Abstract: Soil nitrogen (N) transformation is a key component of N cycling in forest ecosystems. Soil organic N and subsequent mineral N (NH₄⁺ and NO₃^-) transformations in forest ecosystems drive forest productivity and control soil mineral N concentrations and their relative abundance (the ratio of NH₄⁺ to NO₃^-). Increasing atmospheric N deposition is a key issue in the research area of global change. Nitrogen and phosphorus (P) are generally considered as the most important mineral nutrients for plant growth. The rapidly increasing N deposition by anthropogenic activities may increase soil N availability, and cause P deficiency in tropical regions, leading to imbalances in soil mineral nutrition. Changes in N and P inputs caused by intensified human activities have become one of the important factors affecting soil N cycling of forest ecosystems. Therefore, knowledge on the responses of soil N transformation processes to N and P inputs is crucial for understanding soil N dynamics under increasing N deposition. Numerous studies have been conducted over the past decades to examine the effects of N and P inputs on soil N transformation in forests, but without a unified conclusion and overall understanding of this issue, which is particularly prominent in tropical forests with N enrichment and P limitation. Here, we synthesized the studies on soil N transformation and loss following N and P addition in tropical forests, in order to outline the general understanding of this issue and identify the underlying mechanisms. We found that long-term N addition tends to accelerate soil N transformation in tropical forests, increases available N concentration and enhances N loss (through gaseous emissions and leaching), and weakens microbial N retention by decreasing microbial biomass. By contrast, P addition tends to simulate nutrient cycling between plants and soil, reduces the rate of soil N loss, and thus facilitates N retention in tropical forest. We found that N and P addition may not have an interactive effect on soil N transformations in tropical forests. We identified the major possible mechanisms driving changes in soil N transformation by N and/or P addition. Nitrogen and P addition can affect soil N cycling by directly altering the quality of soil organic matter and litterfall, soil conditions, and microbial communities (e.g., biomass, community composition, enzyme activity). In addition, we pointed out the limitations of the previous studies and proposed further research direction on soil N transformation in tropical forests.

Key words: nutrient addition, forest soil, tropical forest, nitrogen transformation.