| The Three Gorges Reservoir Area is an important ecological barrier zone for middle-upper reaches of the Yangtze River,and has important function on ecological security of the Yangtze Basin.However,the pressure induced by Three Gorges Dam project on ecological environment that leads to the problem of agricultural non-point source pollution is very intense because of the special ecological environmental condition,population resource,socioeconomic status.In this hilly region with concentrated rainfall,agricultural diffuse pollutants moved and transferred very easily from the headstream of catchment along with precipitation and flowed into the Three Gorges Reservoir along with last runoff.As an independent water-collecting unit,small catchment is the source of non-point source pollution,Research on erosion and prevention and control measures of nitrogen and phosphorus in the Three Gorges Reservoir has important theoretical significance and practical value to protect the ecological environment and sustainable development.Therefore,Wangjiagou small catchment as a typical small agricultural catchment in the Three Gorges Reservoir Area was selected as the study area,where the surface runoff data and shallow groundwater data were obtained by the field long-term monitoring,soil data by experiments and a large number of data by survey.Based on these data,the law and characteristics of dynamic change of nitrogen and phosphorus loss in surface runoff under the impaction of different spatio-temporal variable elements were analyzed in terms of the scale of slope surface and subcatchment,respectively.The influence mechanism of land-use types and spatial patterns of paddy field on nitrogen and phosphorus loss concentration of surface runoff were carried on the thorough analysis.The relationship between nutrient content of nitrogen and phosphorus in soil on slope surface and their loss concentration of surface runoff and shallow groundwater was discussed in this study.Lastly,Ann AGNPS model was used to evaluate the remediation efficiency of different management measures to nitrogen and phosphorus loss load of the small catchment.Based on the above research,the study has important theoretical and practical significance that overall grasping the law of nitrogen and phosphorus loss and mechanism of different influence factors in small catchment,revealing the formation mechanism of agricultural non-point source pollution,and developing effective prevention and control measures.The main results are as follows:1.In the slope scale,surface runoff samples in the monitoring stations of slope and main canals during 24 natural rainfall events between 2014-2015 showed that the biggest spatial difference of TN and TP loss concentration in the different monitoring stations was happened during the spring crop period?from May to July?,and the outstanding performance was that TN and TP loss concentration of paddy field stations were lower than other stations,during autumn crop period?from October to December?there were no significant difference,and during summer idle period?89 months?showed an upward trend.The spatial variations of TN and TP concentration in surface runoff of different spatial monitoring sites in spring crops were the largest,and the performance of paddy field monitoring points was much lower than other monitoring points,the difference in summer idle period gradually narrowed as well.We found that different spatial and temporal variables had different effects on different forms of nitrogen and phosphorus losses: there were significant correlation between TN loss concentration and all spatiotemporal variable factors,the correlation relationship of NO3--N with these factors were similar to TN,but it was found that no significant correlation of slope and rainfall intensity to NO3--N.NH4+-N loss concentration showed significant positive correlation with area ratio of pickle,and significant negative correlation with fertilization intervals.DTP showed no significant correlation with all variable factors.The loss of TN in the slope surface runoff was NO3--N,which showed significantly positive correlation,and with the increase of TN concentration,the proportion of NO3--N increased gradually.The loss concentration of TP was significant positive correlation with slope,and significant negative correlation with area ratio of rice and leaf area index.In the subcatchments scale,the changes of nitrogen and phosphorus loss in surface runoff and the variation of load with time were more obvious,and the differences of nitrogen and phosphorus loss in the two subcatchments were also prominent in the high peak period.Based on the monitoring data in 2015,two subcatchments of TN,NO3--N and TP monthly average loss concentration rose steadily to the first peak of the year between February to May and decreased rapidly between June to July,and rose again to the second peak from Aug.to Oct.within the same year,and then decreased again and floating slightly in the month.Cumulative emission load increment of TN,NO3--N and TP mainly occurred in the middle ten days of May to the last ten-day period of June,and the middle ten days of September to the last ten-day period of October,during which the proportion of cumulative emission load increment of TN,NO3--N and TP in annual emissions were 75%-82%,and emission load of TP in 5 big storm events was 65% of annual emissions.The differences of nitrogen and phosphorus loss in the two subcatchments were also prominent in these two periods.2.According to the quantitative comparison of three kinds of land use types in dryland,mulberry planting and paddy field,the results showed that,the average losses concentration of TN and NO3--N exhibited a biggest difference for different land-use type during the period of spring crops,in which the average loss concentration of TN and NO3--N for dryland(23.67mg·L-1;21.06mg·L-1)was 1.45 times and 1.42 times of the value of interplanting field of mulberry(16.32 mg·L-1;14.79mg·L-1)respectively,and was 3.32 times and 4.94 times of the value of paddy field(7.12 mg·L-1;4.26 mg·L-1)respectively,while the biggest difference during the period of autumn crops was only 4.57 mg·L-1.The average losses concentration of NH4+-N during the period of autumn crops was highest,in which the value of paddy field and dryland were 0.64 mg·L-1 and 0.51 mg·L-1 respectively and the value of interplant mulberry land was 0.33 mg·L-1.The average losses concentration of TP during these three periods for dryland were the highest,in which the value of dryland during the period of spring crops and summer free time were higher than that of paddy filed by above 0.1 mg·L-1.Comparing with four typical slope surface,the difference of TN and TP loss concentration for different land use types was reflected in small to moderate rain events during the period of spring crops and heavy rain events during the period of summer free time.On the slope surface scale,the difference of TN and TP loss concentration during the period of spring crops on the combined slope surfaceand paddy field was lower than the value of the simple slope surface.The lowest TN and TP loss concentration during these three periods was located on the combined slope surface and mulberry intercropped terraces on the upslope and paddy field on the downslope,the value of simple slope surface and dryland terrace were all highest,their biggest difference was 2.66 times and 1.87 times for TN and TP,respectively.On the catchment scale,two kinds distribution models of land use types were that area ratio of dryland was higher?19%?and paddy filed distributed in midslope and stayed away from the outlet in subcatchment B,area ratio of garden and forest land were higher?15% and 5%?and paddy filed distributed in the bottom slope and valleys in subcatchment A.The comparison between these two models showed that TN and NO3--N loss concentration in runoff of rainfall during the period of spring and autumn crops were 38%-56% higher on average.Further analysis of the samples of daily surface runoff during the periods when the difference of land use composition was the biggest?about 6% paddy field had not been altered into dryland in subcatchment A?were that,the average concentration of TN,NO3--N and TP in surface runoff of the subcatchment A were 33.3%,43.7% and 67.9% higher than that of the subcatchment B respectively,from the mid on March to the late December.3.The study took three plots in the same area and gradient with different spatial pattern in the slope area of paddy field.The samples of twelve individual rainfalls and two typical individual rainfalls on the three plots of sloping paddy field were collected from 2014 to 2015.These samples were used to analyze the difference of spatial pattern of sloping paddy field on the concentration of nitrogen and phosphorus in the surface runoff.The samples of daily runoff in subcatchments in 2010 and 2015 were used to analyze the spatial pattern change of subcatchments' paddy field on the concentration of nitrogen and phosphorus in the surface runoff.The results indicated that the paddy field of the overall layout in the field with the high availability at the bottom of the slope were higher than the paddy field at the middle slope in the rain under the condition of different surface runoff of TN,NO3--N and TP on the average removal rates by 12.2%,11.6% and 18.4% respectively.During individual rainfall events,the losses concentration of TN,NO3--N and TP in the paddy fields was different significantly among the three different spatial patterns during the previous period of heavy rain event,but it became different during the next period of moderate rain event.The two subcatchments' spatial pattern change of paddy field had weakened the ability of intercepting the purifying the losses of nitrogen and phosphorus of paddy field in 2015,compared with that in 2010.TN's losses concentration increased significantly during the periods of light and moderate rain event and base flow.TP's losses concentration fluctuated during the event from moderate rain to heavy rain.4.The soil samples collected from different regions in 2014-2015 and the shallow groundwater samples were collected in 2012-2015.We analyzed the characteristics of nitrogen and phosphorus in soil and shallow groundwater of different land-use types in study area.The relationship between soil nitrogen and phosphorus content and the concentration of nitrogen and phosphorus in shallow groundwater and slope surface runoff was analyzed.The results showed that in different land use types,the average content of TN and NO3--N in the soil of terrace was significantly higher than those in the sloping upland?P<0.05?,in which average content of paddy terrace was the highest(1.49 g·kg-1).The average content of TP in the soil of sloping upland and mulberry-sloping upland was significantly higher than those in the other land types.The average content of NO3--N in the soil of dryland terrace was highest in all land use types and its discrete degree was also the largest.The slope land-use types had a greater impact on the concentrations of TN and NO3--N in shallow groundwater,but it had little influence on TP concentration.In addition,the shallow groundwater TN concentration and NO3--N concentration was significant positive correlation,and the average contribution of NO3--N to TN in five wells was from 68% to 79%.And the TN,NO3--N of shallow groundwater monthly average concentration change was basically the same,showing a significant upward trend after the two crop-fertilization stages in spring and autumn.The average content of TN and NO3--N in the slope soil was significantly correlated with the TN and NO3--N concentrations in the shallow groundwater,but there was no significant correlation between the TN and NO3--N concentrations in the surface runoff.When the slope surface runoff TP concentration was above 0.1mg·L-1,the average content of TP had a significantly linear correlation with it.There was an obvious power function relationship between concentrations of TN,NO3--N in surface runoff and concentrations of TN,NO3--N in shallow groundwater,and the correlation of NO3--N concentrations between surface runoff and shallow groundwater was better.5.Based on field investigation,experimental observation and data collection,the AnnAGNPS model database was constructed,and the adaptability of the model was tested using the monitoring data of rainfall events.In general,the simulation results of the model had the best effect on runoff,and the simulation effect of TN was better than that of TP.The simulated R2 values and the measured values of the different outputs were larger than 0.909,which indicated that AnnAGNPS model was suitable for small catchment in Three Gorges Reservoir Area.Simulation results of spatial distribution of output load of different pollutants showed that the annual output load of the unit cell and the TN and TP per unit area were the highest,and the average values were 24.85 t·hm-2·a-1,63.88 and 5.53 kg·hm-2·a-1 respectively.According to the comparison of the effects from different management measures on the reduction of nitrogen and phosphorus,the optimal control scheme based on the existing land use structure was the most obvious.The reduction rate of TN and TP were 549% and 639% respectively.In addition,the rate of dissolved and particulate nitrogen and phosphorus reduction were between 47%-68%,which reflected that the optimal adjustment of land use structure was an important method of agricultural non-point source pollution control.Slope gradient can effectively prevent the loss of particulate nitrogen and phosphorus.Once the dry land slope of 27% of the catchment area was set as the terrace,the output of particulate nitrogen and phosphorus decreased by 44% and 46%,respectively.The technologies of reducing fertilizer and conservation tillage were simple and easy for the reduction rate of total nitrogen and total phosphorus by 30% and 24% respectively,which should be taken as the priority and desirable measures of agricultural non-point source pollution in the Three Gorges Reservoir Area. |
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