Effects Of Biochar On Soil Nitrate And Phosphate Leaching/immobilization And Its Mechanism

Coastal estuary area is an important"sink"of terrestrial nitrogen phosphate and other nutrients,which is vital for the health of marine water bodies.In recent years,with the increasing intensity of human social activities,the nitrogen phosphate"sink"function of the coastal estuary is facing great challenges.A large number of experimental studies in inland cropland have confirmed that the biochar was a promising amendment material to improve the soil retention capacity of nitrate and phosphate and lessen the amount of nitrogen and phosphate loss.However,a large variation was documented among the published studies related to the effect of biochar addition on soil retention capacity of nitrate and phosphate,due to the differences in characteristics of the biochars and conditions of tested soils.As a result,the mechanisms of soil nitrate and phosphate retention capacity affected by biochar addition are still unclear.In addition,studies on the application of biochar in coastal soils and its effects on soil nitrate/phosphate leaching performances are scarce.The potential improvement of nitrogen phosphate"sink"function and its underlying mechanism are also needed to be further revealed in the coastal estuary area.In this study,based on literature investigation,statistical analysis methods of meta-analysis(MA)and the SVM(Support Vector Machine)classification model were used to systematically identify and specify the key biochar and soil properties that affected nitrate leaching and phosphate retention in soil,based on which the underlying mechanisms were also discussed.At the same time,an indoor soil incubation/leaching experiment was conducted to explore the effects of calcium or magnesium-loaded biochar on leaching performances of nitrate and phosphate in two contrasting costal estuary soils and their underlying mechanisms.The results are expected to provide the scientific theoretical basis for the high-efficiency utilization of biochar to improve soil nitrate and phosphate retention capacity.Based on the literature investigation,combined MA with SVM,the following results are obtained:(1)In general,biochar application could significantly reduce the leaching amount of nitrate.Compared with the control group,the leaching amount of nitrate in biochar addition groups significantly decreased by 37.1%.Specifically,among these factors,when the ash content of biochar more than 10%,the higher the value,the smaller the leaching loss of soil nitrate was observed.The biochar group of pyrolysis temperature of400?600?indicated greater potentials to reduce nitrate leaching than other biochar groups,correspondingly.The higher soil clay content and CEC,the more unfavorable for soil nitrate retention capacity improvement.In addition,an optimal application potential of biochar was observed for the application amount of biochar between 1%and 3%.(2)In general,biochar addition could significantly improve soil phosphate retention capacity by 20.8%.Specifically,among these factors,the biochar groups of pyrolysis temperature>600?,specific surface area?100 m² g^-1 demonstrated a bigger improvement of soil phosphate retention capacity than other biochar groups,correspondingly.The higher soil pH,clay and organic matter content,the stronger soil phosphate retention ability improved by biochar addition was found.The biochar application rate showed an insignificant effect on the response of soil phosphate retention capacity to biochar addition.(3)The prediction results of SVM classification model showed that the feedstock,pyrolysis temperature,ash content of biochar,the clay content,pH and organic content of soil were the key factors affecting nitrate leaching and phosphate retention.The results of laboratory incubation/leaching experiment are obtained:(4)Compared with non-mineral-loaded biochar,the specific surface area of mineral-loaded biochar decreased,while the average pore size increased;Ca loading significantly increased the pH and conductivity of biochar,while Mg loading did not significantly increase the pH of biochar,but significantly decreased the electrical conductivity.The XRD and FT-IR results illustrated that different calcium/magnesium crystals and metal-oxygen bonds were formed on the surface of biochar.The results showed that the physical and chemical properties and surface mineral content of biochar were significantly changed by mineral loading.(5)Mineral loaded biochar amendment had no significant effect on nitrate leaching performances in the two tested soils(except for Ca loaded biochar+agricultural soil treatment).However,from the results of soil nitrate content before and after the experiment,the mineral-loaded biochar significantly improved the soil nitrate retention capacity,mainly due to the developed pore structure and high surface adsorption capacity of biochar,as well as the inhibition of nitrogen-related enzymes(such as urease)activities.In addition,the results show that the leaching behavior of nitrate was greatly influenced by the initial soil characteristics.(6)Mineral loaded biochar significantly reduced the risk of phosphate leaching in both tested soils.Moreover,the potential of mineral-loaded biochar to reduce phosphate leaching loss was more significant in agricultural soil than that of wetland soil.This is because the high clay content of wetland soil could weaken the effect of new minerals introduced by mineral loaded biochar addition on phosphate leaching reduction.The mineral loaded biochar amendment markedly increased soil phosphorus content and inhibited phosphorus related enzyme(alkaline phosphatase)activity.(7)Based on the experimental results(5)and(6)mentioned above,it is recommended to apply magnesium-loaded biochar and calcium-loaded biochar to wetland soil and agricultural soil,respectively.