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Summary ± ZusammenfassungThe composition of functional light soil organic matter pools of arable Cambisols with a gradient in clay content was investigated. Soil texture differences originate from increasing loess admixture to the parent material (coarse-grained tertiary sediments). Using density fractionation in combination with ultrasonic dispersion, two types of particulate organic matter (POM) were obtained: (1) free POM and (2) POM occluded in soil aggregates. Both POM fractions were analyzed by elemental analysis (C, N) and CPMAS 13 C NMR spectroscopy. With increasing clay content the amount of organic carbon stored in the occluded POM fraction increased considerably, whereas the amounts of free POM were not related to the soil clay content. With increasing soil clay contents increasing proportions of O-alkyl C and decreasing proportions of aryl C were found for both POM fractions. The occluded POM fraction showed a higher degree of degradation as indicated by lower amounts in O-alkyl carbon. A lower degree of POM degradation was associated with higher clay contents. Higher soil clay contents promoted the conservation of POM with a low degree of alteration. This effect of soil texture was found to be highly significant when the aryl C : O-alkyl C ratio was used as indicator for POM decomposition rather than the alkyl C : O-alkyl C ratio.Menge und Zusammensetzung freier und okkludierter partikulärer organischer Substanz in ackerbaulich genutzten Braunerden mit unterschiedlicher Bodentextur, aufgezeigt mittels Festkörper-13C-NMRSpektroskopie Key words: soil texture / density fractionation / particulate organic matter / organic carbon and nitrogen / CP MAS 13 C NMR spectroscopy PNSS P118/5B

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Grazing is one of the most important factors that may reduce soil organic carbon (SOC) stocks and subsequently aggregate stability in grassland soils. The central aim of this study was to analyse the quality and quantity of SOC fractions and their contribution to aggregate formation, stability and carbon sequestration, as affected by increased inputs of organic matter after grazing exclusion. We applied a combined aggregate size, density and particle size fractionation procedure and aggregate stability measurements to sandy steppe topsoils with different organic matter inputs from different grazing intensities (continuously grazed ¼ Cg, winter grazing ¼ Wg, ungrazed since 1999 ¼ Ug99, ungrazed since 1979 ¼ Ug79). Greater inputs of organic matter led to larger amounts of OC in coarse aggregate size classes (ASC) and especially in light fractions (LF). We found no grazing-induced changes of soil organic matter (SOM) quantity in fine ASC and mineral fractions. SOM quality ( 13 C solid-state NMR spectroscopy, neutral sugar analysis) was comparable between different grazing intensities, but ungrazed plots had slightly more decomposed SOM across all fractions. We found generally greater radiocarbon concentrations in Ug79 compared with Cg. Aggregate stability, analysed as resistance to sonication, was greater in Ug79 compared with Cg. Larger litter inputs in grazing exclosures increased LF quantity, led to faster SOM turnover and resulted in the formation and stabilization of coarse aggregates. Organo-mineral associations turned over faster as indicated by increased radiocarbon concentrations, but the OC content of this pool did not change. To summarize, additional litter inputs were sequestered in the intermediate LF pool and the long-term pool of organo-mineral associations appears to be close to saturation. We conclude that management changes in steppe ecosystems do not necessarily increase carbon sequestration and their assumed potential to act as carbon sinks has to be questioned.