Net Anthropogenic Nitrogen Inputs (NANI) Into The Yangtze River Basin And Analyses On Eco-environmental Impacts

In the past few decades, global nitrogen overload in terrestrial and aquatic ecosystems is an important environmental issue, due mainly to the excessive input of anthropogenic nitrogen. The anthropogenic perturbation of nitrogen cycle continues to cause numerous environmental issues, ranging from severe eutrophication, frequent occurrences of harmful algal blooms to hypoxia. Many academics believe that amounts of riverine nitrogen export is mainly from unreasonable input strongly influenced by human activities in watersheds. Therefore, a quantitative understanding and characterize the sources of anthropogenic nitrogen and ecological and environmental effects of NANI in watersheds has been paid more global attentions recently.A watershed-scale nutrient accounting method-Net anthropogenic Nitrogen Input (NANI), which was originally proposed by Howarth et al. (1996). NANI accounts for the fluxes of atmospheric nitrogen deposition, fertilizer nitrogen application, net food and feed imports and agricultural nitrogen fixation, each of which represents a potentially important source of nitrogen in watersheds. The NANI approach has been widely applied to identify the major sources of anthropogenic nitrogen in many regions and is recognized as an effective tool to estimate human-controlled nitrogen inputs to a watershed. However, the application of NANI method in our country is still in its infancy.Since the policy of reform and opening up, the economy and society has been developed rapidly in the Yangtze River basin, the terrestrial ecological system is strongly influenced by the accelerated urbanization and human activities. Thus, the Yangtze River basin was selected as a typical study area to study the net anthropogenic nitrogen input into the Yangtze River basin and the relationship with riverine nitrogen export during 1980-2012. This work provides a quantitative understanding of NANI in the Yangtze River basin and its effects on riverine DIN export, and helps to develop integrated watershed nitrogen management strategies. The main findings are as following:(1) Significant spatial and temporal variations in NANI was observed (one-way ANOVA,p< 0.01). The total NANI in the entire basin has increased by more than twofold over the past three decades, from 3537.0±615.3 kg N km-2yr-1 in 1980 to 8176.6±1442.1 kg N km-2 yr-1 in 2012. The results showed that NANI increased firstly and then remained stable. The geographic distribution of NANI across the Yangtze River basin was characterized by a gradual increase from the western to eastern subcatchments. NANI in most subcatchments of the Yangtze River basin is relatively high compared with other basins in the world. The greatest NANI was recorded in the TH subcatchment (18574.6 ± 3753.7 kg N km-2 yr-1) and the lowest NANI appeared in the JSJ subcatchment (3054.1 ± 537.1 kg N km-2 yr-1).(2) The application of nitrogenous fertilizer was always the predominant source to NANI, which contributed about 51.1%(40.8%～56.1%) of NANI. The largest percentage appeared in 1995, and then decreased gradually. The proportion of net nitrogen in food and feed in NANI was 26.0%, followed by atmospheric nitrogen deposition (13.2%), agricultural nitrogen fixation (9.7%). Atmospheric deposition has made an increasing contribution to NANI and become the second largest component in the nitrogen budgets of recent years.(3) A regression analysis showed that the riverine DIN export was strongly correlated with NANI and the annual water discharge (R2=0.90, p< 0.01). Annual NANI, retained nitrogen pools, and natural background sources contribute to about 56%,38%, and 6% of riverine DIN export, respectively, suggesting that about 94% of DIN flux is derived from anthropogenic nitrogen sources.(4) NANI in the Yangtze River basin contributed significantly to the occurrence of red tides. The study highlights the importance of controlling NANI including atmospheric nitrogen deposition, net nitrogen in food and feed, and in the Yangtze River basin for protecting the eco-environmental health of the estuarine and coastal ecosystem. The study . forecasted possible future trends of NANI and riverine DIN export based on three scenarios: "status-quo" scenario projects a 75% increase of NANI and a 122% increase of riverine DIN export in the Yangtze River basin; "Tackling" scenario showed that NANI and DIN export would be approximately 21% and 20% less than the baseline, respectively; "climate change" scenario projects an increase of 7.3% for NANI and 4.8% for riverine DIN export in the Yangtze River basin by 2030.