4 结 论

本试验采用氮添加模拟氮沉降，研究了土壤有效氮升高对草地群落特征季节变化的影响，并进一步通过人工降雨试验探究了其对坡面流的水动力学特性的影响。主要结论如下：

1）施氮可显著促进白羊草盖度的增加，当施氮量打破了土壤氮素限制状态后，该促进作用不显著。此外，生长前中期，施氮显著增加了白羊草盖度，而在生长末期，不施氮处理白羊草盖度的增幅较大。对于生物结皮，施氮有利于藻类结皮的生长，而显著抑制苔藓结皮的发育。

2）白羊草群落的坡面流均为层流和缓流。降雨强度的增大减缓了坡面流阻力，导致径流量、平均流速、径流深、水流功率和过水断面单位能均显著增加。随着季节变化，白羊草群落坡面流阻力显著增大，平均流速减缓，径流深增加，径流剪切力和水流功率均降低，过水断面单位能增加。施氮显著改变了白羊草群落坡面流特征，随着施氮量的增加，坡面流阻力显著降低，平均流速加快，径流深变浅，径流剪切力和水流功率均显著增加，过水断面单位能减小。

3）白羊草群落具有良好的调控坡面流的作用。然而，在大气氮沉降不断增加的背景下，白羊草群落的“阻水”作用有减弱的趋势，可能会增加草地群落坡面土壤侵蚀的风险。因此，应采取相应措施减缓大气氮沉降速率，从而间接地为控制土壤侵蚀作出贡献。

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