Relationships between the C:N:P stoichiometric ratios of soil, extracellular enzymes, and microbial biomass along a primary successional sequence in a volcanic desert on Mount Fuji

Primary succession profoundly alters soil environments and microbial communities, but the mechanisms governing microbial stoichiometric regulation during ecosystem development remain unresolved. In this study, we investigated changes in soil resource stoichiometry, extracellular enzyme allocation, and microbial biomass C:N:P ratios as well as their relationships along a primary successional gradient in a volcanic desert on Mount Fuji, Japan. We quantified soil labile and total C, N, and P pools, extracellular enzyme activities associated with C, N, and P acquisition, and microbial biomass stoichiometry across six successional stages (bare ground to mature forest). As succession progressed, labile and total soil C:N and C:P ratios increased (labile C:N: 265 to 948; C:P: 2.4 to 66.5), indicating a shift toward relative N and P limitation. Correspondingly, ecoenzymatic C:N and C:P ratios decreased, reflecting increased microbial investment in N- and P-acquiring enzymes. However, microbial biomass C:N and C:P ratios increased during succession (from 5.1 to 9.8 and 2.5 to 27.2, respectively), demonstrating stoichiometric plasticity rather than homeostasis. Microbial C:N ratios were strongly correlated with soil labile C:N ratios, which was in contrast to the weak relationships with ecoenzymatic stoichiometry. These results indicate a decoupling between ecoenzymatic regulation and microbial biomass stoichiometry. Our findings suggest that in this C-limited volcanic system, extracellular enzyme allocation alone is insufficient for maintaining microbial stoichiometric homeostasis. Instead, microbial biomass stoichiometry during primary succession would reflect an integrated response to changing soil resource stoichiometry and community-level processes such as shift in community structure.