Limited evidence for soil enzyme activities mediating C:N discrepancies between microbial biomass and organic matter in year 145 of a maize‐based agroecosystem experiment

Soil microbial biomass (MB) generally exhibits a narrower C:N than soil organic matter (SOM), creating a stoichiometric discrepancy hypothesized to be reconciled in part by C‐ and N‐acquiring enzyme activities. We used soils from an at‐equilibrium long‐term experiment to evaluate the degree to which MB C:N plasticity and enzyme activities explained MB–SOM C:N discrepancies among total SOM, particulate organic matter (POM), and mineral‐associated organic matter (MAOM) at 0‐ to 17‐ and 17‐ to 30‐cm depths. MB C:N increased with depth from 8.6 to 11.7, whereas total SOM and MAOM C:N were similar (∼13), and POM maintained higher C:N (∼19–20) between depths. MB C:N was unrelated to total SOM C:N, but increased with MAOM C:N at surface depth and declined with POM C:N at subsurface depth. Residual analyses identified limited relationships between enzyme activities and total SOM or POM C:N discrepancies. MAOM C:N residuals showed more frequent and negative relationships, which did not support enzyme‐mediated reconciliation of stoichiometric discrepancies. In contrast, fertilization‐induced changes in C‐ and N‐acquiring enzyme activities were positively associated, but were more strongly expressed through total N‐acquiring enzyme activity under manure and certain aminopeptidase activities under synthetic fertilization. Overall, our results demonstrate that interpreting MB C:N in maize‐based agroecosystems should consider SOM fractions and exert caution in the selection of soil enzyme activities used to infer limitation, because widely used soil enzyme activity ratios alone (i) provide limited inferences to microbial nutrient limitation and (ii) do not appear to reflect activities of related soil enzymes.