Microbial Decomposition of Cellulose in Soil: Insights Into the Roles of Resource Stoichiometry and Water Content

Decomposition kinetics of carbon (C) substrates in soil vary linearly with changing soil conditions until specific thresholds are reached, where metabolic pathways change completely. These thresholds challenge process-based modeling, e.g., by determining whether nitrogen (N) addition promotes or suppresses microbial respiration. Here, we aimed to identify such thresholds in cellulose decomposition imposed by C, N, and oxygen limitation by manipulating resource stoichiometry and water content in controlled experiments. Agricultural soils were incubated for 35 days under different cellulose amendments, at different water contents with or without nutrient addition. Resource stoichiometry coinciding with microbial biomass C/N ratios imposed a clear threshold behaviour on growth dynamics. Under C limitation (resource C/N < 8), cumulative C release scaled with input; whereas under N limitation (C/N >> 8), this relationship broke down. When N-limited, N shortened the exponential growth phase by determining the onset of growth retardation, while N excess (C/N < 8) delayed microbial growth across all stages. In both cases, the onset of growth retardation scaled linearly with resource C/N ratio, but at different rates for C and N limitation. Further, a distinct threshold behaviour was observed for water contents beyond field capacity. In soil with low microbial activity, wetter conditions accelerated growth by reducing resource limitation without changing cumulative C release. The same wetting reduced cumulative C release in soil with higher microbial activity, potentially due to oxygen limitation. These findings underscore the importance of identifying soil condition thresholds, beyond which microbial respiration shifts unpredictably, invalidating linear assumptions in process-based models.