Imbalanced nitrogen and phosphorus deposition in the urban and forest environments in southeast Tibet

In recent decades, high levels of anthropogenic emissions in China have dramatically increased nitrogen (N) deposition and may lead to an imbalance of atmospheric N and phosphorus (P) inputs in terrestrial ecosystems. However, currently the status of N and P deposition in southeast Tibet is poorly understood. Here, we investigated spatial and monthly patterns of N and P bulk deposition based on measurements of dissolved inorganic N (DIN, including ammonium N and nitrate N) and dissolved organic N (DON) and total dissolved P (TDP) in precipitation from March to October 2017. Measurements were made at an urban site in Nyingchi city (NC) and at a forest site in Sejila Mountain (SJL). Over the study period, monthly total dissolved N (the sum of DIN and DON) deposition fluxes totalled 4.6 and 3.6 kg N ha−1 at SJL and NC, respectively, of which dissolved organic nitrogen accounted for 35 and 38%. Monthly averages showed an increasing trend from March to June, and then a decrease during last two months (September and October). At both two sites, the ratios of ammonium to nitrate N in bulk deposition are greater than 1, indicating that reduced N mainly from agricultural sources dominated N deposition in study area. Monthly TDP deposition fluxes totalled 0.68 and 0.58 kg P ha−1 at SJL and NC, respectively, both of which showed an increasing trend from March to May and a decreasing trend from July to October. The N/P ratio was 6.1 and 6.8 at NC and SJL, respectively.