Nitrogen Sources And Exports In Jiulong River Watershed

Excessive reactive nitrogen (N) additions from human activities have resulted in serious and long-term environmental consequences for large regions of the Earth. National and international policies attempt to reduce these impacts on ecosystem health. Understanding of N sources and exports pattern is important for selecting the proper remedial strategies for controlling nutrient losses from watershed.In this study, the N sources and exports were determined for Jiulong River Watershed, an agricultural hill-land watershed with an area of 1.47×104 km2 in coastal region of southeast China. Water quality monitoring, field measurement, mechanism model and GIS technique were linked to estimate N flux of atmospheric deposition, streamflow discharge, leaching, denitrification, and ammonia volatilization. Nitrate isotope tracing, N budgets, and probabilistic risk assessment (PRA) were also used to identify N sources and primary path of N losses. The work provides a sound understanding of the sources of N to landscapes and the associated N fluxes in exports, and highlights how anthropogenic activities impact N cycling in coastal watershed.First, spatio-temporal distributions of atmospheric N deposition were examined for Jiulong River Watershed. The annual deposition atmospheric N flux amounted to 14.9 kg N/hm2, of which dry deposition and wet deposition accounts for 34% and 66%, respectively. About 80% of atmospheric deposition occurred in spring and summer. Ammonium, nitrate and dissolved organic N formed 31%, 24% and 45% of dry deposition flux, 39%, 25% and 36% of wet deposition flux for the whole watershed, respectively. In deposition the spatial pattern of N species was ostensibly linked to local pollution sources. The spatio-temporal variation of atmospheric N deposition indicated that higher ammonia volatilization from fertilizer application in the growing season, and livestock productions together provided the larger N source. Isotope value of nitrateδ15N implied that nitrate in rain droplets mainly originated from agricultural activities, and fuel combustion. Second, annual and seasonal patterns of N loss in streamflow were evaluated based on monitoring data of water quality and flow in 2002–2005. N loss in stormflow was positively related to the ratio of arable land, and varied greatly among the representative subwatersheds, reflecting the differences in precipitation, land cover, and N inputs. Stormflow contributes about 75% of the annual N export (67.1 kg N/(hm2·y)) in Wuchuan subwatershed, a small agricultural watershed in up-stream of Jiulong River. Streamflow export was greatest in spring (43%) and summer (42%) when larger fertilizer used for growing crops, and recorded rainfall contributes about 75% of total rainfall in 2004. The riverine export of dissolved total nitrogen (DTN), dissolved inorganic nitrogen (DIN) formed 83% and 68% of the total flux, respectively. Nitrate in streamflow contributed approximately two-thirds of DIN export, and its potential sources are inorganic N fertilizer and seasonal application of manure. Jiulong River provided DIN transport of 14.6×103 t N/y to estuary, of which more than 60% occurred during March to August (wet season) inducing by higher flow.Third, N leaching in Wuchuan subwatershed was evaluated using shallow water...