Preliminary results concerning the spatio-temporal pattern and mechanism of nitrogen sources and exports in the Jiulong River watershed
九龙江流域氮的源汇时空模式与机理初探

Excessive reactive nitrogen(N)additions from human activities have resulted in serious and long-term environmental consequences for large regions of the Earth.An understanding of Nsources and their export pattern is important for selecting the proper remedial strategies to control nutrient losses from the watershed.In this study,the Nsources and exports were determined for the Jiulong River Watershed(JRW),a coastal watershed with an area of 1.47×104km2 in the southeast of China;and the Wuchuan subwatershed(1.88 km2),a small agricultural watershed in the up-stream Jiulong River.Water quality monitoring,field measurements,mechanism models and GIStechniques were linked to estimate the Nflux of atmospheric deposition,streamflow discharge,leaching,denitrification,and ammonia volatilization.Two scale watershed Nbudgets based on 2004 data were used to identify Nsources and the primary path of Nlosses.The Nbudget for JRWshowed that fertilizer and animal feedstuffs contributed 86% of the total Ninput(125.6 kg·hm-2);while streamflow discharge and ammonia volatilization contributed 82% of the total Noutput(72.9 kg·hm-2).More than half of the anthropogenic Ninput was lost to the air and water.About 14.5% of the Ninput was discharged to rivers and subsequently delivered to the estuary.The annual deposition flux of atmospheric Namounted to 14.9 kg·hm-2,of which dry deposition and wet deposition accounted for 34% and 66%,respectively.About 80% of atmospheric deposition occurred in spring and summer,indicating that higher ammonia volatilization from fertilizer application in the growing season,and animal feeding together provided the largest Nsource.The Jiulong River provided dissolved inorganic Ntransport of 11.5 kg·hm-2 to the estuary,of which more than 90% occurred from March through November(wet season)resulting from 89% of the total flow.Compared with the baseflow,stormflow contributed about 75% of the annual Nexport(67.1 kg·hm-2)in the Wuchuan subwatershed.There,streamflow export was greater in spring(43%)and summer(42%)when more fertilizer was used for growing crops.Annual Nleaching(averaged 27.5 kg·hm-2),accounting for 9% of the total Ninput,was dominated by ammonium(40%)rather than the nitrate form(20%),as a result of the acidic soil being positively charged at pH<5.The annual flux of denitrification in JRWwas 7.7 kg·hm-2.However,commercial fertilizer use and animal feeding contributed 50% and 39%,respectively,to the high volatilization rate of 42.1 kg·hm-2.Reducing the Ninput by a more efficient use of fertilizers during the growing season(nutrient management),and trapping storm runoff during the wet season(hydrological control)offer the keys to eliminating Npollution in this region.