| The Three Gorges Dam plays an important role in national economy,water resources security,and ecological barrier.In order to provide theoretical basis for preventing and controlling soil and water loss,controlling nutrient loss on WLFZ and reducing agricultural non-point source pollution in the Three Gorges Reservoir Area,distribution of nitrogen and phosphorous content and loss characteristics were studied in the WLFZ of the Three Gorges Reservoir area.Soil and water samples from three different parent material area(purple sandstone,granite,limestone)of different water-level were collected and field runoff scouring experiments were conducted in the widespread purple sandstone area in the lowest water level period.Then bibliometric was used to analyze changes of nitrogen and phosphorus contents and occurrence patterns in different research areas before and after impoundment of the Three Gorges Dam.The results are as follows:(1)The soil bulk density distributed between 1.35~1.53 g/cm3,the average is 1.45±0.06 g/cm3,the total porosity distributed between 41.87~49.81 %,the average was 46.79±2.57 %.With the increase of water level,sand content(28.62~51.74 %)and soil bulk density increased gradually,the change trend of total porosity,total porosity just on the contrary.With the increase of water level,the surface soil pH decreased gradually(6.87~6.57),the soil organic matter,total nitrogen increased;the soil organic matter,total nitrogen and total phosphorus distributed between 8.54~22.85 g/kg 0.15~0.76 g/kg,0.16~0.29 g/kg,and the mean value were 14.22±9.68 g/kg,0.51±0.18 g/kg and 0.22±0.04 g/kg.Total nitrogen and total phosphorus content in water increased with the decrease of water level,and the total nitrogen content of different parent materials in water showed significant differences but total phosphorus in water samples did not show significant difference.(2)The change trend of the concentration of nitrogen and phosphorus in the runoff with time was roughly the same,both reached the maximum concentration in the initial scouring process,then decreased gradually,and gradually stabilized after a certain time.The initial concentration of nitrogen and phosphorus were located between 50.44~66.09 mg/L and 68.56~85.06 mg/L,nitrate nitrogen and ammonium nitrogen concentration were distributed between 16.91~20.57 mg/L and 22.41~34.26 mg/L.By using power function and exponential function respectively for fitting the concentration of nitrogen and phosphorus,the results demonstrated that the exponential function fitted better.The grass buffer zone interception efficiency of nitrogen and phosphorus was TN> NO3--N >NH4+-N >TP;the grass buffer zone interception efficiency of nitrogen was more than 85.7 % and the interception efficiency for total phosphorus was among 61.3%,which was greatly affected by different grass buffer zone width and scouring intensity.With the increase of grass buffer zone width,the interception efficiency for nitrogen and phosphorus increased.However,with the increase of scouring intensity,the interception efficiency for nitrogen and phosphorus decreased.By comparing the content of nitrogen and phosphorus in the soil at different soil depth before and after scouring,it was found that nitrogen and phosphorus were enriched in the soil surface at the end of scouring,which indicated that the nitrogen and phosphorus in the grass interception were mostly stored by the soil.The change rate of nitrogen and phosphorus were negatively correlated with scouring intensity and soil depth.With the increase of scouring intensity and soil depth,the abundant degree of nitrogen and phosphorus in soil layer decreased.(3)At different grass buffer zone width,runoff producing time with the increase of erosion intensity decreased,runoff rate significantly increased with scouring intensity increased.High scouring intensity(90,120 mm/h)compared to small erosion intensity(60 mm/h)22.9 % reduction of runoff time was 40.8 %(grass buffer zone width 5 m)and 16.3 %,69.5 %(grass buffer zone width 10 m),stable runoff rate in high scouring intensity were 1.48,3.10 times(grass buffer zone width 5 m)and 1.54,2.40(grass buffer zone width 10 m)to that of the 60 mm/h.The cumulative runoff increased with scouring process,there was a significant linear relationship between the cumulative runoff and scouring time.The response of different depth of soil moisture content to runoff was significantly different in the experimental plot.Slope surface soil moisture increased,an increase of around 0.10 v/v,30 cm depth of soil moisture content increased slowly,increase range distributed between 0.009~0.012 v/v.Soil water content in the depth of 10 cm of downhill increase the most,0.121 v/v,when the scouring intensity increased,the degree of soil moisture content on the response of runoff increased the most obvious.(4)After the officially impoundment of the Three Gorges Reservoir,total nitrogen and phosphorus decreased in different degree compared with the impoundment before,nitrogen and phosphorus content in the Hubei section is lower than that of Chongqing.And with the increase of continuous flooding,fluctuating with the total phosphorus content showed a decreasing trend.The different fractions of nitrogen and phosphorus are the important factors affecting their adsorption and release.Fe-Al/P is the lowest occurrence form of TP,but it also affects the phosphorus release from sediments to overlying water.Ca-P is the highest form of occurrence in TP.It belongs to stable phosphorus and is not easily absorbed and utilized by organisms.OSF-N and CF-N content accounted for most of the available nitrogen TF-N,while IEF-N and IMOF-N accounted for a relatively small proportion. |
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