Soil mineral–associated organic carbon fraction maintains quantitatively but not biochemically after cropland abandonment

Abandonment is a strategy applied to increase soil organic C (SOC) in degraded cropland, but such efforts may fail because of microbial N limitation after abandonment in the absence of fertilization. In this study, we investigated the associations between SOC and microbial necromass C (MNC) dynamics in bulk soil and particle-size pools with N availability in a cropland abandonment chronosequence on the Loess Plateau. The total SOC, total MNC, and their particulate fractions (> 0.05 mm) in soil declined in the first eight years after cropland abandonment, but increased thereafter. By the 23rd year, the SOC content in abandoned soils increased towards the levels of cropland (16.5 g kg^–1) but were still far lower than those of natural vegetation (21.5 g kg^–1). The mineral–associated SOC (< 0.05 mm) content maintained after abandonment; but by contrast, the mineral-associated MNC profoundly decreased. This indicated that the reduction in MNC in this fraction was compensated for by plant-derived substances from the particulate fraction. Enzymatic stoichiometry analysis identified microbial N limitations in abandoned soils compared with cropland soils. As such, microbial N limitation led to increases in mineralization and/or decreases in synthesis of MNC in both particulate and mineral-associated fractions after abandonment, attributable to the decreased total SOC. Across the abandonment chronosequence, up to 20 % of particulate SOC was derived from microbes, whereas more than half of mineral-associated SOC came from plants. These findings challenge the general consensus that particulate SOC is dominated by plant residues whereas the mineral-associated fraction contains mainly microbially derived substances. The MNC contained a smaller proportion of fungal substances in mineral-associated fractions compared to particulate fractions, reflecting microbial ecological niche differentiation in the SOC formation between particle-size fractions. In conclusion, cropland abandonment decreased MNC accumulation because of microbial N limitation, and the mineral-associated SOC was stable in quantity but not in its source composition.