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Loss of soil carbon and nitrogen rather than variations in soil particle size distribution decreased microbial residues during conversion of grassland to croplands

Conversion of grassland into cropland always induces a great degradation of soil organic carbon (SOC). However, during this process information on microbial residue C (MR‐C) and its contribution to SOC with increasing conversion age remain scarce. The relative contribution of soil properties to the accumulation of amino sugars is still poorly understood. Here, we studied variations in soil amino sugars and MR‐C, and their contribution to SOC with increasing conversion age (5 (M05), 16 (M16) and 34 (M34) years, respectively) from grassland to cropland. In contrast to the contents of glucosamine, fungal glucosamine, galactosamine, muramic acid, total amino sugars and MR‐C in grassland, those in M05 decreased in the range of 14.8–37.5%, and those in M34 decreased in the range of 31.8–65.1%, respectively. The contents of amino sugars and MR‐C did not differ between M16 and M34. The similar contribution of MR‐C to SOC (44.6–46.3%) in grassland, M16 and M34 is likely to result from a balance between microbial and non‐degraded plant residues. Decreasing ratios of fungal to bacterial residue C with conversion age indicated a decreased relative contribution of fungal residues to SOC, which probably resulted from the decreased percentage of macroaggregates. MR‐C and SOC decreased and reached a steady status in a few years (≧16 a) after conversion of grassland to cropland. The redundancy analysis model revealed that total nitrogen (TN), pH, SOC and available nitrogen (AN) played a bigger role than particle size distribution, in terms of explaining the contribution of amino sugars to soil carbon.