Effects of nitrogen deposition and fertilization on N transformations in forest soils: a review

Understanding how process-specific nitrogen (N) transformations in natural forest soils are modified by N deposition and fertilization is critically important to gain mechanistic insights on the links between global N deposition and N enrichment and loss in forest soils. Here we identify the general characteristics and the main mechanisms of N deposition- and fertilization-induced modifications in multiple N transformations, including N immobilization, N mineralization, nitrification (autotrophic nitrification and heterotrophic nitrification), and denitrification, in forest soils by literature survey. Overall, N status, soil C/N ratios, C availability, and soil pH are key factors separately and/or interactively affecting the effects of N deposition and fertilization on forest soil N transformations. In the N-limited stage, N deposition and fertilization can act as a stimulator of N mineralization by removing microbial N limitation and reducing the C/N ratios of the substrate being decomposed. In the N-unlimited stage, N added to forest ecosystems can retard N mineralization, which may primarily be a result of decreased microbial activity due to soil acidification and low C availability. The changes in N mineralization may drive a corresponding change in N immobilization, autotrophic nitrification, and denitrification. Despite the fact that ammonia-oxidizing archaea (AOA) has a higher affinity than ammonia-oxidizers (AOB) for low-concentration ammonia (NH3), low NH3 availability may still limit the rate of ammonia oxidation (autotrophic nitrification) in acidic forest soils even in the case of high NH4 (+) input. Heterotrophic nitrification, however, may be favored if soil C/N ratios and pH decrease with N deposition and fertilization. The responses of denitrification and N2O emission to N deposition and fertilization in forests may be nonlinear, with a trend of stimulation in the short term but a decline over time, partly because soil pH has a contrast effect on denitrification capacity and N2O emission. There are various effects of N deposition and fertilization on forest soil N transformations; thus, their responses to N deposition are still not well characterized and understood. N deposition- and fertilization-induced modifications in soil N transformations have important implications for N enrichment, N loss, and soil acidification in forest ecosystems. In the future, more research is required to investigate on dissimilatory nitrate reduction to ammonium (DNRA) process and link microbial community characteristics and functions of microbial extracellular enzymes with these rate processes in forest soils to narrow the uncertainty in evaluating and predicting ecosystem responses to global N deposition.