Soil depth mediates contrasting effects of nitrogen and phosphorus inputs on microbial carbon use efficiency in a subtropical forest

Abstract

Forest soils serve as vital terrestrial carbon sinks, in which microbial carbon use efficiency (CUE) plays a central role in regulating soil carbon storage. While anthropogenic deposition exerts profound influences on forest carbon cycling, the depth-dependent responses of microbial CUE to nitrogen (N) and phosphorus (P) deposition remain poorly understood. Here, we examined how soil microbial CUE responded to N and P inputs along a soil profile (0–80 cm) in a subtropical forest, using a long-term simulated N and P deposition experiment. Our results demonstrated a consistent increase in microbial CUE with depth across all treatments. Specifically, N addition significantly enhanced CUE by 27 % and 10 % at the depth of 40–60 cm and 60–80 cm, respectively, relative to the non-N group (which controlled for P variation). Furthermore, when compared directly to control, N addition increased CUE by 12.7 % and 13.5 % at these depths. In contrast, P addition suppressed CUE by 14 % and 16 % at the depth of 0–20 cm and 20–40 cm, respectively, compared to the non-P group (which controlled for N variation). Similarly, direct comparison with the control showed that P addition decreased CUE by 8.1 % and 10.9 % at these topsoil layers. Regression analyses revealed significant associations between microbial CUE and soil pH, dissolved organic C, dissolved organic N, and available P. Variance partitioning analysis further elucidated that microbial CUE was predominantly governed by the interplay of soil substrates (primarily carbon and nutrient availability) and microbial enzymatic activities in surface layers, and N input was a major environmental driver for CUE variation in deep soils. These findings uncover contrasting nutrient–depth interactions in regulating microbial CUE and underscore the pivotal role of subsoil C dynamics under escalating N deposition