Simulation Studies On Colloidal Phosphorus Transport From Purple Soil In Central Hilly Area Of Sichuan,China

The purple soil area is an important grain-producing area in Sichuan province.Due to the special physical and chemical properties of purple soil and the large precipitation in the study area,soil and water loss in farmland is serious,which easily leads to the loss of phosphorus(P),which not only reduces the soil fertility and fertilizer utilization rate in this area,but also make a threat to the water.In this study,the characteristics and mechanism of colloid phosphorus(CP)loss in purple soil in the research area were systematically studied.The study enriches the research on phosphorus in the study area,that provide the basis reference for the comprehensive understanding on P forms of environmental behavior and purple soil p loss risk,at the same time that also can provide the theory evidence and theoretical support for sewage sludge management and agricultural sustainable development.The results are shown below.(1)In the farmland profile of purple soil in the hilly area of Sichuan,the soil layer was dominated by soil particles with particle size of 0.2-2 mm,and the contribution to the content of total phosphorus(TP)and(AP)available phosphorus in the whole soil were large.About 38%-50%of TP and AP in the whole soil came from the part of 0.2-2 mm soil particles.The particle size of the colloidal part of purple soil<0.002 mm accounted for the smallest proportion of the whole soil,but its TP and AP contents were the highest,ranging from 1.07-1.73 g/kg and 135.78-288.15 mg/kg,and that decreased with the deepening of the soil layer.The content of water dispersble colloid(WDC)in the 0-10 cm soil layer was significantly higher than that in other soil layers.The content of WDC in purple soil was about twice the content size<0.002 mm.The content of WDC could be estimated by particle size<0.002 mm.CP_MRP was the main component of CP in purple soil.The content of inorganic P in CP decreased with the increase of soil depth,except for O-P.Long-term fertilization can increase the content of inorganic P in the colloid part of the topsoil layer(0-20 cm).P adsorption capacity of soil colloid part was biggest,the adsorption capacity between 222.63 to 555.56 mg kg^-1,and the desorption capacity was high too.So it's easy to cause CP leached down and released into the water.In addition,the P fertilizer can increase the desorption capacity of big size particles in 0 to 10 cm soil larger,which increasing the slope p loss risk.(2)The concentration of various forms of P in purple soil leaching increased with the P levels and decreased with the extension of culture time.CP was dominant in the leaching solution and was greatly affected by p H.Application of P fertilizer had a great impact on the contents of CP and DP in the soil profile,both of which decrease significantly with the increase of soil depth.Application of P fertilizer would cause the CP in the soil to move down about 15-20 cm,and promote the migration capacity of CP in the soil profile to be stronger than that of DP.P fertilizer application and cultivation time in soil may be important mechanism of CP migration and release in soil leaching.In addition,the CP concentration in leaching in the 40 cm soil layer can be predicted according to the content of CP in the topsoil layer when the leaching water is difficult to collect.P leaching can also be reduced by keeping P in the soil for more than 14 days.(3)The moisture infiltration process of purple soil moved forward in a rectangular trapezoidal shape at the beginning of the wetting front with the extension of time.The average infiltration rate of water between 0 and 15 cm was 1.37 cm min^-1,after 15 cm,the average infiltration rate was 0.34 cm min^-1.The content of CP and DP in the soil trough decreased rapidly at first and then tended to be stable,and the content of DP was lower than that of CP.The migration distance of CP in the horizontal direction was within 25 cm,with a large decrease;The migration distance of DP in the horizontal direction was within20 cm,with a small reduction.There was a significant correlation between the content of water dispersed P in soil and the lateral permeability,and the greater the P levels,the greater the correlation was.The lateral permeability had a great influence on CP in the process of horizontal lateral permeability.The application of P fertilizer is the main driving source of P lateral infiltration in the horizontal soil tank.The contribution of fertilization to CP content in soil was greater than that to DP.(4)The application of P fertilizer had little effect on the surface runoff on the purple slope,but had a significant effect on the sediment yield.The initial sediment yield was reduced by 49%-68%after application P fertilizer.The water dispersible total phosphorus(WTP)concentration and loss load in the runoff increased significantly after fertilization,the increase of WTP concentration was 1.8-2.9 times that of control treatment,and the increase of loss load was 1.6-3.2 times that of control treatment.Therefore,the surface runoff carried a great loss of P after the application of P fertilizer,and the degree of threat to the environment is also great.CP loss in the runoff was the main factor,accounting for72.12%-80.71%of the WTP loss in the runoff.The CP concentration in runoff increased by up to 20.9 times after applying P.The losses of WTP,CP and DP increased by 24%-92%,5%-109%and 47%-76%,respectively.Water dispersive phosphorus loss in sediment was less,the loss coefficient was 0.01%-0.04%,which was much lower than that of surface runoff.During the whole process of rainfall,the main way of slope water dispersive P loss was runoff.Phosphorus loss in runoff accounts for 63%-95%of the total loss,and the contribution rate of CP loss in runoff to the total CP loss is 62%-85%.There was a certain correlation among various P forms,especially between WTP and CP,showing a very significant correlation,and the gray correlation degree was high.According to the optimal simulation,the total P loss load was in a binomial relationship with the P levels,and the fitting coefficient R² was very high.Moreover,it is feasible to calculate the amount of CP loss based on the P levels.(5)The maximum adsorption capacity of P on purple soil increased with the increase of biochar.Under the treatment of C1.5%,C3.0%and C5.0%,the maximum adsorption capacity was 3.4 times,8.2 times and 10.6 times of the control treatment,respectively.The application of biochar significantly reduced the concentration of P in leachate and the content of water dispersed P in soil profile.The application of biochar will increase the runoff and sediment.The average amount of runoff increased by 20.16%,and the average sediment yield increased by 5.5-11.27 times.The loss of P in runoff and sediment were mainly dominated by CP,with each group accounting for more than 50%.The loss of P could be controlled indirectly by the prevention and control of the loss of CP.The application of biochar can effectively reduce the proportion of CP in WTP and weaken the damage caused by the strong migration of CP.In addition,the addition of 1.5%and 3.0%biochar had the best effect on controlling P loss,but increased the yield of sediment,which was unfavorable for the control of total phosphorus.Therefore,biochar can effectively change the loss forms of P in the soil,reduce the proportion of CP,and avoid water pollution in a larger range.But it will aggravate the soil erosion of purple soil and bring difficulties to soil erosion prevention and control.