Extracellular enzymes are the major mediators of plant residue and organic matter decomposition in soil, frequently associated with microbial metabolic processes and the biochemical cycling of nutrients in soil ecosystems. However, the dynamic trends and driving factors of extracellular enzymes and their stoichiometry during plant residue transformation remain to be further studied. Here, we investigated the dynamics of extracellular enzymes and enzymatic stoichiometry in the “litter-soil” transformation interface soil (TIS) layer, an essential occurrence layer for microbially-mediated C transformation. The results indicated an unbalanced relationship between substrate resource supply and microbial metabolic demand. Microbial metabolism was limited by C (C/N-acquiring enzymes > 1) and P (N/P-acquiring enzymes < 1) throughout the observed stages of plant residue transformation. The initially higher extracellular enzyme activity reflected the availability of the active components (dissolved carbon (DC), nitrogen (DN), microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP)) in the substrate and the higher intensity of microbial metabolism. With the transformation of plant residues, the active fraction ceased to be the predominant microbial C source, forcing the secretion of C-acquiring enzymes and N-acquiring enzymes to obtain C sources and N nutrients from refractory substrates. Moreover, C/N-acquiring enzymes decreased, while C/P-acquiring enzymes and N/P-acquiring enzymes subsequently increased, which suggested that the microbial demand for N gradually increased and for P relatively decreased. Soil microorganisms can be forced into dormancy or intracellular mineralization due to the lack of substrate resources, so microbial biomass and extracellular enzyme activities decreased significantly compared to initial values. In summary, the results indicated that soil nutrients indirectly contribute to extracellular enzymes and their stoichiometry by affecting microbial activities. Furthermore, extracellular enzymes and their stoichiometry were more sensitive to the response of soil microbial biomass carbon.

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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 vacations during plant residue decomposition. Methods We investigated in a 512-days culture experiment with a perennial C3 herb (St.B, S. bungeana) to trace the formations of muramic acid (MurA) vs. Glucosamine (GluN), and investigate 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 MurA changed from fluctuating variations to a significant increase. Bacterial necromass has been in a balance of accumulation and decomposition at early and middle periods. The entombing effect in the later stage resulted in bacterial necromass accumulation. While in the case of microbial metabolism limitation, GluN will lose its physicochemical protection and be degraded. 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 microbial necromass only plays an indirect role. The priming effecting caused by one-time input of high C/N ratio plant residues leads to the absence of SOC accumulation in the short term. 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.