| Choosing two pieces of typical plantation (pinus tabuleaefomis forest and the mixed Quercus acutissima and Platycladus orientalis forest) as research object, setting up sample plot and observing ecological factor of precipitation, litter and otherwise, and by the methods of simulated rainfall in both the indoor soil boxes and the field runoff plot, hydrological and ecological characters of typical forest ecosystem and biogeochemical cycle mechanisms of nonpoint source pollutant of N, P and K in the representative forest ecosystem and the intrinsic rules of the release and migration of nonpoint source pollutant transport to runoff on a slope scale and the influences of forest plant on nonpoint source pollutant transport have been analyzed in Miyun reservoir watershed of Beijing Mountain Area. At the same time, mechanisms of ecological adjustment nonpoint source pollution by forest plant have been revealed also in the paper. On the base of before research, nonpoint pollution of watershed scale has been simulated by SWAT model. The main conclusion is as follows:The results showed that canopy interception, throughfall and stemflow of pinus tabuleaefomis forest and the mixed Quercus acutissima and Platycladus orientalis forest account for 7.23～87.39%, 12.56～91.58%, 0.007～1.77% and 0.53～33.67%, 64.57%～93.71%, 1.76%～5.76% of the rainfall input in open field, respectively. The average canopy interception rates are 31.67% and 18.94%. The maximum water holding capacity of the litters of the two stands are 2.45mm and 1.27mm, respectively. The maximum and the effective soil water storage capacity of 0～30cm soil layer in pinus tabuleaefomis plantation, the mixed Quercus acutissima and Platycladus orientalis forest, shrub forest plots and wild grass ground are 204.90mm and 68.23mm, 160.80mm and 41.87mm, 161.10mm and 39.90mm, 123.45mm and 12.36mm, respectively. The layer of canopy, litter and soil of forest ecosystem can change and adjust hydrologic process and decrease the original power of nonpoint sourse pollution process effectively.When the precipitation pass through the canopy of pinus tabuleaefomis, the content of N, P, K of throughfall and stemflow are increased in compassion with that of the precipitation. Annual N, P and K return through rain leaching amount to 31.5636kg·hm-2 in a 33-year-old pinus tabuleaefomis. Amount of annual litter and annual N, P and K return through litterfall is 2193.75kg·hm-2 and 18.0012kg·hm-2, respectively. The stand has the litter storage of 13.70t·hm-2 and the total accumulation of N, P and K in the litter layer is 162.436kg·hm-2. The total biomass storage and the total accumulation of N, P and K in the tree layer of pinus tabuleaefomis are 82152.01kg·hm-2 and 612.3715kg·hm-2, respectively. The annual total absorption of N, P and K are 126.5755kg·hm-2, in which the annual retention are 77.0107kg·hm-2, and the annual return through litterfall and rain leaching are 49.5648kg·hm-2.When the precipitation pass through the canopy and trunk of the mixed Quercus acutissima and Platycladus orientalis forest, the content of N, P and K of throughfall are increased and the content of N and K of stemflow are also increased, but P is appreciably decreased in compassion with that of the precipitation. Annual N, P and K return through rain leaching amount to 16.4797kg·hm-2. Amount of annual litter and annual N, P and K return through litterfall is 1455.71kg·hm-2 and 14.0620kg·hm-2 in a 12-year-old mixed Quercus acutissima and Platycladus orientalis forest, respectively. The stand has the litter storage of 5.66t·hm-2 and the total accumulation of N, P and K in the litter layer is 50.719kg·hm-2. The total biomass storage and the total accumulation of N, P and K in the tree layer of the mixed Quercus acutissima and Platycladus orientalis forest are 31624.1329kg·hm-2 and 273.2513kg·hm-2, respectively. The annual total absorption of N, P and K are 54.9578kg·hm-2, in which the annual retention are 24.4161kg·hm-2, and the annual return through litterfall and rain leaching are 30.5417kg·hm-2. The study showed that forest ecological system has powerful ability of absorbing and transforming pollutant of nonpoint source.The results of simulated rainfall experiments in laboratory showed that rainfall intensities, surface slope and antecedent soil water content significantly affect the solute transport on slope. The content and loss of N, P and K in runoff enhance generally with the increasing of rainfall intensities, surface slope and antecedent soil water content. The results of simulated rainfall experiments in the field plots showed that the consumption of litterfall coverage on runoff and sediment is more distinct with the increasing of litterfall coverage. Litterfall coverage has double effect on the loss of pollutants of N, P and K. One aspect, litterfall coverage increases the interaction between rain runoff and topsoil, aggravates releasing of P and K from topsoil to runoff, enhances the concentration of P and K in runoff and the content of N, P and K in sediment. On the other hand, litterfall coverage reduces the splashing ability of raindrop, dams and slows up occurrence of runoff and sediment. Consequently, litterfall coverage lessens the loss total quantity of pollutants of nonpoint source.The results of SWAT model simulation showed that loss of non-point pollutant can be reduced effectively with the increasing area of forest plant. Non-point pollutant modulus of landuse type of low-density plant is more than forest landuse. Sediment, organic nitrogen load, organophosphrus load, nitrate nitrogen load and sediment adsorption phosphorus show power function and exponential function increment trend with the increasing of precipitation, respectively.Research indicated the plant fence and river forest buffer system control nonpoint source pollution effectively.
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