Different contributing processes in bacterial vs. fungal necromass affect soil carbon fractions during plant residue transformation.

Aims: Recent research has suggested that microbial necromass has a disproportionate influence on soil organic C accumulation. But few field studies have followed the bacterial and fungal necromass changes during plant residue decomposition. How bacterial and fungal necromass drives soil C fractions; what role of soil C fractions play; and the critical factors which influence their formation have remained poorly understood. Methods: In a 512-days culture experiment with a perennial C3 herb (St.B, S. bungeana) we traced the formation of muramic acid (MurA) vs. Glucosamine (GluN), and investigated the relationships between MurA, GluN and soil C fractions. Results: The results showed that the bacteria community dominates the decomposition process due to soil pH (> 7) and microbial metabolic C-, P-limitations. The dynamics of bacterial necromass over the time-course of the experiment changed from fluctuating variations to a significant increase. This means that the bacterial necromass has been in a balance of accumulation and decomposition at early and middle periods. Significant accumulation only occurred in the later stages, which was attributed to the involvement of bacteria in more microbial necromass degradation. In this process, the entombing effect of bacterial necromass was more dramatic than their metabolic consumption. While in the case of microbial metabolism limitation, fungal necromass will lose its physicochemical protection of the mineral particles, and thus be degraded and utilized. The priming effecting caused by the one-time input of plant residues with high C/N ratios and soil mineralization resulted in the absence of SOC accumulation in the short term. Microorganisms regulate the turnover of POC, MAOC and MBC by microbial biomass and necromass. The utilization of soil C fractions is the direct cause of SOC decline, while the turnover of microbial necromass only plays an indirect role. Soil pH and microbial biomass stoichiometry as the critical factors in changes in bacterial and fungal necromass. Lower living microbial biomass production and the dynamics of microbial necromass indicate that microbial necromass is constantly broken down as an available C source. Conclusions: With the synergistic effect of soil C fractions, the production of microbial biomass and the degradation of cellular residues maintain microbial stoichiometric homeostasis. In addition to soil pH, microbial biomass stoichiometry co-determines microbial necromass formation. [ABSTRACT FROM AUTHOR]