Parallel patterns of microbial carbon and nitrogen use efficiency in response to nitrogen addition across soil depths in a Castanopsis faberi forest

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Microbial carbon use efficiency (CUE) and nitrogen use efficiency (NUE) are key parameters that determine the fate of carbon (C) and nitrogen (N) in soils. However, the responses of microbial CUE and NUE to N deposition across soil depths remain poorly understood. In this study, we conducted an N-addition experiment in a Castanopsis faberi forest. Soil samples were collected from the topsoil (0–10 cm) and subsoil (20–30 cm) under three N addition levels to investigate depth-specific responses of microbial CUE and NUE. Changes in soil C fractions, microbial biomass, enzyme activities, and microbial community composition were assessed. Results showed a significant positive correlation between CUE and NUE, indicating a closed relationship between microbial C and N metabolism. Nitrogen addition reduced CUE and NUE at both depths, though the negative effects were less pronounced in the subsoil. Nitrogen addition significantly altered soil C fractions: labile C and particulate organic C increased in the topsoil, whereas mineral-associated organic C and recalcitrant C decreased in the subsoil. Microbial CUE and NUE were significantly correlated with the quantity of soil C fractions. Compared to the topsoil, the subsoil had lower CUE, NUE and dissolved organic C content but a higher C/N ratio, suggesting that differences in CUE and NUE between soil depths are regulated by both the quantity and quality of soil C. These findings reveal a depth-dependent yet directionally consistent effect of N addition on microbial resource-use efficiency, offering new insights into C–N coupling processes in forest ecosystems.