Conversion of grassland into cropland affects microbial residue carbon retention in both surface and subsurface soils of a temperate agroecosystem

We evaluated how microbial residues and their contributions to soil carbon (C) stocks changed with long-term (50 years) conversion of native grassland to cropland in the profiles (0–15, 15–30, 30–60, 60–90 cm) of chernozem. The conversion of grassland into arable land led to substantial depletion of microbial residues down to 90-cm depth, indicating the potential vulnerability of the microbial-derived C due to land-use change. Moreover, losses of microbial residue C at depths below 30 cm were much higher than that of total soil C after several decades’ cultivation. This demonstrated that the decline of total soil C pool after grassland conversion was selectively removing microbial residue C. Lower ratio of fungal to bacterial residue C after grassland conversion suggested a shift in the composition of microbial residue C, with potential consequences for changes in soil C quality and stock associated with land cultivation. Collectively, our findings foster the importance of microbial-derived organic C for soil C stock maintenance and emphasize the necessity to take subsurface soils into account when evaluating the role of microbial-derived C to soil C losses under land-use change.