Applicability Of Revised DNDC Model To Simulate Phosphorus Migration On The Slope Farmland Of Purple Soil

The hilly area of purple soil in the Sichuan Basin is located in the transition zone of the second and third geographic platform of China.It is the forefront of the ecological barrier in the upper reaches of the Yangtze River and has the special ecological sensitivity.Slope farmland is one of the main agricultural land types in the hilly areas of purple soil,and it is also an important source of soil erosion and non-point source phosphorus pollution due to heavy rainfall and frequent tillage.Although there have been abundant research results on the migration process and flux of non-point source phosphorus in purple soil,the simulation of phosphorus migration in the slope farmland of purple soil still needs to be further studied.The process model DNDC is one of the most successful biogeochemical models in the world with not only solid physical chemistry theoretical foundation,but also a wide range of applications.This study aimed to improve phosphorus cycle module of the DNDC model based on the DNDC9.5 source code.Then simulation and validation of the revised DNDC model would be conducted based on the long-term observation data of the slope farmland in Yanting Agro-ecological Experimental Station;and analyses of sensitive parameters would be carried through the predictive simulation.The main results and conclusions of this study are as follows:(1)The DNDC model has been widely used and verified internationally,but there is still a lack of research report on simulation of phosphorus migration.Based on the source code of DNDC9.5,this master thesis has revised the DNDC model through analyzing and improving the related modules of phosphorus migration to solve the problem of phosphorus transpiration simulation.The soil erosion equation(Rose equation)at the field scale and the equation of phosphorus enrichment in sediment had been added to the model to build a phosphorus migration module,with coupling original phosphorus cycle module of DNDC.Finally,Parameter calibration and verification have been performed based on localized data to make the revised DNDC model has a function of field-scale simulation on phosphorus migration.(2)The revised model can respond well to fertilization,tillage and harvest.After fertilization,the content of dissolved phosphorus increases in the topsoil.Due to the adsorption of the soil,the adsorbed phosphorus in the topsoil will also increases;after tillage,the content of adsorbed phosphorus in the topsoil will drop sharply as the topsoil is mixed with the subsoil;after harvest,the organic phosphorus will increase as the residual amount of straws and roots decomposed by microorganisms and converted into organic phosphorus.(3)The revised model has a high accuracy for the simulation of surface runoff,sediment yield,and phosphorus loss in the field scale.Therefore,the revised DNDC model has been proved suitable for the simulation of phosphorus migration on the slope farmland of purple soil.In 2015-2017,the simulated results of surface runoff are in great agreement with the actual observations(NSE:0.82,R~2:0.90);the simulated values of sediment yield are close to the observations(NSE:0.84,R~2:0.89);the simulated phosphorus losses are in nice agreement with the observed values(NSE:0.74,R~2:0.79).(4)The revised DNDC model was used to predicate the phosphorus loss from 2018to 2019,and the results showed that the phosphorus loss was positively correlated with the annual rainfall.However,comparing the simulation results of 2016 and 2018,it was found that the phosphorus loss was lower in 2016 with higher annual rainfall was lower than in 2018 with lower rainfall.It may be owing to that 2015 was a wet year,with heavy rainfall and sediment yield,and the amount of soil phosphorus loss was very high,resulting in a decrease of phosphorus content in the top soil.However,2017 was an extremely dry year with less soil phosphorus loss,leading to soil phosphorus accumulation.Therefore,there is no positive correlation between phosphorus loss and rainfall in 2016 and 2018.(5)Sensitivity analysis of the revised model showed that the change of the sediment carrying efficiency parameter of the bare soil will have a greater impact on the amount of sediment yield,which will affect the amount of phosphorus loss;the change in fertilization amount will seriously affect the phosphorus loss in runoff,because fertilization will directly increase the phosphorus content of the surface soil layer;the tillage depth is negatively correlated with the amount of phosphorus loss,mainly because the phosphorus content shows a decreasing trend from top to bottom in the soil profile,so that the phosphorus in the upper layer will decrease after tillage;and slope is another important factor influencing sediment yield and phosphorus loss.