Effects of Nitrogen Deposition on Carbon Sequestration in China as Simulated by Process-Based Ecosystem Models

During the past decades, nitrogen deposition in China has been substantially increased, with mean values higher than those in the United States and Europe. The elevated nitrogen input, as important part of global changes, has greatly altered ecosystem structure and functioning, and thus exerted significant impacts on carbon cycling. In this study, three process-based ecosystem models (BIOME-BGC, DLEM and TEM) are used to explore the effects of increased nitrogen deposition and its combination with other environmental factors on C dynamic of China's terrestrial ecosystems. To illustrate the interactive effects of N deposition with ecosystem components, our simulations also consider multiple changes in climate, CO2, O3 concentration and land use & land cover (LULC) patterns. The model results indicate that raised nitrogen deposition level has enhanced net primary productivity and carbon storage in China during the year 1961 to 2005. Over the past 45 years, nitrogen amendment in China plays a vital role to accumulate carbon into ecosystems, considerably offsetting C release through climatic extremes, LULC changes and increased O3 concentration. It is proved that N deposition and its interaction with other driving forces accelerate carbon sequestration as the greatest contributor, although there apparently exist large uncertainties due to model assumptions and reliability of input dataset. Model inter-comparisons also reveal that N amendment imposes greater impacts on vegetation carbon pool than that on soil carbon pool. Among the models, the net effect of elevated nitrogen deposition on annual NPP ranges from 0.1 to 0.5 Pg C/yr in China from the year 1961 to 2005, while total carbon storage of terrestrial ecosystems has been enhanced by an average of about 5 Pg C in the same period. Key words: nitrogen deposition, carbon sequestration, China, process-based ecosystem models, model inter- comparison