Alteration of soil organic matter pools and aggregation in semi-arid steppe topsoils as driven by organic matter input

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 = Cg, winter grazing = Wg, ungrazed since 1999 = Ug99, ungrazed since 1979 = 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 (¹³C 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.