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通过流量和水质的同步监测,在径流小区和流域尺度上研究了极端降水事件对氮、磷农田土壤养分流失的影响。研究结果表明:在极端降水事件频发的6￣8月期间,坡地农田随地表径流流失的养分占全年总量的50%以上。水稻田的养分流失占全年的60%左右。而在极端降水量较大的年份,养分流失量也显著高于多年平均值。一年中3次最大降雨事件中输出的氮占全年总量的35.7%￣52.4%,输出的磷占全年总量的46.8%￣66.4%。强降雨过程中小流域出口处的氮、磷浓度随水文过程线的上升而增加,在退水过程虽逐渐下降,但长时间平稳地保持在一个较高的水平。伴随着大流量出现的养分浓度升高导致大量的氮、磷在极短的时间内流失,4次极端降水事件中氮、磷从小流域的输出分别为全年总量的29.2%和44.5%。根据气象预报优化农事活动管理措施,尽量减少在极端降水事件频发期进行翻耕、施肥等农事活动,是减少农田生态系统养分输出和降低农业非点源污染的有效途径。

Effects of Extreme Rainfall on the Export of Nutrients from Agricultural Land

In this study, plot and watershed scale experiments were conducted to study the effect of some extraordinary rainfalls on the export of nutrients from agricultural land. The results of three-year plot experiments show that more than 50% of annual nitrogen and phosphorus losses via surface runoff happened between June and August, when the monsoon and typhoon rains are heavy and frequent. During the three largest storms, 35.7%-52.4% of the annual N and 46.8%-66.4% of the annual P were exported by surface runoff from different land use types. Nutrient concentrations increased with hydrological process during storm flow, decreased with the prolongation of storms yet remained at higher concentrations compared with base flow time. About 30% of N and more than 40% of the annual P loads were exported from the watershed by the four largest storms. Reducing agricultural practices such as tillage and fertilization during frequent heavy rainfall period are effective ways for mitigating agricultural nonpoint source pollution.