九龙江流域氮的源汇时空模式与机理初探

综合运用定位监测、野外试验、模型模拟与GIS技术等手段和方法,定量研究了南亚热带地区九龙江流域和五川小流域氮的大气沉降、河流输送(地表径流)、淋失、反硝化和氨挥发等输入输出(源汇)时空模式与机理.结果表明,九龙江流域氮"源"以化肥与饲料输入为主(占总输入125.6kg·hm-2的86%).氮"汇"以氨挥发和河流输送为主(占总输出72.9kg·hm-2的82%).氮输入后50%以上进入大气和水环境,14.5%通过河流输送至河口与近海.大气氮沉降通量为14.9kg·hm-2,其中干沉降占34%,湿沉降占66%,形成1:2的干湿沉降结构;源于化肥施用与畜禽养殖引起的强烈氨挥发,氮沉降集中在春夏两季(占全年80%),且以铵态氮为主(39%以上).氮的径流输出及河流输送受人为氮输入与水文条件的双重控制,2004年九龙江向厦门海域输送无机氮11.5kg·hm-2,其中90%发生在春夏秋季(同期流量占全年89%);五川小流域总氯径流输出负荷为67.1kg·hm-2,其中85%发生在施肥量大、降雨集中的春夏两季(作物生长期);基流与降雨径流分别贡献25%和75%.总氮淋失负荷为27.5kg·hm-2,占总输入的9%;pH<5的酸性土壤带正电荷导致氮淋失以铵态氮为主(约占40%).九龙江流域反硝化通量为7.7 kg.hm-2,而氮挥发高达42.1kg·hm-2,氨挥发主要来自化肥施用与畜禽养殖(分别贡献50%和39%).减少春夏时期肥料氮的输入(养分管理),有效截留雨季的降雨径流(水文控制)是该流域氮素管理的关键.

Preliminary results concerning the spatio-temporal pattern and mechanism of nitrogen sources and exports in the 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.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.