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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 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]

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. [ABSTRACT FROM AUTHOR]

Soil extracellular enzyme stoichiometry (EES) reflects the biogeochemical balance between microbial metabolic requirements and environmental nutrient availability. Recent research suggests that EES well effect on soil microbial metabolic limitations (SMMLs), however, few field studies have explicitly tested this based on a herbaceous successional chronosequence. We used the EES models to identify the response of SMMLs, and investigated the potential implications of microbial nutritional limitations across the time series (herbaceous succession) and space (transformation interface soil [TIS] and underlying topsoil [UTS] layer) in the grassland restoration series. We show that soil microorganisms were generally limited by C, both in the TIS and UTS. Microbial C-limitation exhibited a unimodal direction, peaking in intermediate successional stages, however, P-limitation presented the opposite trend. During herbaceous succession, microbial P-limitation was more substantial than that in N-limitation. SMMLs gradually transferred from P- to N- and back to P-limitation at later successional stages in the TIS layer. Furthermore, we demonstrate that biotic factors, soil basic index, and soil nutrients explained 92.2 % of the variations in microbial C-limitation and 84.4 % of the variations in microbial P-limitation. Multi–interaction factors exhibited the most significant relative influences of 65.11 % (TIS) and 43 % (UTS) on the SMMLs. Microbial C-limitation was induced by the imbalance between C supply and microbial C demand, whereas the changes in microbial P-limitation were due to the changes in the competition for P between plants and microorganisms. Overall, our findings provide support for microbial C- and P-limitation in the process of herbaceous succession during the restoration. We also highlight the possibility of additive effects on soil SMMLs via interactions of vegetation composition, soil properties, and microbial nutritional demands, which might constrain soil microbial metabolism requirements despite greater living root and litter resource inputs. [Display omitted] • Soil microorganisms were generally limited by C and P during the herbaceous succession. • In the TIS layer, SMMLs gradually transferred from P- to N- and back to P-limitation. • Multi–interaction factors exhibited the most significant relative influences of 65.11 % (TIS) and 43 % (UTS) on the SMMLs. • Microbial C-limitation was induced by the imbalance between plant residure C supply and microbial C demand. • The changes in microbial P-limitation were due to the changes in the competition for P between plants and microorganisms. [ABSTRACT FROM AUTHOR]

[Display omitted] • Strong microbial-substrate interactions exist in transformation interface soil layers. • Microbial metabolism was limited by C and P during litter decomposition. • Microbial non-homeostatic behavior corresponded to the transition stage. • The effect of microbial factors on enzyme stoichiometry was more stable. The relationships between soil eco-enzymatic stoichiometry and substrate stoichiometry could be used to explain nutrient cycling processes regulated by microbial biomass stoichiometry. But what are the relationships between the stoichiometry of resources, microorganisms, and enzymes? We do not have a clear understanding yet, particularly in the different stages of litter decomposition. By analyzing the response of extracellular enzymatic stoichiometry to the imbalance between microbial and substrate stoichiometry, we explore the microbial-mediated litter transformation and microbial adaptation mechanisms in the transformation interface soil (TIS) layer. The results showed that soil microbial metabolism was limited by C and P during decomposition, peaking in the transition (middle) stage and then gradually decreasing. According to the judgment of microbial homeostasis (effect of substrate stoichiometry on microbial biomass stoichiometry), the microbial communities shift from homeostatic in the early stage to non-homeostatic in the middle stage and then return to homeostasis in the later stage. During the litter decomposition, the non-homeostatic period corresponded to the transition stage. That means severe microbial metabolic limitation is a potential determinant of microbial homeostasis. Soil microorganisms adapt and alleviate microbial metabolic limitations by adjusting extracellular enzyme activity and microbial non-homeostatic behavior when the available resources fail to satisfy microbial requirements. [ABSTRACT FROM AUTHOR]