Biochar‐Induced Shifts in Resource Stoichiometry Shape Potential Nitrogen Limitation and Estimated Microbial Carbon Use Efficiency in Tropical Soils

ABSTRACT Biochar is widely recognized as a promising amendment for enhancing soil carbon sequestration and microbial functioning, yet how biochar properties and application levels jointly influence potential microbial nutrient limitation and estimated microbial carbon use efficiency (CUE) remains unclear, especially in tropical farmland soils. Here, a 2‐year field experiment was conducted to examine the effects of two types (RB, rice hull biochar and PB, peanut shell biochar) and four application levels (10, 20, 40, and 60 t/ha), with 0 t/ha serving as the control, on labile substrates, microbial biomass, extracellular enzyme activities (EEAs), stoichiometric imbalance, vector‐based indicators of potential nutrient limitation, and estimated microbial CUE. Increasing biochar application levels consistently enhanced substrate availability, with dissolved organic nitrogen (DON) showing a stronger response than dissolved organic carbon (DOC), particularly under PB, thereby decreasing the DOC:DON ratio. Biochar application also increased microbial biomass C, N, and P, and synchronously stimulated C‐, N‐, and P‐acquiring enzyme activities by 15.61%–159.88%, 9.73%–201.21%, and 9.98%–162.96%, respectively; thus, ecoenzymatic stoichiometry remained largely unchanged. Vector angles remained below 45° across all treatments, indicating persistent potential relative N rather than P limitation, but their responses to biochar showed no clear monotonic pattern. Biochar type had a stronger influence than application level on stoichiometric imbalance and estimated microbial CUE. PB increased DON availability and was associated with a lower DOC:DON ratio, reduced C:N imbalance, and higher estimated microbial CUE. Exploratory path analysis associated biochar type mainly with C:N imbalance and estimated microbial CUE, while application level was associated with labile substrates, EEA, and microbial biomass. Overall, our results provide new insights into how biochar quality and application level jointly shape microbial resource stoichiometry, microbial functioning, and soil C cycling in tropical farmlands.