植被覆盖度和综合治理对纸坊沟流域土壤氮素流失的影响

摘要

同小流域土壤侵蚀一样,小流域土壤氮素随洪流流失也受到植被覆盖度的影响,通常经过调整小流域内土地利用结构以达到控制水土流失.该研究以8.27 km2纸坊沟流域和1:400比例流域模型为研究对象,研究植被覆盖度和综合治理对纸坊沟流域土壤氮素流失的影响.结果表明:在模拟降雨下,当流域植被覆盖度分别为60%、40%、20%和0时,流域模型铵态氮流失量分别为87.08、44.31、25.16和13.71 kg/km2,硝态氮为85.50、74.05、63.95和56.23 kg/km2,全氮为0.81、1.18、1.98和7.51 t/km2;在自然降雨下,1998年与1992年相比,全流域年土壤侵蚀量为1 086 t/km2和1 119 t/km2,氮素流失量为8 758.5和7 562.2 kg,减少了15.8%,其中农地减少了52.0%.流域对降水中的矿质氮具有过滤作用,硝态氮的过滤作用明显高于铵态氮.洪流泥沙中＜20 μm微团聚体富集造成了泥沙有机质和全氮的富集.植被覆盖虽能有效地减少流域土壤侵蚀和全氮的流失,却能增加土壤矿质氮的流失.坡地退耕还林草可显著减少流域土壤氮素流失.%Soil erosion and nutrient loss due to erosion are world-wide problems. Similar to soil loss byerosion, soil nitrogen (N) loss by erosion in small catchments is affected by vegetation coverage. Thepractice of comprehensive management for catchments mainly by adjusting cropland, grassland and wood-land areas was widely adopted to reduce soil and water loss in catchments of the Chinese Loess Plateau.Three experiments under natural and artificial rainfall conditions on N loss by erosion for a model catch-ment and for an actual catchment in Zhifanggou of Ansai County in China was performed to determine therelationships between comprehensive management and N loss by runoff in small catchments. The resultsfor vegetation coverage of 60%, 40%, 20% and 0 show that runoff loss of ammonium, nitrate, and total Nwere 87.08, 44.31, 25.16, 13.71 kg/km2; 85.50 74.06, 63.95, 56.23 kg/km2; and 0.18, 1.18, 1.98, 7.51 t/km2, respectively. Due to reduction in the size of cropped area on steeply sloping land, soil N loss byerosion in the catchments was decreased by 15.8% as compared with that in 1992, i.e., from 8 758.5 kg in1992 to 7 562.2 kg in 1998. Whereas, catchments act as a filter for ammonium and nitrate in rain, thecatchment filtering effects on nitrate is remarkably higher than that on ammonium. The enrichment of ＜20 μrn aggregate in sediment results in the enrichment of organic matter and total N in flood sediment.Greater vegetation coverage can effectively decrease soil erosion and total N loss. However, soil mineralN loss increased as vegetation coverage increased.