| Purple soil is the main agricultural soil type in the Three Gorges Reservoir area.The dry sloping land of purple soil accounts for about 80%of the total cultivated land area in the Three Gorges Reservoir area.The slope of the reservoir area is large,the rainfall is rich,the soil develops rapidly,and the soil layer is shallow,which leads to the serious erosion of purple soil in the reservoir area,especially the concentrated rainstorm in the reservoir area.Due to the large population and less land in the Three Gorges Reservoir area,the high reclamation index and multiple cropping index,the relatively backward development of agricultural economy,the large proportion of sloping farmland and the low degree of mechanization,the food security situation is grim.In recent years,with the rapid expansion of cities and the continuous reduction of arable land,a large number of people rely on water and fertilizer input to alleviate the pressure of food supply,which directly leads to the increase of agricultural input,resulting in soil acidification,waste of resources,weakening of soil microbial activity,imbalance of soil nutrients,increase of heavy metal activity,eutrophication of water body and other potential hazards,seriously affecting the production and ecological functions of purple soil.Therefore,it is of great significance to study reasonable fertilization measures to reduce soil nutrient loss and improve crop yield for the prevention and control of agricultural non-point source pollution and agricultural green development in the Three Gorges Reservoir area.At present,the research on soil nutrient loss and control technology has made more progress at home and abroad.However,the effects of chemical fertilizer reduction combined with biochar and straw mulching on soil surface runoff and soil interflow,and the characteristics of annual nitrogen and phosphorus loss fluxes are not very clear.Therefore,through field plot experiment,five treatments were set up under 15°slope for three consecutive years（from October 2017 to October2020）,including no fertilization（CK treatment）,conventional fertilization,optimized fertilization,chemical fertilizer reduction combined with biochar,chemical fertilizer reduction combined with straw mulching,to explore the interannual nitrogen migration in runoff of Purple Soil Upland in the Three Gorges Reservoir area under different fertilization treatments The purpose is to provide theoretical basis for the prevention and control of soil nitrogen and phosphorus loss in the reservoir area.At the same time,the water quality of the outlet section of shipanqiu small watershed in the Three Gorges Reservoir area from November 2017 to November 2020 was continuously monitored,in order to grasp the annual variation characteristics and loss amount of nitrogen and phosphorus in surface runoff under different land use modes in the small watershed,and provide scientific basis for the evaluation and prevention of non-point source pollution in the reservoir area.The main results are as follows（1）From 2018 to 2020,the total runoff yield of CK treatment was the highest（13872.22 L）,and that of straw mulching treatment was the lowest（2967.11 L）,The results showed that the effect of chemical fertilizer reduction and straw mulching treatment on reducing surface runoff was the most significant.The total runoff yield of chemical fertilizer reduction combined with biochar treatment was basically the same as that of conventional treatment,but increased by 18.16%compared with the optimized treatment,and the surface runoff of each fertilization treatment in2020 was significantly lower than that of 2018 and 2019;the order of total runoff yield of each fertilization treatment from 2018 to 2020 was as follows The results showed that:biochar>optimized treatment>straw mulching>conventional treatment>CK treatment.Among them,the runoff of straw mulched soil increased by 20.42%and 12.13%compared with conventional treatment and optimized treatment.In 2020,compared with the previous two years,all fertilization treatments increased soil runoff,and straw treatment increased significantly;straw treatment can reduce the sediment yield of surface runoff,while biochar can increase the sediment yield of surface runoff.（2）Soil flow is the main way of soil total nitrogen loss.In the surface runoff,the total nitrogen loss flux in 2018 and 2019 is much higher than that in 2020.In 2018 and 2019,particulate nitrogen is the main loss way of surface runoff,while in 2020,the loss amount and loss ability of particulate nitrogen are weakened;in the middle soil flow,the total nitrogen loss flux is much higher than that of surface runoff.From 2018 to 2020,the proportion of total nitrate nitrogen loss flux and total nitrogen loss flux under different fertilization treatments were more than 50%,and the proportion of straw treatment reached the highest,69.21%,followed by optimized treatment,62.28%.However,the proportion of the total loss fluxes of particulate nitrogen and total nitrogen was less than 10%,and straw treatment could effectively reduce the loss of particulate nitrogen in surface runoff,but nitrate nitrogen was the key factor in the total loss of total nitrogen and soil flow.（3）The loss of all forms of phosphorus in surface runoff is mainly particulate phosphorus,and the proportion of particulate phosphorus loss flux to total phosphorus is more than 50%in2018-2020,and the loss flux of all forms of phosphorus is low in 2020.For orthophosphate,the loss flux of conventional treatment was the lowest in 2020,while the loss flux of conventional treatment was the highest in 2018.Particulate nitrogen is the key factor of phosphorus loss in surface runoff.Straw treatment can effectively reduce the loss of phosphorus in surface runoff,while biochar can increase the loss of phosphorus in surface runoff.In 2018 and 2019,particulate phosphorus was the main way of phosphorus loss,and the highest proportion of particulate phosphorus loss flux and total phosphorus reached 69.43%,but in 2020,middle soil flow was the main way of nitrogen loss,and the highest proportion of particulate phosphorus was only 3.22%.（4）In 2018-2020,the yields of rape and maize of all fertilization treatments were significantly higher than CK treatment,and there was no significant difference among all treatments except CK treatment,which indicated that the yield of rape and maize would not be significantly reduced by reducing chemical fertilizer combined with biochar and straw mulching,and excessive fertilization could not significantly improve crop yield.Appropriate amount of reducing fertilizer or combined application of biochar and straw mulching could improve crop yield To increase production.Among them,the total yield of rape in 2018-2020 was conventional treatment>optimized treatment>straw mulching>biochar treatment>CK treatment,and the total yield of Maize in2018-2020 was optimized treatment>conventional treatment>straw mulching>biochar treatment>CK treatment.In 2018-2020,the reduction of chemical fertilizer combined with biochar and straw can increase the content of soil nitrogen forms compared with chemical fertilizer alone.The annual average content of total nitrogen in biochar treatment and straw treatment was significantly higher than that in conventional treatment.The annual average content of alkali hydrolyzable nitrogen in straw treatment was the highest,followed by biochar treatment,and the optimal treatment was the lowest.There was no significant difference in the annual average content of soil nitrate nitrogen among conventional treatment,biochar treatment and straw treatment,and it was significantly higher than that of CK treatment.Compared with no fertilizer and only chemical fertilizer,reducing fertilizer combined with straw and biochar can effectively maintain or even increase the content of soil phosphorus.The annual average content of total phosphorus in straw soil was the highest,which was 0.702 g·kg-1.The annual average content of available phosphorus in different fertilization treatments was straw treatment,biochar treatment,conventional treatment,optimized treatment and CK treatment.（5）The monthly average loss concentration of all forms of nitrogen in shipanqiu small watershed was mainly concentrated in fertilization season and harvest period.The highest monthly average loss concentration of total nitrogen,nitrate nitrogen and ammonium nitrogen appeared in September 2019,which were 5.534 mg·L-1,4.216 mg·L-1 and 0.346 mg·L-1,respectively.The annual average emission concentrations of ammonium nitrogen,total nitrogen,nitrate nitrogen and ammonium nitrogen showed a trend of increase and then decrease,and reached the highest in 2019.Total nitrogen was significantly correlated with nitrate nitrogen and ammonium nitrogen（P<0.01）.The total nitrogen loss flux was the highest in 2019 and the lowest in 2020.The order of the emission concentration of each form of phosphorus from large to small was total phosphorus,particulate nitrogen,soluble total phosphorus and orthophosphate.The annual average loss concentrations of total phosphorus,soluble total phosphorus and orthophosphate were the lowest in 2019,and the monthly average emission concentration of particulate nitrogen was the highest in November 2017,which was 3.30 times of the average emission concentration in 2018.The total phosphorus loss flux was the highest in 2019,which was 1.11 times and 1.13 times of that in 2018 and 2020,respectively.The loss flux of total soluble phosphorus in small watershed was much higher than that of other forms of phosphorus.It can be seen that the soluble total phosphorus is an important factor of phosphorus loss in small watershed.Considering the characteristics of runoff and sediment yield,soil nitrogen and phosphorus loss,crop yield and soil nutrients,the best fertilization treatment was reducing chemical fertilizer combined with straw mulching.At the same time,reasonable fertilization methods and land use types should be adopted to reduce the output of nitrogen and phosphorus nutrients in small watershed. |
