Research On N And P Loss Load And Nutrient Balance Under Farming-mulberry System On Purple Soil Sloping Cropland Of Three Gorge Reservoir Area

Nitrogen（N）and phosphorus（P）,as essential nutrient elements for organisms,are also important factors causing agricultural non-point source pollution.The loss of soil nutrients（especially N and P）is the direct cause of agricultural non-point source pollution.The soil environment is the main place where the loss of nitrogen and phosphorus occurs.N and P transport’s biogeochemical processes are complex in soil,including N and P accumulation,adsorption/desorption,and transportation with water.Excess nitrogen and phosphorus nutrients in soil enter water bodies through runoff,sediment transport and leaching,posing risks to human and environmental health.The rainfall-runoff process is the most important natural reason for non-point source pollution in case of unreasonable production and living activities of human beings.The surface runoff and subsurface leaching,often occurring simultaneously,are two main hydrological pathways for transporting nitrogen and phosphorus from the agro-ecosystem to the aquatic ecosystem.Studies show that the loss of N and P through subsurface runoff might be high and cause environmental pollution.A great deal of research has been carried out on the migration process,mechanism,influencing factors,loss and collection methods of N and P,but there are still some deficiencies in the research on the loss of N and P under different farming-mulberry configurations in the purple soil sloping cropland:（1）Collection methods of subsurface runoff still need to be further explored.Presently,the commonly used subsurface runoff collection methods are the porous corrugated tile method,external collection trough method,seepage plate collection method,lysimeters method,and ion exchange resin method and so on.These methods have some limitations in sampling position,collecting volume,sampling depth,the precision of sampling,sampling efficiency,and soil disturbance;（2）Characteristics of nutrient loss in different soil layers are not clear.Due to the limitations of current collection methods,stratified quantitative studies on soil moisture,sediment and nutrient transport are still lacking.It is difficult to accurately reflect the spatial characteristics of soil nutrient transport on natural sloping land;（3）The effects of terraces and mulberry ridge on different soil layers’soil water,sediment and nutrient transport in sloping farmland are less studied.Most of the existing studies are based on runoff plots on the single continuous topographic slope,which cannot reflect the soil and nutrient loss characteristic on terraces of natural sloping farmland.At the same time,the effects of terrace and mulberry ridge on the transport of water,sediment and nutrients in different soil layers are still unclear and need to be further explored.In this study,two kinds of field location monitoring and collection methods（U-trough and seepage plate collection methods）were used to study different planting seasons（S₁:maize planting season,S₂:summer fallow season,S₃:mustard planting season,S₄:winter fallow season）under different natural rainfall grades（TR:torrential rain,HR:heavy rain,MR:moderate rain,LR:light rain）and different farming-mulberry configurations（single terrace+bare ridge;single terrace+mulberry ridge;double terraces+bare ridge;double terraces+mulberry ridge;three terraces+bare ridge;three terraces+mulberry ridge）.The migrating loss loads of nitrogen and phosphorus in different soil depths（0 cm,20 cm,40 cm and 60 cm）were monitored and analyzed.The nutrient balance analysis of N and P’s input and output in each crop was carried out with the absorption capacity of different plant roots to different depths of soil N and P nutrients.The optimal fertilizer application amount suitable for each crop in each farming configuration system was calculated.This study aims to provide a theoretical reference for the prevention and control of agricultural non-point source pollution in purple soil slope farmland to some extent.The main results and conclusion of this study are as follows:（1）This study found that the U-trough collection method was a reliable method for monitoring and collecting soil moisture,sediment and nutrient migration intensity due to its advantages of convenient sampling,a large amount of collection,no disturbance to soil and high sampling sensitivity to small grades of rainfall.The runoff sample volume,sampling probability,nutrient concentration and migration intensity of the U-trough collection method and the improved seepage plate were compared.Both collection methods can reflect the characteristics of migration intensity of runoff,sediment and nutrients in the soil to a certain extent.However,the average amount of subsurface runoff collected by the U-trough method is nearly 10 times that by the seepage plate method.U-trough method can observe and collect more than 95.0%of the annual rainfall-runoff events and is more sensitive to the small and medium-sized rainfall-runoff events.Except for sediment,the cumulative annual loss intensities of subsurface runoff,TN,and TP under both collection methods showed an insignificant difference from the actual loss intensity at the plot’s bottom outlet（P>0.05）.The values of the U-trough collection method are closer to the actual values.（2）Each soil layer has its unique way of intercepting the runoff,sediment and nutrients.The soil layer’s ability to intercept water flow,sediment,and nutrients decrease with soil depth.From the perspective of annual net migration intensity,cultivated horizon（L₁）is the net loss source of sediments,N and P nutrients.The maize season was the main period of runoff,sediment and nutrient loss.The loss amount of runoff,sediment and nutrient increased significantly with the increase of rainfall intensity.Soil layers intercept about 70%of the annual cumulative precipitation;The annual cumulative loss amount of subsurface runoff is about three times that of surface runoff loss.The rainfall-runoff coefficient increased significantly with the increase of rainfall grade.The contribution of each rainfall pattern to annual surface runoff loss is in the sequence of heavy rain（HR）>Moderate rain（MR）>Torrential rain（TR）>Light rain（LR）.The contribution order of various rainfall patterns is in the sequence of MR>HR>TR>LR,to the annual subsurface runoff loss.The annual surface sediment loss intensity is 746.90 kg*ha^-1*yr^-1.The sediment migration intensity from the L₁ layer down to the lower soil layer is 621.29 kg*ha^-1*yr^-1,of which the bottom soil layers can intercept 90.1%.The annual surface（L₀）loss of TN,TP was 6.05kg*ha^-1*yr^-1 and 1.10 kg*ha^-1*yr^-1.The TN and TP intensities of infiltration loss of the L₁ layer are as high as 228.82 kg*ha^-1*yr^-1 and3.55 kg*ha^-1*yr^-1,of which 48.6%and 84.7%can be intercepted by various bottom layers,respectively.The contribution order of rainfall patterns to TN and TP annual surface migration intensity is in the sequence of MR>TR>HR>LR.The loss of phosphorus was mainly particle phosphorus,and the loss intensity of particle phosphorus increased significantly with the increase of rainfall intensity.（3）The terraces and mulberry ridge have significant effects on the control of soil erosion and nutrient migration.N and P’s surface loss,especially PP,was significantly reduced by terraces and mulberry ridges;in contrast,N underground’s loss load increased.The annual surface loss loads of runoff,sediment,TN and TP are 46.4～70.2mm,445.71-746.90 kg*ha^-1*yr^-1,3.94-6.05 kg*ha^-1*yr^-1and 504.47～1081.00 g*ha^-1*yr^-1,respectively.Simultaneously,the annual underground infiltration loss loads of subsurface flow,sediment,TN and TP are 220.8～250.5mm,41.45-65.88 kg*ha^-1*yr^-1,90.81～117.62 kg*ha^-1*yr^-1 and 374.86-549.25 g*ha^-1*yr^-1,respectively.With the increase of terrace number,the surface runoff,sediment,TN and TP migration loads,and the subsurface flow,sediment and TP infiltration loss load in the underground all showed a decreasing trend.The TN underground leakage load increases with the increase of the terrace number.The surface runoff,sediment,TN and TP transport loads,as well as subsurface flow,sediment,TN and TP loss loads underground under mulberry ridge treatments,were lower than those under the corresponding bare ridge treatments.The order of contribution of rainfall patterns to the annual runoff and TP loss load is as follows:HR>TR>MR>LR,and the contribution order of rainfall patterns to subsurface runoff loss was MR>HR>TR>LR;The contribution order of rainfall patterns to the annual transport load of surface and underground sediment and the underground loss load of TP is TR>HR>MR>LR;The contribution order of rainfall patterns to TN surface migration load is MR>TR>HR>LR;The contribution order of rainfall patterns to TN underground leakage load is HR>MR>TR>LR.However,in some TR events,mulberry ridge treatments’interception effect is significantly reduced,leading to the aggravation of surface erosion of sediment,N and P.With the increase of the number of surface terraces,there was no significant difference between the surface loss and underground loss load of NO₃^--N（P>0.05）.With the increase of terraces,the surface loss loads of NH₄⁺-N gradually decreased;the difference among different treatments was not significant（P>0.05）.PP/TP and PO₄^3--P/TP were generally lower in mulberry ridge treatments than in bare ridge treatments.The maize planting season is the main period of surface and subsurface runoff,sediment and nutrient migration,in which the surface loss loads of runoff,sediment,TN and TP accounted for 67.5%,76.0%,45.0%and 70.1%of the annual losses,respectively,while infiltration loss loads of subsurface flow,sediment,TN and TP accounted for 78.0%,69.8%,71.3%and 68.8%of the annual losses underground,respectively.（4）The surplus N and P accumulate in the soil to increase the soil N and P stock and increase N and P loss risk,thus leading to agricultural non-point source pollution.The main sources of N and P input in the agricultural system are chemical fertilizer,returned straw and atmospheric deposition.Chemical fertilizer was the main source of annual N and P input,accounting for 81.8%-84.7%and92.3%-93.6%of exogenous N and P input,respectively.The contribution rates of returned straw to soil N and P input were 12.6%～14.6%and 6.2%-7.5%,respectively.The main consumption items of N and P in the agricultural system are plant absorption,transported loss by runoff and sediment and gas volatilization.Plant absorption and transported loss by sediment and runoff are the main ways of nitrogen phosphorus expenditure.The annual N and P uptake and utilization of plants accounted for71.6%-79.7%and 88.4%-94.2%of the total spending.The amount of N and P used by crops increased with terraces.N and P consumption amount in mulberry ridge treatments was higher than that of corresponding terrace’s bare treatments.The total N and P consumption in each planting season decreased with the increase of table surface in bare ridge treatment,while the total N and P consumption in mulberry ridge treatment showed an opposite trend and significantly increased with the increase of terraces.N and P consumption amount of mulberry ridge treatment was higher than that of corresponding bare ridge treatments.The amount of nitrogen lost through soil erosion accounted for 9.2%～12.7%of the annual total N input.The N input of returned straw to the field accounted for 12.6%～14.6%of the total annual N input.During the study period,the annual surplus of soil N and P were 431.12～510.43 kg*ha^-1*yr^-1 and 91.79～108.51 kg*ha^-1*yr^-1,respectively.（5）Optimal fertilization can effectively reduce the amount of chemical fertilizer input and decrease the economic cost.Simultaneously,it can significantly reduce the enrichment of N and P in the soil,reduce the risk of non-point source pollution,and has potential ecological benefits.In this study,the average maximum root absorption depth of maize and mustard was 36.6-39.4 cm and12.8～13.6 cm,respectively.According to the input-loss balance analysis of N and P in the corresponding soil layer,the optimal fertilizer application amounts were 272.63-307.46 kg*ha^-1*yr^-1（N）and 68.23～76.38 kg*ha^-1*yr^-1（P₂O₅）for maize,and 116.93～171.65 kg*ha^-1*yr^-1（N）and87.19～115.54 kg*ha^-1*yr^-1（P₂O₅）for mustard,respectively.N and P’s optimal theoretical inputs for each maize season were 6.8～17.4%and 36.3-43.1%lower than the current N and P application,respectively.The average reduction of fertilizer N and P was 11.3%and 39.9%,respectively.The theoretical input of N and P in mustard decreased by 66.3%-77.1%and 34.2%-45.3%,respectively,compared with the current input;The average decrease of fertilizer N and P was 73.2%and 38.8%,respectively.The annual application amount of N and P could be reduced by 44.0%～52.3%.The N and P surplus could decrease by 81.4～90.3%and 86.6%～90.2%,respectively.In conclusion,the U-trough collection method,with advantages of convenient sampling,a large amount of collection,no disturbance to the soil,high sampling sensitivity to small-sized rainfall,and real-time monitoring and collecting subsurface runoff,is a reliable method of monitoring and collecting the subsurface runoff,sediment and nutrient migration.Each soil layer has an evident ability to intercept and enrich runoff,sediment,and nutrients;each soil layer’s ability to intercept water flow,sediment and nutrients decrease with the increase of soil depth.The terraces and mulberry ridge had a better control effect on N and P loss in sloping farmland.According to the nutrient absorption capacity of crops at different depths,this study obtained the optimized fertilizer application amount for different crop seasons through the balance analysis of N and P input（chemical fertilizer,straw returning,atmospheric deposition）and N and P consumption（crop absorption and utilization,runoff and sediment loss of N and P loss,gaseous volatilization）.The test runoff field had been built entirely until February 28,2018（including a three-month stable period）.The soil structure might not have been fully recovered when the test was carried out.Hence,the test results need to be further verified.This study’s next steps were to continually carry out this study and further study the adaptability of the U-trough collection method in different soil and land use types.Due to topography limitation,there is no analysis of different terraces’slope lengths and slopes.Meanwhile,due to the short slope length,the setting of each terrace’s relative height difference is low.Therefore,it is necessary to further study the terrace setting’s influence mechanism with different slope lengths,different slopes and different relative heights on the surface and underground loss of soil N and P.