| Rainfall is the driving force for runoff and sediment production.Runoff(surface runoff and sub-surface runoff)is one of the main causes of nitrogen and phosphorus loss in slopes,which leads to pollution of water in downstream rivers and reservoirs.Therefore,research on the regulation of runoff and nitrogen and phosphorus loss and migration mechanism of forest vegetation upstream of the reservoir has been widely concerned.In this study,the Yaoxiang watershed in the upper reaches of the Huangqian Reservoir was used as the research area.The soil of Quercus acutissima forest,Robinia pseudoacacia forest and Quercus acutissima-Robinia pseudoacacia mixed forest was selected as the research object.The grassland was used as the control to study the forestland and the simulated rainfall experiment method was used to study the characteristics of the runoff and nitrogen and phosphorus loss in the forest.In order to provide theoretical basis for the control of non-point source pollution and the allocation of water source forest and its benefit evaluation,this study discussed the proportional load and transport law of nitrogen and phosphorus loss between surface runoff and sub-surface runoff in forestland.At the same time,it has important reference value to deeply study the nitrogen and phosphorus migration in slope land and enrich the contents of soil science research.The main findings are as follows:(1)There are significant differences in runoff and nitrogen and phosphorus loss in forestland and grassland between surface runoff and sub-surface runoff.The surface runoff process from both forestland and grassland showed the characteristics of increasing and tended to be stable.The sub-surface runoff generation time was obviously lagged behind the surface runoff,and the runoff fluctuation of the sub-surface runoff is relatively smaller and more stable than that of the surface runoff.TN and TP concentrations with surface runoff in forestland and grassland were high initially but tended to be stable as the rainfall continued.TN and TP concentrations with sub-surface runoff remained relatively stable in the wholerunoff process.There was a good power function relationship between TN loss concentration and rainfall time in surface runoff.(2)Compared with grassland,forestland can effectively reduce runoff and nitrogen and phosphorus loss,and control non-point source pollution.In the simulated rainfall experiment,the surface runoff of Quercus acutissima forest,Robinia pseudoacacia forest and Quercus acutissima-Robinia pseudoacacia mixed forest was 34.14%,38.63% and 53.49% lower than that of grassland,respectively.The average TN concentration of surface runoff in the Quercus acutissima forest,Robinia pseudoacacia forest and Quercus acutissima-Robinia pseudoacacia mixed forest was 20.32%,25.00% and 26.83% lower than that of grassland,respectively.The TN loss with surface runoff was 52.48%,46.04% and 34.04% of the grassland,respectively.The average output concentration of TN with sub-surface runoff in Quercus acutissima forest,Robinia pseudoacacia forest and Quercus acutissima-Robinia pseudoacacia mixed forest was 11.50%,16.48% and 17.60% lower than that in grassland,respectively.The total TN loss content in Quercus acutissima forest,Robinia pseudoacacia forest and Quercus acutissima-Robinia pseudoacacia mixed forest was 62.40%,57.29% and46.36% of the grassland,respectively.The average TP concentration of surface runoff in Quercus acutissima forest,Robinia pseudoacacia forest and Quercus acutissima-Robinia pseudoacacia mixed forest was 36.47%,43.53% and 58.82% lower than that of grassland,respectively.The TP loss with surface runoff was 42.12%,34.63% and 19.27% of the grassland,respectively.The total TP loss of forestland was lower than that of grassland.The TP loss in grassland was 1.58 times,2.18 times and 2.15 times that of Quercus acutissima forest,Robinia pseudoacacia forest and Quercus acutissima-Robinia pseudoacacia mixed forest.The total TP loss in Quercus acutissima forest,Robinia pseudoacacia forest and Quercus acutissima-Robinia pseudoacacia mixed forest was 55.32%,64.02% and 77.43%,respectively,lower than that of grassland.Compared with grassland,the effect of forestland on nitrogen and phosphorus regulation is better,and the control effect of Quercus acutissima-Robinia pseudoacacia mixed forest is the best.(3)The loss of runoff and nitrogen and phosphorus in forestland and grassland is mainly due to the loss of surface runoff,but the loss of sub-surface runoff cannot be ignored.The surface runoff of Quercus acutissima forest,Robinia pseudoacacia forest,Quercus acutissima-Robinia pseudoacacia mixed forest and grassland accounted for 63.84%,59.87%,53.07%and 81.42% of the total runoff,respectively.The sub-surface runoff in the Quercus acutissima forest,Robinia pseudoacacia forest,Quercus acutissima-Robinia pseudoacacia mixed forest and grassland accounted for 36.16%,40.13%,46.93% and 18.58%,respectively.The proportion of sub-surface runoff in forestland soil is higher than that in grassland,which indicates that the forestland soil is favorable to infiltration of rain water,among which the Quercus acutissima-Robinia pseudoacacia mixed forest has the best infiltration ability.The TN loss with surface runoff in Quercus acutissima forest,Robinia pseudoacacia forest,Quercus acutissima-Robinia pseudoacacia mixed forest and grassland accounted for 75.06%,71.71%,65.52% and 89.24% of total TN loss,respectively.The TN loss with sub-surface runoff in Quercus acutissima forest,Robinia pseudoacacia forest,Quercus acutissima-Robinia pseudoacacia mixed forest and grassland accounted for 24.94%,28.29%,34.48%and 10.76% of total TN loss,respectively.The TP loss with surface runoff in Quercus acutissima forest,Robinia pseudoacacia forest,Quercus acutissima-Robinia pseudoacacia mixed forest and grassland accounted for 82.86%,84.57 %,75.00% and 87.88 % of total TP loss,respectively.The TP loss with sub-surface runoff in Quercus acutissima forest,Robinia pseudoacacia forest,Quercus acutissima-Robinia pseudoacacia mixed forest and grassland accounted for 17.14%,15.43 %,25.00% and 12.12% of total TP loss,respectively.(4)With the increase of rain intensity,runoff and nitrogen and phosphorus loss increased,and the surface runoff proportion of runoff and nitrogen and phosphorus loss increased.With the increase of rainfall intensity,the proportion of forestland surface runoff increased from45.66% to 62.38%,and the proportion of sub-surface runoff decreased from 54.34% to37.62%.The runoff generation time of surface runoff and sub-surface runoff decreased with the increase of rainfall intensity.With the increase of rainfall intensity,the TN loss content in forestland increased 1.05 ~ 2.78 times,the TN loss content in surface runoff increased 1.27 ~3.74 times,and TN loss content in sub-surface runoff increased about 0.74~1.39 times.With the increase of rainfall intensity,the proportion of TN loss with surface runoff in forestland increased from 59.62% to 74.29%,and the proportion of TN loss with sub-surface runoff decreased from 40.68% to 25.71%.With the increase of rainfall intensity,the TP loss contentin forestland increased 1.06 ~ 5.73 times,the TP loss content in surface runoff increased 1.23~ 7.68 times,and TP loss content in sub-surface runoff increased about 0.68~1.33 times.With the increase of rainfall intensity,the proportion of TP loss with surface runoff in forestland increased from 69.34% to 89.38%,and the proportion of TP loss with sub-surface runoff decreased from 30.66% to 10.62%. |
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