WET NITROGEN DEPOSITION AND CAUSAL FACTORS IN THE FOREST ECOSYSTEMS OF SOUTHEAST TIBET, CHINA

A continuous three-year observation (from May 2012 to September 2014) was conducted to characterize the different forms of nitrogen deposition for a forest ecosystem in the southeastern part of the Tibet Plateau in China. Total nitrogen (TN) throughfall deposition varied from 6.9 kg N?ha?1?yr?1 to 8.8 kg N?ha?1?yr?1 (average: 7.5 kg N?ha?1?yr?1). NH4+-N was the dominant form of total inorganic nitrogen (TIN) deposition, accounting for 53% of TN deposition in throughfall, while NO3?-N accounted for 22% and dissolved organic nitrogen accounted for 25%. Seasonal variability indicated that TN deposition peaked in summer. Monthly average temperature was positively correlated with NO3?-N and TIN deposition (p < 0.05), and cumulative precipitation was positively correlated with NO3?-N, TN, and TIN deposition (p < 0.05). Additionally, NH4+-N and monthly cumulative precipitation accounted for 90.2% of the variation of the TN deposition flux in the forest throughfall. Thus, elevated deposition of various forms of nitrogen likely threaten forest ecosystems via cumulative precipitation, influenced by average temperature to a lesser extent. Finally, TN deposition in this region is mostly controlled by agricultural activities rather than industrial activities or transportation.