Turnover of fungal glucosamine and bacterial muramic acid in comparison with soil organic carbon in two arable soils with distinct fungal communities

Microbial necromass carbon (MNC) can contribute 50 % or more to soil organic C (SOC) and may thus be crucial for C sequestration in soil. However, it is not known how persistent MNC is and whether the turnover of fungal and bacterial necromass C differs from that of SOC in this respect. The current study therefore investigates the turnover times of fungal glucosamine (GlcN) and bacterial muramic acid (MurN) in two soils from the long-term Darmstadt fertilization trial with distinct fungal communities. The soil with inorganic fertilization and straw return (MIN) contains significantly more saprotrophic fungi than the organically managed soil with cattle farmyard manure fertilization (FYM). The soil organic carbon (SOC) turnover time was 10.0 years in the FYM soil, 16 % longer than the 8.6 years in the MIN soil. In contrast, the microbial biomass C (MBC) turnover time of 147 days in the FYM soil was more than twice the 67 days found in the MIN soil. The turnover time of fungal GlcN and MurN varied around 6.3 years in the FYM soil and around 4.9 years in the MIN soil. In contrast to plant residues, fungal GlcN and MurN are constantly recycled in the microbial biomass during growth, which results in shorter turnover times compared to SOC. The different conversion factors from amino sugars to necromass currently used have only minor effects on the estimates of turnover times. The main drivers for the turnover of MBC and MNC in soil are microbial C use efficiency (CUE) and C input. There is particularly a serious lack in knowledge on the CUE values of partly decomposed organic fertilizers such as FYM. Future studies also need to more accurately estimate quantity and quality of the C input by straw, harvest residues, roots, rhizodeposits, and organic fertilizers.