Impacts of nitrogen deposition on soil nitrogen cycle in forest ecosystems: A review

Abstract

Atmospheric nitrogen (N) deposition has accelerated in the last several decades due to anthropogenic activities, such as nitrogen fertilization, N-fixing plants cultivation and fossil fuel and biomass combustion. Increasing N deposition has become one of the important factors regulating N cycle in forest ecosystems. Forest ecosystems can retain part of deposited N in soil by biotic and abiotic mechanisms, but when N inputs exceed the capacity of soil retention, N losses will aggravate in terms of N oxide emission and/or nitrate leaching. The excess N input has threatened ecosystem health via acidification and eutrophication, causing declines in terrestrial biodiversity and forest productivity in forest ecosystems of Europe and North America. Recently, China has become one of the three areas that undergo severe N deposition in the world. Impacts of N deposition on soil N cycle in Chinese forest ecosystems have received increasing concern. In this paper, we reviewed the processes of soil N cycle and their responses to atmospheric N deposition based on available literature. The objective is to enhance our understanding on how N deposition affects soil N cycle in forest ecosystems and provide scientific information for sustainable forest management. The review mainly includes the following four aspects: (1) processes of soil N cycle and their controlling factors. These processes include biological N fixation (BNF), decomposition, mineralization, nitrification, denitrification, N oxide emission and NO3−N leaching. The controlling factors of these processes are complicated and interactional. Only one of these factors altered may affect soil N cycle. For example, C/N is the factor that controls BNF, decomposition, mineralization and NO3−N leaching. (2) Research methods and current results about studies are related to the impact of N deposition on soil N cycle in forest ecosystems. In general, the research methods are long-term simulated N deposition experiment, N deposition gradient method, roof clean rain method and 15N tracing method. Effects of N deposition on soil N cycle vary depending on different initial N statuses and lengths of experiment. In “N-limited” forests, N deposition tended to have positive effect on soil N cycling processes, such as accelerating litter decomposition rate and N mineralization rate. However, such result generally showed in short-term fertilization experiments. In some long-term fertilization experiments, it showed that the negative effects would rise when the forests reached N saturation. Compared to “N-limited” forests in temperate region, N deposition tended to have negative or neutral effects in “N-rich” tropical forest. For example, N deposition promoted nitrification process in tropical forests. (3) Possible mechanisms for the effect of N deposition on soil N cycle: N deposition can affect soil N cycle through altering the chemical characteristic of forest substrates, the biomass and community composition of plant and microorganism. (4) Current problems and future research needs for the study about the effect of N deposition on soil N cycle: What role does regional diversity, changes in forest type, and interaction of carbon (C), N and phosphorus (P) play on the effect of N deposition on soil N cycle in forest ecosystems deserve our further study in the future.