350 Freising-Weihenstephan, Germany

2 CNRS, Laboratoire de Biogéochimie des Milieux Continentaux, Centre INRA de Versailles-Grignon,F-78850 Thiverval-Grignon, France

3 Institut für Bodenökologie, GSF-Forschungszentrum für Umwelt und Gesundheit, Ingolstädter Landstr. 1, 85764 Neuherberg,Germany

Dedicated to Professor Dr. Burkhard Sattelmacher, who passed away on November 21, 2005

Accepted November 27, 2006

SummaryThe application of 13C-labeled litter enables to study decom-position processes as well as the allocation of litter-derivedcarbon to different soil C pools. 13Carbon-labeled mustardlitter was used in order to compare decomposition processesin an agricultural cropland with high-yield (HY) and low-yield(LY) areas, the latter being characterized by a finer textureand a lower organic-C (OC) content. After tracer application,13C concentrations were monitored in topsoil samples in par-ticulate organic matter (POM) and in fine mineral fractions(silt- and clay-sized fractions).After 568 d, approximately 5% and 10% of the initial 13Camount were found in POM fractions of LY and HY areas,respectively. Higher amounts were found in POM occluded inaggregates than in free POM. Medium-term (0.5–2 y) storageof the initial 13C in fine silt- and clay-sized fractions amountsto 10% in HY and LY soils, with faster enrichment but alsofaster disappearance of the 13C signal from LY soils. Amountsof 80%–90% of the added 13C were mineralized or leached inthe observed period. Decomposition of free POM was faster

in HY than in LY areas during the first year, but the remaining13C amounts in occluded-POM fractions were higher in HYsoils after 568 d. High-yield and low-yield areas showeddifferent 13C dynamics in fine mineral fractions. In LY soils,13C amounts and concentrations in mineral-associated frac-tions increased within 160 d after application and decreasedin the following time period. In HY areas, a significantincrease in 13C amounts did not occur until after 568 d. Theresults indicate initially faster decomposition processes in HYthan in LY areas due to different soil conditions, such as soiltexture and water regime. The higher silt and clay contents ofLY areas seem to promote a faster aggregate formation andturnover, leading to a closer contact between POM andmineral surfaces in this area. This favors the OC storage infine mineral fractions in the medium term. Lower aggregateformation and turnover in the coarser textured HY soil leadsto a delayed C stabilization in silt- and clay-sized fractions.

Key words: soil organic matter / particulate organic matter / physicalfractionation / organic carbon and nitrogen / 13C-tracer techniques

2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1436-8730/07/0102-123

J. Plant Nutr. Soil Sci. 2007, 170, 123–133 DOI: 10.1002/jpln.200625071 123

1 Introduction

On agricultural fields, low-yield (LY) and high-yield (HY) areascan occur in proximity. Soil texture as well as slope positioncan be responsible for these differences (Kölbl et al., 2006;Sehy et al., 2003), leading to spatial variations in litter decom-position and subsequent nutrient supply for crops.

To study soil-organic-matter (SOM) turnover in soils showingcontrasting agricultural yield, particle-size and density fractio-nation may be most appropriate. Particle-size fractionationbased on the concept that SOM fractions associated with par-ticles of different size and different mineralogical compositiondiffer in structure and function and therefore turnover differ-ently (Christensen, 1992, 2001). Density fractionation isapplied to isolate SOM, which is not firmly associated withsoil minerals. The light fraction is composed of less decom-posed plant and animal residues, whereas the heavy fraction

(sediment) contains organo-mineral complexes in whichSOM is more processed (Christensen, 1992). The light, parti-culate organic-matter (POM) fractions were suggested asindicators for the labile (active) SOM pool. The heavy frac-tions are attributed to the stable (passive) SOM pool (Chris-tensen, 1996; Elliott et al., 1996). Golchin et al. (1994, 1995)isolated two structurally defined POM fractions: a fractionoccluded within aggregates and a free fraction. These frac-tions are supposed to represent functionally different SOMpools, caused by spatial differences within the soil matrix.

Application of 13C-labeled litter in combination with particle-size and density fractionation allows to trace the incorpora-tion of the released OC into POM and mineral-associatedorganic-matter fractions (Buyanovsky et al., 1994; Hassink,1997; Aita et al., 1997). For example, Aita et al. (1997) andHaynes (1997) analyzed the short-term (2 y) decompositionof labeled plant residues in the field by separating the soilsamples into particle-size fractions. A similar study by Magidet al. (2002) indicated the transfer of carbon from large lightfractions to fine heavy organo-mineral fractions. But the* Correspondence: Dr. A. Kölbl; e-mail: [email protected]

translocation of litter-derived OC through free and intra-aggregate POM fractions and to fine mineral–associated OMdepending on soil texture and aggregation has rarely beenreported (Kölbl et al., 2006).

At the experimental farm of the Munich Research Network onAgroecosystems (FAM), high-yield and low-yield areas of anagricultural cropland were found to be due to differences insoil texture and slope position and therefore different waterregimes. Schloter et al. (2003) found a much higher potentialfor ammonification at HY plots compared to the LY plots. Thesoil microbial biomass, quoted as microbial C, was higher inHY soils (Tab. 1) and showed a faster turnover of added Cand N, indicating a higher microbial activity at this site (Wes-sels Perelo and Munch, 2005). Sehy et al. (2003) measuredsignificantly higher N2O emissions in the HY area. Higher soilwater contents and a higher nutrient supply as a result of lat-eral transport processes from the surrounding slopes due tothe location of the HY plots in a footslope position were foundto be responsible for faster turnover processes (Sehy et al.,2003). In a previous study (Kölbl et al., 2006), we investigateddecomposition dynamics of 15N-labeled mustard litter in HYand LY soil, focussing on the 15N movement from free POMto intra-aggregate POM and to fine mineral fractions. Theauthors described a faster cycling of 15N through the soil frac-tions in HY than in LY areas with stabilization of mustard-de-rived N in fine mineral fractions.

The present study elucidates differences in the turnover ofmustard-derived 13C in the HY and LY plots. The main objec-tive is to identify how far small-scale differences in local char-acteristics (mainly due to topography and texture) influenceturnover and storage of added 13C in the different POM andfine mineral fractions in the medium-term (0–2 y).

2 Materials and methods

2.1 Site description and sampling

The experiment was carried out at the Research StationScheyern of the Munich Research Network on Agroeco-systems (FAM), 40 km north of Munich in southern Germany(445–498 m asl). The sampling sites were located in a hillylandscape derived from tertiary sediments and partly coveredby loess. Further details about the area are given in Schröderet al. (2002).

The study sites were established in a field managed accord-ing to conventional agricultural practices. Within the studysite, one area of low yield (LY, 85% average yield) and onearea of high-yield (HY, >115% average yield) based on long-term observations of crop yields of the whole field werechosen. High-yield areas are situated in a footslope positionand thus receiving lateral water and nutrient supply (Sehyet al., 2003), whereas LY areas are localized on a roundedhilltop. Soil characteristics are given in Tab. 1. Two plots of4 × 4 m2, one amended with 15N- and 13C-labeled mustard(Sinapis alba) in October 1999, and an unlabeled control plotwere set up each in the LY and HY area. The mustard mate-rial had a d13C value of 101‰ of dry matter. At the beginning

of the experiment, the soil in the plots was removed to adepth of 15 cm and sieved (15 mm) through a compost siev-ing machine. The mustard material was chopped to 6 mm.Mustard and soil material were thoroughly mixed with a ratioof 1.37 mg mustard (g dry soil)–1 (corresponding to 508 lg C(g dry soil)–1 and 28.5 lg N (g dry soil)–1) and returned intothe plots. The subsequent soil management in the investi-gated period is shown in Tab. 2. Further information about theexperimental design is given in Schloter et al. (2003), Sehy etal. (2003), and Wessels Perelo and Munch (2005), andresults of the 15N labeling were presented in Kölbl et al.(2006).

Sampling was carried out at four dates corresponding to 161,283, 420, and 568 days after application (Tab. 2). Bulk soilsamples were taken from the upper 15 cm of the labeled andof the control plot of both HY and LY areas in four replicates,resulting in 16 soil samples per sampling date. Due to the plotdesign, the four replicates were taken as subsamples fromeach of the four plots, resulting in nonindependent replica-tions. Nevertheless, this approach was chosen to investigatethe variability of litter decomposition within each plot. The soilsamples were air-dried and gently sieved to a size of<2.0 mm.

2.2 Physical fractionation

A physical fractionation procedure was applied to both,labeled and unlabeled soil material, using air-dried aggre-gates sieved to a size of <2.0 mm as described in detail inKölbl and Kögel-Knabner (2004). Briefly, in a first fractiona-tion step, the free-POM fraction was separated in a Na-poly-tungstate solution (q = 1.8 g cm–3). To obtain the POM frac-tion occluded in aggregates, the heavy fraction of the firstfractionation step was treated by ultrasound with an energyof 450 J mL–1. This energy was sufficient to disrupt all aggre-gates and to obtain similar clay yields compared to standardtexture analysis as described by Schmidt et al. (1999). With a

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Table 1: Soil characteristics of the topsoils (0–15 cm) of both areas;data from own measurements and FAM database.

High-yield area Low-yield area

Soil type Eutric Regosol* Eutric Cambisol*

pH (CaCl2) 5.9 6.1

OC (mg g–1) 17 14

Ntot (mg g–1) 1.7 1.5

Cmic (lg g–1)** 268 ± 62 211 ± 45

Soil texture (mg g–1)

Clay <2 lm 150 200

Silt 2–63 lm 490 510

Sand 63–2000 lm 360 290

* World Reference Base for Soil Resources (IUSS Working GroupReference Base, 1998).

** Cmic: microbial C (data taken from Wessels Perelo and Munch,2005).

124 Kölbl, von Lützow, Rumpel, Munch, Kögel-Knabner J. Plant Nutr. Soil Sci. 2007, 170, 123–133

subsequent density-fractionation step (Na-polytungstatesolution), the occluded-POM fraction, flooding on the solutionwas obtained. The heavy fraction of the density-fractionationprocedure—mineral particles and organo-mineral com-plexes—was sieved at 20 lm to obtain the sand and coarse-silt fraction. Finer particles were separated by sedimentationprocedures in medium silt– (20–6.3 lm), fine silt–(6.3–2.0 lm), coarse clay– (2.0–0.2 lm), and fine clay–sizedfractions (<0.2 lm). All mineral fractions were washed withbidistilled water, freeze-dried, weighed, and ground for furtheranalyses.

2.3 Chemical analyses

Bulk soil material and physical fractions were analyzed induplicate for total carbon and nitrogen (Ntot) concentrationsby dry combustion at 950°C with a “Vario EL” elementalanalyzer (Elementar Analysensysteme GmbH, Hanau,Germany). Since all samples were free of carbonates, themeasured total carbon content was equivalent to the organic-carbon (OC) content. Determination of the stable-carbon-iso-tope ratio (d13C) of soil fractions was done using an elementalanalyzer coupled to an isotope-ratio mass spectrometer(Sira 10) at the Laboratoire de Biogéochimie des MilieuxContinentaux, Centre INRA de Versailles-Grignon. The preci-sion of the isotope analyses was 0.3‰.

The isotopic composition of whole soil and SOM fractionswere expressed in delta (d) units:

d � ��Rsample��RPDB � 1� × 1000�� (1)

RSample is the sample ratio of 13C : 12C and RPDB = 0.0112372(based on the Pee Dee Belemnite standard). Therefore, all13C values are reported as the difference in parts per thou-sand (‰) from the defined international PDB standard.

No soil samples were taken directly after the initiation of theexperiment. Therefore, the initial 13C label of the soil wascalculated according to the following assumptions: Mustardlitter incorporated into the soil at time 0 of the experiment isneither occluded in soil aggregates nor associated with min-eral particles. Consequently, the total 13C label at time 0 isattributed to the free POM, and it is assumed that all othersoil fractions are not enriched in 13C at the beginning of theexperiment. In the control plots, the amount of free-POM-OCwas determined to 0.394 mg POM-OC (g soil)–1 in the HYand 0.458 mg POM-OC (g soil)–1 in the LY plots in April 2000(Tab. 3). The d13C natural abundances of POM were–26.1‰. Addition of 0.508 mg mustard-OC with d13C valuesof 101‰ at the beginning of the experiment in October 1999leads to estimated d13C values in free POM of 45.5‰ in HYand 40.7‰ in LY plots.

2.4 Data representation and statistical analysis

Statistical analyses were carried out using the software SPSS12.0 (SPSS Inc.). The nonparametric Mann-Whitney-Utest for non-normal distributed datasets, which requires onlyfour replications in order to demonstrate significant differ-ences (Hurlbert, 1984), was used for differentiation of soilfractions between HY and LY areas at the different samplingdates.


Table 2: Soil management in the investigated period (ANU: ammonium-nitrate-urea solution).

Days after startof experiment

Soil management

Oct 22, 1999 0 Application of labeled mustard litter, tillage and sowing of winter wheat

Mar 24, 2000 156 Fertilizer ANU (50 kg N ha–1)

Apr 3, 2000 161 Soil sampling

May 9, 2000 202 Fertilizer ANU (65 kg N ha–1)

May 29, 2000 222 Fertilizer ANU (45 kg N ha–1)

Aug 3, 2000 283 Soil sampling

Aug 13, 2000 298 Harvest of winter wheat, tillage and sowing of mustard (green manure cropping)

Aug 24, 2000 309 Cattle manure (17.5 m3 ha–1)

Dec 18, 2000 420 Soil sampling

May 5, 2001 568 Soil sampling

Table 3: Estimated δ13C enrichments of free POM at the beginning of the experiment in October 1999—values for high- and low-yield plots.

InitialPOMfree-OC

InitialPOMfree-δ13C

AddedMustard-OC

AddedMustard-δ13C

TotalPOMfree-OC

δ13C-enrichmentPOMfree

mg (g soil)–1 ‰ mg (g soil)–1 ‰ mg (g soil)–1 ‰

HY 0.394 –26.1 0.508 +101.0 0.902 +45.5

LY 0.458 –26.1 0.508 +101.0 0.966 +40.7

J. Plant Nutr. Soil Sci. 2007, 170, 123–133 Dynamics of 13C-labeled mustard litter 125

Organic-C storage capacities of particles <20 lm were calcu-lated using the correlations between the amounts of C and Nassociated with the clay and silt fraction and the percentageof soil particles in this size fraction found by Hassink (1997)for uncultivated and grassland soils:

C�g kg�1� � 4�09 � 0�37 × % �particles � 20 lm�� (2)

Decomposition of labeled free-POM fractions was describedwith a two-component model (Voroney et al., 1989):

y = a × e(–k1×t1)+ b × e (–k2×t2), (3)

using the software SigmaPlot 2000, version 6.0 (SPSS Inc.).Terms a and b describe the fast and slowly decomposableOM pool, respectively, while k displays the turnover rate atthe respective time t. This equation allows describing thekinetics of two-phase decomposition processes. Based onEq. 3, half-life periods of the labeled substrates were calcu-lated with t1/2 = ln2 / k.

3 Results

3.1 Mass distribution, OC and N in soil fractions

In both HY and LY areas, the sand + coarse-silt fraction(2000–20 lm) accounts for approximately 50% of the bulksoil (Tab. 4). Corresponding to the soil-texture data (Tab. 1),the LY area has higher amounts of clay-sized fraction (28%)compared to HY plots (23%). Fine- and medium-silt fractionsamount to 25% in the LY and 29% in the HY area.

The OC and N concentrations of the particle-size fractions showdifferences between both areas, which are more pronounced inthe finer-particle-size fractions. Clay-sized fractions havebetween 40 and 44 mg OC g–1 in HY, but only 28 to 29 mg OCg–1 in LY plots. Fine-silt fractions have 24 mg OC g–1 in HY and19 mg OC g–1 in LYareas, whereas coarser fractions show verylow OC concentrations between 1 and 3 mg OC g–1 (HY), and 1and 2 mg OC g–1 (LY). Nitrogen has a comparable distributionwith highest N concentrations in the clay-sized fractions andhigher values in HY compared to LY plots. In summary, higherC and N concentrations were found in fine-particle-size frac-tions in HY soils, having lower proportions of clay. In both HYand LY areas, the contribution of OC allocated to individualfractions to the bulk soil shows that 75% of the mineral-boundOC is stored in the clay-sized fractions, 13%–14% is storedin the fine-silt fraction, and all coarser fractions togetheramount to 11%–12% of the OC storage.

Mass distribution as well as the OC and N concentrations ofboth POM fractions show a pronounced variation in time(Tab. 5). Masses of the free POM vary between 3 and 27 mg(g soil)–1, masses of the occluded POM are a little higher,with values between 9 and 34 mg (g soil)–1. The OC concen-trations show variations between 110 and 381 mg g–1 in free-POM and between 336 and 453 mg g–1 in occluded-POMfractions. Nitrogen concentrations range from 6.1 to 16.1 mgg–1 and from 18.3 to 23.3 mg g–1 in free and occluded POM,respectively. Thus, OC and N concentrations are noticeablyhigher in occluded than in free POM. The contribution ofPOM-OC to the OC content of the bulk soil varies between0.07 and 0.72 mg (g soil)–1 for free POM and between 0.31and 1.39 mg (g soil)–1 for occluded POM. This correspondsto 0.6%–7% of TOC for free POM and 2.5%–11% of TOC foroccluded POM.


Table 4: Mass distribution, OC and N concentrations of soil fractions, and contribution of OC of a specific fraction to the bulk soil. Means ± SE inbrackets, n = 16 (four replicates at four sampling dates).

High yield Low yield

Soilfraction

massg (g soil)–1

OCmg(g fraction)–1

Nmg(g fraction)–1

C:N OCmg(g soil)–1

OC% ofbulk soil

massg (g soil)–1

OCmg(g fraction)–1

Nmg(g fraction)–1

C:N OCmg(g soil)–1

OC% ofbulksoil

0–0.2 lm 0.060 44.4 6.59 6.7 2.66 21.1 0.103 28.2 4.52 6.2 2.90 28.3

(±0.005) (±0.82) (±0.09) (±0.000) (±0.47) (±0.01)

0.2–2.0 lm 0.169 40.2 5.20 7.7 6.79 53.9 0.174 28.7 4.21 6.8 4.99 48.6

(±0.006) (±0.52) (±0.14) (±0.001) (±0.28) (±0.09)

2.0–6.3 lm 0.072 23.7 2.31 10.3 1.71 13.5 0.068 19.1 2.06 9.3 1.30 12.6

(±0.002) (±1.01) (±0.11) (±0.001) (±0.60) (±0.05)

6.3–20 lm 0.221 3.1 0.44 7.1 0.69 5.5 0.189 2.3 0.33 7.0 0.43 4.2

(±0.008) (±0.21) (±0.04) (±0.001) (±0.13) (±0.01)

20–200 lm 0.330 1.9 0.27 7.0 0.62 4.9 0.335 1.2 0.15 8.0 0.41 4.0

(±0.004) (±0.13) (±0.01) (±0.001) (±0.05) (±0.01)

200–2000 lm 0.144 1.0 0.13 7.7 0.14 1.1 0.126 1.9 0.13 14.6 0.24 2.3

(±0.001) (±0.05) (±0.01) (±0.002) (±0.08) (±0.01)

total 12.61 100 10.27 100


Significant differences between HY and LY areas are onlyfound in the free-POM fraction at the first three samplingdates. Masses, OC and N concentrations showed no definitetrend over the period of time, but emphasized the seasonaldynamics especially of the free-POM fraction over the year.

Total amounts of OC in particle-size fractions were 12.6 mgOC (g soil)–1 and 10.3 mg OC (g soil)–1 in HY and LY soils, re-spectively (Tab. 4). Total OC amounts in POM fractions (POMfree + POM occl.) add up to 1.7 mg OC (g soil)–1 in HY soil(568 d) and 1.5 mg OC (g soil)–1 in LY soil (420 d) (Tab. 5).Thus, total OC contents recovered after physical fractionationachieved 14.3 mg OC (g soil)–1 and 11.8 mg OC (g soil)–1 inHY and LY soils, respectively. In contrast, OC contents of thebulk soil material were 17 mg OC (g soil)–1 in HY and 14 mgOC (g soil)–1 in LY areas (Tab. 1). As a result, 2–3 mg OC(g soil)–1 were lost during the fractionation procedure, equiva-lent to 16% OC loss in both soils.

3.2 13C distribution in soil fractions

The development of the d13C values of free and occludedPOM over time in HY and LY areas is shown in Fig. 1. After161 d, the relatively high 13C enrichment of the free POMdecreased from 45.5‰ to –11.0‰ in the HY, and from 40.7‰to 0.8 ‰ in the LY plot. Within the next 407 d, a slower declineto –24‰ was found in both areas. The occluded POMshowed the highest 13C enrichments after 161 d in LY, butonly after 420 d in HY areas, with –15.4‰ and –14.5‰, re-

spectively. At 568 d, the 13C concentration declined in bothareas to give –24‰.

To take into account the seasonal dynamics, changes inmass and OC concentration of POM fractions over time(Tab. 5) were used to calculate the contribution of POM-13Cto the bulk soil. In this step, natural abundance of 13C in eachfraction was subtracted from the total 13C amount, so thatonly added 13C amounts are described below. Therefore, andin order to consider the selective 13C enrichment in fine-parti-cle sizes (Wynn et al., 2005), we rearranged Eq. 1 and calcu-lated total 13C amounts of each fraction as follows:

13C �mg g�1� � 10001

� d × RPDB

1000� RPDB

� � 1 (4)

After this step, we subtracted the results of unlabeled fractionsfrom labeled fractions. Initially added 13C amounts of free-POMfractions were determined using the calculated values shown inTab. 3. In HY, for example, the d13C enrichment of +45.5‰ led toa total 13C concentration of 11.61 mg g–1. In the unlabeled frac-tion, having a d13C value of –26.1‰, the 13C concentration was10.83 mg g–1. Thus, the concentration of added 13C in freePOM amounted to 0.78 mg g–1. Considering the amount ofPOMfree-OC of 0.902 mg (g soil)–1, the amount of added 13Cbased on the bulk soil was 0.72 mg (kg soil)–1.

Based on the calculations described above, an initial amountof 0.72 mg added 13C (kg soil)–1 was found for free-POM frac-tions at time 0 in both areas (Fig. 2). The subsequent strong


Table 5: Mass distribution, OC and N concentrations of POM fractions, and contribution of OC of a specific POM fraction to the bulk soildepending on sampling date. Means ± SE in brackets, n = 4. Significant differences between HY and LY of each fraction within each samplingdate are indicated by ** (p < 0.01).

POM free POM occluded

days after startof experiment

massg(g soil)–1

OCmg (g dryweight)–1

Nmg (g dryweight)–1

C : N OCmg(g soil)–1

massg(g soil)–1

OCmg (g dryweight)–1

Nmg (g dryweight)–1

C : N OCmg(g soil)–1

HY 161 0.00098** 381.2** 16.05 23.8 0.372 0.00087 353.8 21.34 16.6 0.309

(±0.00017) (±7.0) (±1.14) (±0.00026) (±12.3) (±0.81)

283 0.00219** 109.7** 6.13** 17.9 0.241 0.00175 360.8** 21.96 16.4 0.630

(±0.00053) (±20.9) (±0.86) (±0.00027) (±12.4) (±0.80)

420 0.00028** 255.5** 10.49 24.4 0.072 0.00095** 452.9 19.65 23.0 0.432

(±0.00000) (±18.4) (±1.05) (±0.00010) (±18.7) (±1.14)

568 0.00226 122.1 6.56 18.6 0.276 0.00336** 414.9 21.41 19.4 1.393

(±0.00057) (±21.4) (±1.02) (±0.00051) (±27.9) (±1.79)

LY 161 0.00270 268.5 12.78 21.0 0.724 0.00163 336.5 20.06 16.8 0.549

(±0.00013) (±18.3) (±0.51) (±0.00010) (±22.9) (±0.43)

283 0.00076 224.1 9.77 22.9 0.171 0.00233 405.9 23.27 17.4 0.944

(±0.00012) (±31.5) (±1.17) (±0.00004) (±6.5) (±0.35)

420 0.00094 322.8 10.57 30.5 0.303 0.00278 449.2 18.30 24.5 1.250

(±0.00012) (±22.3) (±1.26) (±0.00029) (±16.9) (±1.77)

568 0.00180 219.4 9.93 22.1 0.396 0.00132 440.0 18.73 23.5 0.581

(±0.00086) (±62.5) (±2.04) (±0.00042) (±37.2) (±2.16)


decrease in the first 161 days led to 0.07 and 0.23 mg 13C (kgsoil)–1 in free POM of HY and LY areas, respectively, demon-strating an initially faster mineralization of the applied mustardat the HY area. However, after 568 d, only 0.01 mg 13C (kgsoil)–1 were found in free POM of both areas, indicatingequally cumulative mineralization. The calculated half-life per-iod of t1/2 < 1 d in HY of free POM underlined a fast decompo-sition in the first days after litter application. The remainingOC-label was decomposed more slowly with t1/2 = 139 d. Thecalculated half-life period of free-POM-13C in the LY soils ist1/2 = 87 d throughout the observed period without distinctionbetween two different decomposition phases.

At the HYarea, the 13C label of occluded POM increased slowlybut continuously after mustard application, reaching 0.06 mg13C (kg soil)–1 at the last sampling date. At the LY site, highestamounts of 0.07 mg 13C (kg soil)–1 were found as occludedPOM as early as the 2nd sampling date, followed by a continu-ous decrease to 0.02 mg 13C (kg soil)–1 after 568 d.

The d13C values in clay and silt fractions (Fig. 3) showed nearlyno response to the added 13C with time, but a rather good differ-entiation of typical d13C values for each particle-size class (clay-size fraction had less negative values compared to silt-size frac-tions). This was visible in both HY and LY areas. Significant dif-ferences between HYand LYareas in fine-clay and fine-silt frac-tions due to 13C addition can be attributed to different site condi-tions, e.g., different soil texture.

The contribution of mineral-associated 13C to the bulk soilunderlines the very low 13C enrichment in these fractions(Fig. 4). In both HY and LY areas, not more than 0.04 mg 13C(kg soil)–1 was found in fine mineral-associated fractions.Highest values were detected in the fine- and coarse-clayfraction 161 d after the start of the experiment. It is remark-able that in fine-clay fractions of LY areas, a slightly higherand more or less constant 13C amount in the first 420 d afterlitter application was measured, followed by a decrease atthe last sampling date. The coarse-clay fraction showed adecrease in 13C amount already after 160 d. In contrast, inHY plots an increase in 13C amounts was detected only at theend of the observed period for all analyzed fractions. Only thefine-clay fraction showed a first maximum of 13C amount after161 d.

A compilation of the changes of 13C amounts in different frac-tions reveals that after 283 d, between 13% and 17% of theapplied 13C amount was found in the analyzed fractions,most of it in the POM fractions (Tab. 6). In the LY area, thetotal amount of 13C in the first 161 days after the start of theexperiment was higher than in the HY area, mainly becauseof the higher values in the POM fractions. After 161 d, thestrong decrease in 13C amounts proceeded in the LY area,leading to similar amounts after 283 d in both areas. Between283 and 420 d after the start of the experiment, both areasshow no further decrease in 13C amounts. After 568 d, the LYarea shows a decline of 13C, mainly due to a loss of 13C in the


13

**

**

**


-40

-30

-20

-10

0

10

20

30

40

50

0 200 400 600

Time [days]

POM free

POM occl.

-40

-30

-20

-10

0

10

20

30

40

50

0 200 400 600

Time [days]

POM free

POM occl.

δ13C

[‰

]

**

**

**


-40

-30

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-10

0

10

20

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-20

-10

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10

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40

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Time [days]

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δ13C

[‰

]

Figure 1: Time course of d13Cvalues of POM fractions in high-and low-yield areas. Means ± SD(n = 4). Significant differencesbetween HY and LY of each fractionare indicated by ** (p < 0.05).

0.00

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]

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0.70

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**

**

**

Loss of added POM 13C:

y = 0.57 e (-98.6 t1) + 0.15 e (-0.005 t2)

r2 = 0.99


y = 0.39 e (-0.008 t1) + 0.33 e (-0.008 t2)

r2 = 0.97

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**

**

**

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**


y = 0.57 e (-98.6 t1) + 0.15 e (-0.005 t2)

r2 = 0.99


y = 0.39 e (-0.008 t1) + 0.33 e (-0.008 t2)

r2 = 0.97

Figure 2: Time course of 13Camounts of labeled POM fractionsin high- and low-yield areas. Means± SD (n = 4). Significant differencesbetween HY and LY of each fractionare indicated by ** (p < 0.05).



Table 6: Summarized data for the distribution of 13C label between the fractions given in mg (kg soil)–1 as shown in Fig. 2 and Fig. 4 and also asa percentage of the total 13C label added. Significant differences between HY and LY of each fraction are indicated by ** (p < 0.05).

High yield13C mg (kg soil)–1

(% of total 13C label added)

Low yield13C mg (kg soil)–1

(% of total 13C label added)

days 0 161 283 420 568 0 161 283 420 568

POM free 0.723 0.067** 0.051 0.009 0.012 0.724 0.225 0.030 0.014 0.012

(100) (9.3) (7.1) (1.2) (1.7) (100) (31.1) (4.1) (1.9) (1.7)

POM occl. 0 0.020 0.033** 0.062 0.058** 0 0.073 0.045 0.056 0.021

(0) (2.8) (4.6) (8.6) (8.0) (0) (10.1) (6.2) (7.7) (2.9)

6.3–20 lm 0 0.001 0 0 0.003 0 0 0.003 0.004 0.001

(0) (0.1) (0.0) (0.0) (0.4) (0) (0.0) (0.4) (0.6) (0.1)

2.0–6.3 lm 0 0 0** 0.002** 0.028** 0 0.004 0.005 0.009 0.004

(0) (0) (0) (0.3) (3.9) (0) (0.6) (0.7) (1.2) (0.6)

0.2–2.0 lm 0 0** 0 0 0.016 0 0.022 0.002 0.001 0.005

(0) (0) (0) (0) (2.2) (0) (3.0) (0.3) (0.1) (0.7)

0–0.2 lm 0 0.022** 0.008** 0.016** 0.023 0 0.036 0.037 0.036 0.018

(0) (3.0) (1.1) (2.2) (3.2) (0) (5.0) (5.1) (5.0) (2.5)

Total 0.723 0.110 0.092 0.089 0.140 0.724 0.360 0.122 0.120 0.061

% of added 13C (100) (15.2) (12.8) (12.3) (19.4) (100) (49.8) (16.8) (16.5) (8.5)

-28

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**** **

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High yield Low yieldδ1

3C

[‰

]

δ13C

[‰

]

**** **

**

****

Figure 3: 13C concentrations ofmineral fractions in high- and low-yield areas. Means ± SD (n = 4).Significant differences between HYand LY of each fraction are indi-cated by ** (p < 0.05).

2

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** ** **

**

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**


µm

Figure 4: Time course of 13Camounts of mineral fractions inhigh- and low-yield areas. Means± SD (n = 4). Significant differencesbetween HY and LY of each fractionare indicated by ** (p < 0.05).


occluded-POM fraction. In HY areas, even an increase intotal 13C was measured, due to an increase in mineral-asso-ciated fractions. This resulted in higher 13C amounts in HYthan in LY areas.

4 Discussion

4.1 Organic-C storage in soil fractions

A comparison of the sum of OC contents of the different soilfractions with the bulk soil OC content (Tabs. 1, 4, and 5)shows that 84% OC were recovered in the different fractions.An amount of 16% OC was lost during the fractionation pro-cedure in both soils. This can be explained by leaching of dis-solved organic matter (DOC), elution of microbial biomass,and of other organic compounds that are not resistant againsttreatment with Na-polytungstate solution, ultrasonic disper-sion, and repeated washing procedures.

The proportion of OC in particle-size fractions showed theimportance of the clay fractions for OC storage. In both HYand LY areas, about 75% of the mineral-bound OC wasstored in the fine- and coarse-clay fractions (Tab. 4). In thesoil of the HY area, the amounts of clay were lower, and theconcentrations of OC in the clay-sized fractions were highercompared to the LY area, indicating a higher degree of OCstorage in the fine mineral fractions and therefore a lowercapacity for storing additional amounts of OC at this site. Incontrast, LY soils have lower OC concentrations because ofdilution effects due to higher clay contents. This offers a high-er potential for storing new OC.

Organic-C storage capacities were calculated according toHassink (1997), who found close correlations between theamounts of C and N associated with the clay and silt fractionand the percentage of soil mineral particles in this size frac-tion (Eq. 2). At the HY site, particles <20 lm had an OCstorage capacity of 23.4 g kg–1 and LY sites of 23.8 g kg–1.Measured OC amounts of particles <20 lm (Tab. 4) were11.9 g kg–1 and 9.6 g kg–1 for HY and LY soils, respectively.The difference between actual values and the calculated OCstorage capacity was 49% for HY and 60% for LY soils. Thelower degree of OC saturation in fine mineral fractions at theLY site may be due to a stronger aggregation and thus loweraccessibility of particle surfaces for OC binding or to lowerOC input because of less crop residues at the LY site.Hassink (1997) found that cultivated arable soils generallycontained less than 100% but more than 60% of the amountof C in the fraction <20 lm. Compared to these values, thesoils of our study show a relatively low C saturation. This canbe explained by the long-term intensive cultivation and theaccompanied frequent disturbances due to tillage of thesesites, being responsible for a lower stabilization of C inorgano-mineral complexes.

The mass distribution and OC concentrations of the POMfractions showed a high variation in time due to the seasonalinput of fresh litter and varying decomposition over the year(Tab. 5). It is therefore necessary to take masses, OC and13C concentrations into account at each sampling date.

Comparing free- and occluded-POM fractions, concentra-tions and amounts of OC and N were noticeably higher inoccluded than in free POM (Tab. 5). This indicates that thesoils at the FAM research station are generally well-aggre-gated, with a high capacity to store POM inside the aggre-gates (Kölbl and Kögel-Knabner, 2004). The amount ofPOM-OC occluded in aggregates was higher in LY than inHY soils. The POM fractions did not show major differencesin the C : N ratio (Tab. 5). The LY soils have a higher clay con-tent and thus have a higher capacity to store POM by occlu-sion in aggregates. This is consistent with Franzluebbers andArshad (1997) as well as Kölbl and Kögel-Knabner (2004),who showed that the soil clay content plays an important rolein stabilization of OM, especially by physical protection ofPOM in aggregates.

4.2 13C distribution in POM fractions

4.2.1 Turnover of free POM

Figures 1 and 2 show a strong decline in free-POM-13C con-centrations and free-POM-13C contributions to bulk soil withinthe first 161 d after the start of the experiment, followed by adistinctly slower decrease in the succeeding 407 d. Thedecomposition kinetics was fitted with a two-componentequation (Eq. 3) of the decomposition of free POM (Fig. 2). Inthe first phase of decomposition, a preferential decay of eas-ily decomposable substrates occurred. In this period, 59% to88% of the labeled free POM was decomposed relativelyquickly with half-life periods of t1/2 < 1 d in HY and t1/2 = 87 din LY soils. This points to a faster decomposition of free POMin the first days after litter application in the HY soil. Theremaining more stable organic materials were decomposedmore slowly with t1/2 = 139 d at the HY site. At the LY site,decomposition proceeded with the same rate of t1/2 = 87 d.This emphasizes a slower decomposition of labeled freePOM in the HY soil in the second phase, but constant decom-position rates in the LY soil. This result is in good agreementwith the decline of POM-15N (Kölbl et al., 2006), showingfaster decomposition processes in HY than in LY soils in thefirst days after litter application, but slower decompositionrates compared to LY soils in the second phase.

Wessels (2003) assumed that the microbial population in theLY area was more exposed to desiccation stress throughoutthe year than that in the HY area due to differences in land-scape position. This may lead to periodically reduced SOMturnover in LY areas, but also to reduced crop growth andsubsequently reduced litter input and lower SOM contents.Voroney et al. (1989) differentiated between two decomposi-tion phases and found that mineralization of C and N derivedfrom wheat straw was greater at a site with a moister environ-ment in the short term. This observation is in line with ourresults, showing initially higher decomposition rates in HYsoils, characterized by higher soil water contents as a resultof lateral transport processes due to the location of the HYplot in a footslope position (Sehy et al., 2003).

The decomposition of free POM was consistent with changesof the amount of microbial biomass at both sites. WesselsPerelo and Munch (2005) showed in HY and LY plots a signifi-



cant increase in biomass C mostly derived from mustard litterwithin a few days after the start of the experiment, with highervalues in HY than in LY plots. Additionally, the authorsshowed that incorporation of substrate C per unit biomassoccurred faster and to a greater extend in the HY soils andshowed a faster turnover, indicating a more active microbialpopulation. Between November 1999 and April 2000, rela-tively high concentrations of CO2 and 13CO2 were measured,confirming a rapid initial fresh-litter decomposition and miner-alization (Sehy, 2003). In our study, the fast decomposition ofmustard litter in the HY soil and the accompanying higheramount of microbial-biomass C derived from mustard littermay have led to a higher proportion of 13C fixed in the micro-bial-biomass pool. Due to the physical fractionation proce-dure, we assume that most of the microbial biomass wasdestroyed and washed away together with dissolved organiccarbon (DOC), so that these pools were not detected in ourstudy.

4.2.2 Aggregate turnover

After 161 d, 10% of the added 13C were found as occluded-POM fraction in aggregates of the LY, but only 3% in aggre-gates of the HY area (Tab. 6). This indicates a relatively fastformation of new aggregates and physical protection of litter-derived C inside these aggregates as occluded POM, whichis explainable in view of the more pronounced aggregation ofthe finer textured material of the LY soil. In HY soils, theocclusion of 13C was much slower, reaching the highest value420 d after start of the experiment. This points to a sloweraggregate formation and turnover compared to LY soils.Within the first 420 d after the start of the experiment, theamount of 13C in occluded POM achieved in both areas high-er values than in the free POM fraction (Fig. 2). In the follow-ing period, the amount of occluded-POM-13C tended todecrease in the LY soil, indicating a release of the occluded-POM-C upon the turnover of the macroaggregates in which itwas temporarily stabilized (Six et al., 1999; Denef et al.,2001). At the same time, new POM-C is occluded upon for-mation of new aggregates. The higher amount of mustard-derived POM occluded and released in the observed periodindicates a faster aggregate turnover in the LY soil. Aggre-gate turnover is supported by both the finer textured soilmaterial and the most likely more pronounced drying-wettingevents of the LY plot situated on a hilltop. Dry-wet cycles arealso known to kill 15%–30% of the microbial biomass (Bales-dent et al., 2000) and may also reduce earthworm activities,which in turn slows down POM decomposition. Hence, higheramounts of occluded POM support a close contact betweenPOM and mineral surfaces. This is corroborated by Christen-sen (1996) who assumed that physical protection of POMand its role in aggregate formation and stabilization is crucialin regulating the proportion of new SOM that is further trans-formed and stabilized in clay- and silt-sized organomineralcomplexes.

4.3 13C in mineral fractions

During the observed period, higher quantities of 13C weredetected in POM than in mineral fractions (Tab. 6). Coarse-

silt- and sand-sized fractions had very low C contents andtherefore play a minor role in the turnover of the labeled mus-tard litter. Starting from 0.72 mg 13C (kg soil)–1 in POM free5% (LY) to 10% (HY) of the applied 13C remained in the POMfractions, and less than 10% of the applied 13C were found inthe fine-silt and clay-sized fractions. Consequently, 80% (HY)to 90% (LY) of the labeled material were mineralized or lea-ched within the first 568 d after application, and only smallamounts accumulated in the fine mineral fractions. It can beassumed that the turnover of 13C derived from mustard litteris mainly associated with POM turnover and hardly affectedby the fine mineral fractions during the 2-year period. This isin contrast to the allocation of litter-derived 15N (Kölbl et al.,2006), showing lower losses of labeled material in the ob-served period (85% in HY and 78% in LY soils) and higherrecoveries in the fine mineral fraction (12% in HY and 26% inLY soils).

161 days after litter application, 13C in fine mineral fractionsreached higher amounts in LY than in HY areas (Fig. 4), indi-cating a higher OC storage potential of fine mineral particlesof the finer textured LY soil. Between 161 and 568 d, 13Camounts in all analyzed mineral fractions remain constant ordecreased in LY soils, showing a similar trend as theoccluded-POM fraction (Fig. 2). This indicates that these 13Cpools are not stable and disappeared because of mineraliza-tion of partly decomposed OM. According to Aita et al.(1997), the quantity of 13C increases or remains constant infiner fractions as long as the easily decomposable litter frac-tion can provide 13C. In our study, the quantity of 13C in thefine mineral fractions decreased as the litter fraction, andespecially the occluded-POM fraction, became depleted.This underlines the low stability of young mineral-associatedOC. In contrast to the LY soil, the fine mineral fractions of theHY area as a whole showed very low amounts of 13C that in-creased only towards the end of the observation period(Fig. 4). This agrees with the slow increase of occluded POM(Fig. 2), leading to a delayed closer contact between POMand the mineral fraction.

To summarize, the higher fine-silt and clay content in LY soil,the calculated lower degree of OC saturation of the particles<20 lm (see section 4.1), and the slower release of OC fromthe POM fraction may have led to a larger storage of mus-tard-derived C in fine mineral fractions in the medium term(0.5–2 y). In the HY soil, a lower clay content, higher degreeof OC saturation in fine mineral fractions, and a less intensiveaggregate turnover may be responsible for a delayed closecontact between POM and mineral surfaces, resulting in adelayed appearance of mustard-derived C in fine mineralfractions.

4.4 Total 13C amounts

Comparing HY and LY areas (Tab. 6), smaller amounts oftotal 13C were recovered in HY soil, possibly due to higher mi-neralization directly after start of the experiment and maybedue to the higher leaching of DOC because of higher soilmoisture in this area. This leads to smaller 13C amounts inthe POM fractions in this area. It is also emphasized that theoccluded-POM fraction stored more 13C in the LY area



compared to the HY area during the first year. After the sec-ond year, reversed ratios were found, with higher amounts ofoccluded-POM-13C in HY than in LY areas due to the sloweraggregate turnover. The amounts of free-POM-13C were sim-ilar at both sites at the end of the observed period, indicatingthat this fraction is not influenced by site conditions in thelong term (>2 y).

Only small amounts of the total 13C label were found in thefine fractions, with the fine clay predominating over all othermineral fractions during the whole experiment (Tab. 6). In theLY area, more of the 13C label showed up immediately in theclay fractions and was stored during this first year. Higher 13Cenrichments in fine mineral fractions of the LY area corre-spond to slower OM turnover through the microbial biomass(Wessels Perelo and Munch, 2005) and to higher 13Camounts in the occluded POM. We assume that this resultsin a closer contact with mineral particles due to enhancedaggregate turnover. In the HY area, the slightly increasing13C amounts at the end of the investigated period led to ahigher recovery of added 13C compared to LY soils. This find-ing may derive from slower turnover of occluded POM, butalso from pools not investigated in this study, like microbial-biomass C or DOC.

An earlier study on the same sites showed also a fastercycling of 15N through the soil fractions in HY than in LY areas(Kölbl et al., 2006). This underlines the more intensive POM-decomposition processes in the HY soils. But in contrast to13C, higher total amounts of 15N were recovered in the ob-served period, particularly in the fine mineral fractions.Although 13C is recovered shortly after litter incorporation infine mineral fractions of the LY soil, the absolute amounts areconsiderably lower, and the stabilization in fine mineral frac-tion seems to be less effective compared to 15N releasedfrom the same mustard litter. It can be concluded that N isrecycled more efficiently in the plant-soil system and can bestabilized in fine mineral fractions over longer time periods.This is in accordance with Aita et al. (1997), who found moreresidual 15N than 13C 574 d after application of labeled straw,because C is lost to CO2 by mineralization, whilst N is usedand recycled by microorganisms during decomposition.

4.5 Conclusions

Analyses of short-term decomposition of 13C-labeled mustardlitter in high- and low-yield field areas of an agricultural crop-land lead to a better understanding of small-scale differencesin OM turnover. Our study shows initially faster POM-decom-position processes in HY than in LY areas, mainly due tohigher total organic-carbon and microbial-biomass contents,lower position on the slope accompanied by higher soil moist-ure, and lower silt and clay contents. These small-scaledifferences in the local soil characteristics result in initiallyfaster decomposition of labeled mustard litter in HY than inLY areas.

In LY soils, higher amounts of POM-13C occluded in aggre-gates were found within the first year after mustard-litterapplication, attributable to a faster aggregate formation andturnover, and most likely leading to a closer contact between

POM and mineral surfaces in this area. Higher amounts of13C were found in fine mineral fractions in LY soils within thefirst year. The higher silt and clay contents of LY areas seemto promote the stabilization of mustard-derived OC in finemineral fractions in the medium-term (0.5–2 y).

For HY areas, a faster decomposition of POM in combinationwith lower storage of the added OC in fine mineral fractionsleads to enhancedmineralization in the first year after mus-tard-litter application, maybe due to a higher microbial-bio-mass population. In the second year after start of the experi-ment, the delayed POM-13C occlusion due to the sloweraggregate turnover in HY soils is most likely the reason forincreasing recoveries of 13C at the end of the observedperiod. Higher 13C amounts recovered in HY soils 568 d afterthe start of the experiment indicate a better stabilization ofmustard-derived OC in the long term (>2 y). This may indi-cate a more sustained stabilization of fresh OM in fine mineralfractions of soils with lower aggregate turnover. If we assumea regular annual input of crop residues due to sustainableagricultural management, the slower turnover of residual OCin the HY soils may be the reason for the higher SOM con-tents measured in this area.

Acknowledgments

We thank Jens Leifeld and Miguel Jimenez for their activitiesat the beginning of the study, including experimental setupand soil sampling. We also acknowledge Solvejg Roller,Elfriede Schörk, and Valerie Poteau for technical assistance.Two anonymous reviewers provided helpful comments on themanuscript. The scientific activities for the Munich ResearchNetwork on Agroecosystems (FAM) were financially sup-ported by the Federal Ministry of Research and Technology(BMBF 0339370). Rent and operating expenses of theexperimental farm in Scheyern were paid by the BavarianState Ministry for Education and Culture, Science and Art.Costs for isotopical labeling of the mustard litter were pro-vided by the GSF-Research Centre, Neuherberg.

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