Deep soil microbial carbon use efficiency responds stronger to nitrogen deposition than top soil in tropical forests, southern China

Background and aims: Soil microbial carbon use efficiency (CUE), a key parameter determining soil C fate, has been associated with soil nitrogen (N) availability. However, the responses of forest soil CUE to aggregated N deposition are not clear. Moreover, it remains unclear whether responses may be mediated by soil depth. It is crucial to investigate the patterns and drivers of CUE with N deposition in forest top and deep soil. Methods: This study examined the responses of microbial CUE to short-term N addition treatments in the top (0–10 cm) and deep (60–80 cm) soils from two tropical forests in Hainan, southern China. Additionally, a meta-analysis was conducted to reveal a global pattern of forest soil microbial CUE response to N deposition. Results: The addition of N in the two tropical forests did not change topsoil CUE, whereas deep soil CUE increased by 25.5% (0.44 and 0.55 without and with N addition) on average. In the two soil depths, changes in CUE to N addition were negatively correlated with Fungi: Bacteria ratio (F: B). The negative relationship between resource limitation and CUE was found to be present with C limitation in top soil and N limitation in deep soil. N cycling enzyme activities played a vital role in regulating microbial resource limitation and CUE. The decrease in F: B and C: N ratios with the addition of N partly explained the stronger response of CUE in deep soil. At the global scale, the addition of N had no effect on the forest top soil CUE. However, the significant decline in pH and microbial biomass carbon with N deposition could reduce microbial CUE. Conclusion: The addition of N facilitated carbon storage in deep soil but had no effects on topsoil carbon storage in a tropical forest. Our results highlighted that the soil depth-driven variation in soil C: N and F: B can regulate microbial CUE, which could provide a basis for understanding the soil C cycle in tropical forest ecosystems.