Soil microbial phosphorus limitation constrains carbon use efficiency in subtropical forests

Microbial carbon use efficiency (CUE) is a vital parameter that determines soil's ability to sequester organic C, yet the response of CUE to land use intensification and the underlying mechanisms remain poorly understood, leading to large uncertainties in developing strategies to mitigate soil C losses. We investigated how legacies of land use (cropland and forest) intensity and climate affect microbial CUE along a subtropical climate gradient in southwest China. Our findings showed that microbial utilization of organic C for growth and C losses through respiration varied proportionally between land–use types, resulting in similar CUE values. However, microbial CUE was more sensitive to climate in managed ecosystems, being higher in cropland soils than in forest soils under warmer and wetter climate conditions. This indicates that intensive land use management increases the sensitivity of microbial CUE to climate change. In forest soils, CUE was constrained by low phosphorus (P) availability and was enhanced after P addition, whereas CUE in cropland soils showed no response, indicating that chronic P limitation is a key regulator of microbial metabolism in forests but not in adjacent croplands. Collectively, our work suggests that land use conversion does not necessarily alter microbial CUE, and highlights the importance of microbial P limitation in regulating and predicting forest soil organic C dynamics, particularly against the background of increasing P limitation induced by global changes such as nitrogen deposition and warming.