Strong response of microbial carbon use efficiency to nitrogen deposition in deep soil of two tropical forests, southern China

Background and Aims : Soil microbial carbon use efficiency (CUE) directly impacts the carbon balance between the atmosphere and soil. Atmospheric N deposition may alter soil microbial CUE due to the effect of nitrogen availability on microbial metabolism. However, how N deposition affects the forest ecosystem soil microbial CUE is still not clear. 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: Forest topsoil microbial CUE did not significantly respond to N addition at the global scale and in Hainan. However, microbial CUE increased by 25% (on average) in the deep soil layers with N addition. The increased bacterial abundance and growth rate with N addition partially explained the higher soil microbial CUE of deep soil. We also found that when microbes were N-limited, high soil organic carbon content resulted in low microbial CUE, while the secretion of more N-cycling enzymes promoted microbial CUE. On a global scale, significant decreases in pH and microbial biomass carbon would result in topsoil microbial CUE reduction with N deposition. Conclusion: Deep soil could store more C with a higher CUE under N deposition in the N-limited tropical forest. Soil microbial nutrients limitation should take into consideration in predicting the feedback of soil C stock to global N deposition.