Fungi contribute more than bacteria to soil organic matter through necromass accumulation under different agricultural practices during the early pedogenesis of a Mollisol

• Young soils developed from the parent material of a Mollisol under re-vegetation and cropping for eight years.
• Amino sugars and PLFAs measured to separate living microbial biomass and microbial necromass.
• Fungal necromass dominates in SOC over bacterial necromass despite of higher bacterial biomass than fungal biomass.
• Contribution of microbial necromass varied with land use and the amount of organic C inputs.

Living and dead microbial organisms contribute to the sequestration of soil organic carbon (SOC). The contributions of different community compositions are not well understood, particularly at the initial stage of soil development. Using an eight-year field experiment established on exposed parent material (PM) of a Mollisol, our objectives were (1) to differentiate microbial biomass and necromass of different microbial communities, and (2) to elucidate their contributions to SOC under different agricultural practices compared to PM and an arable Mollisol without C amendment (MO). The field treatments included two no-tilled soils supporting perennial plants (Alfalfa, and natural fallow), and four tilled soils under rotation between maize and soybean in alternate years, with or without chemical fertilization and crop residue amendment. Bacterial and fungal derived necromass were estimated by comparing amino sugars (ASs) contained in living and dead cell walls and phospholipid fatty acids (PLFAs) contained in living cell membranes, assuming that the conversion factor between cell membrane and wall for all microbes was one. The ratio of living microbial biomass estimated in ASs to that as indicated by total PLFAs was 0.76–0.87, indicating a high reliability of the estimation. Microbial biomass parameters in the field treatments were lower than those in MO and higher than those in PM. Both PLFAs and ASs demonstrated that bacteria dominated over fungi (70.2% v.s. 12.6%), but the fungal derived necromass were larger than bacterial derived necromass (70.7% v.s. 25.9%) in the studied soils. The microbial contribution to SOC was larger in necromass than in living biomass. The contribution of fungal derived necromass to SOC was dominant in soils and showed the same order among the soils as the fungal biomass, i.e. being larger under alfalfa than under natural fallow and in the tilled soils with organic C amendment than those without organic C input. These results suggested that only shift in fungal community due to land use change and organic C input could influence microbial contribution to soil organic carbon stabilization.