Abstract: In a simulated rainfall experiment on Loess slope land, this paper determined the rainfall, surface runoff and the effective depth of interaction (EDI) between rainfall and soil mineral nitrogen, and studied the effects of grass coverage on the EDI and the runoff loss of soil mineral nitrogen. The results showed that with the increase of EDI, soil nitrogen in deeper layers could be released into surface runoff through dissolution and desorption. The higher the grass coverage, the deeper the EDI was. Grass coverage promoted the interaction between surface runoff and surface soil. On the slope land with 60％, 80％ and 100％ of grass coverage, the mean content of runoff mineral nitrogen increased by 34.52％, 32.67％ and 6.00％, while surface runoff decreased by 4.72％, 9.84％ and 12.89％, and eroded sediment decreased by 83.55％, 87.11％ and 89.01％, respectively, compared with bare slope land. The total runoff loss of soil mineral nitrogen on the lands with 60％, 80％, and 100％ of grass coverage was 95.73％, 109.04%, and 84.05％ of that on bare land, respectively. Grass cover had dual effects on the surface runoff of soil mineral nitrogen. On one hand, it enhanced the influx of soil mineral nitrogen to surface runoff, and on the other hand, it markedly decreased the runoff, resulting in the decrease of soil mineral nitrogen loss through runoff and sediment. These two distinct factors codetermined the total runoff loss of soil mineral nitrogen.

Key words: Spodoptera litura, Microplitis sp., Spodoptera litura nuclear polyhedrosis virus, Virus transmission