Abstract: Aims Microbial nitrogen use efficiency (NUE) refers to the ratio of nitrogen used by microorganisms for growth to total organic nitrogen absorbed, which is a key regulator of the soil nitrogen cycle. The 18O-H2O method and eco-enzyme stoichiometric model are the most common techniques employed to estimate microbial NUE. However, the impact of nitrogen enrichment on microbial NUE in forest soils remains to be elucidated, which would siginificantly impede the accurate prediction of soil nitrogen supply. Methods In this study, we sampled the soils from subtropical Castanopsis faberi forests and used urea to simulate atmospheric nitrogen deposition. In total, three levels of nitrogen additions were set up, including control (CK, 0 kg N·hm–2·a–1), low nitrogen (40 kg N·hm–2·a–1), and high nitrogen (HN, 80 kg N·hm–2·a–1) treatment. The 18O-H2O method and eco-enzyme stoichiometric model were used to compare the differences in soil microbial NUE under different nitrogen enrichment levels. Furthermore, soil physicochemical properties, microbial biomass, extracellular enzyme activity, and stoichiometric imbalance were also measured to explore the factors that influence the response of microbial NUE to nitrogen enrichment. Important findings The results showed that the microbial NUE, as determined by two different methods, exhibited a decline with the increase of nitrogen enrichment. The vector angle values of different nitrogen treatments were all greater than 55°, indicating that soil microorganisms in this study area were generally phosphorus limited, but nitrogen enrichment did not exacerbate the phosphorus limitation. On the contrary, nitrogen enrichment has a significant negative effect on microbial carbon and nitrogen imbalance, and a positive correlation was observed between microbial NUE and microbial carbon and nitrogen imbalance. In summary, the nitrogen enrichment may have a detrimental effect on soil nitrogen storage in subtropical forests, highlighting the necessity to enhance soil nitrogen storage by means of regulating the soil stoichiometric balance to in these forests.

Key words: microbial nitrogen use efficiency, nitrogen addition, ecological stoichiometry, isotope labeling, forest