Long-term nitrogen addition increased soil microbial carbon use efficiency in subalpine forests on the eastern edge of the Qinghai–Tibet Plateau

Abstract Aims Nitrogen (N) deposition increased forest carbon (C) sink significantly, hence exploring the microscopic mechanisms is critical to predicting future global ecosystem C cycle, especially the effects of enhanced N deposition on soil microbial carbon use efficiency (CUE), which still unclear.Methods We evaluated the responses of soil microbial CUE to long-term (5 years) N addition in an evergreen broad-leaved forest and a mature coniferous forest by using a 13C isotope tracing method.Results The results showed that the soil microbial CUE ranged from 0.38 to 0.51, which was smaller than the results obtained from the previous studies based the same method and forest type. In evergreen broad-leaved forest, the microbial CUE had no significant changes in the low N-addition treatment, but it was increased by 9.23% and 12.69% in medium and high N-addition treatments compared to the control. In coniferous forest, soil microbial CUE was increased by 14.64%, 21.89% and 24.34% in low, medium and high N-addition treatments, respectively. Moreover, the soil C:P and N:P are negatively relate to soil microbial CUE.Conclusions Our findings indicate that the enhancing N deposition can increase soil microbial CUE and ultimately promote C sequestration, especially in coniferous forest. The imbalance of soil stoichiometry is the main impact factor of CUE under N addition. However, we speculate that the key to enhance forest soil C sequestration is to promote the decomposition rate of litter and thus increase the available carbon content.