Effect Of Biochar And Organic Liquid Fertilizer Application On Phosphorus Availability And Risk Of Phosphorus Loss In Red Soils Of Navel Orange Orchards Under Simulated Acid Rain

The red soil in citrus orchards in the southern hilly areas is generally"acidic","sticky","slabby"and"thin".Acid rain causes the red soil to become"more acidic",resulting in a reduction in the availability of phosphorus in the red soil,increasing the risk of phosphorus loss,which in turn causes eutrophication of water bodies and leads to serious agricultural surface pollution problems.Currently the issue of rainfall-driven phosphorus loss is more often studied in terms of the intensity and type of rainfall and physical properties such as slope,but it is not common to explore phosphorus loss in terms of the water chemistry of rainfall.Studies have shown that the application of biochar can activate soil accumulated phosphorus and reduce phosphorus runoff losses,and the application of organic liquid fertilizer can improve the quality of navel orange fruit,as well as offering significant advantages in terms of improving fertilizer application efficiency and saving labour costs in southern orange orchards,but the changes in phosphorus availability and risk of loss following the application of biochar and organic liquid fertilizer to red soils are unclear.Therefore,this study systematically investigated the effects of applying fruit charcoal and organic liquid fertilizer on the phosphorus availability and loss risk of red soil in southern orange orchards under simulated acid rainfall deposition,using red soil in navel orange orchards as the research object,through soil column leaching and indoor constant temperature incubation tests.In this experiment,four acid rain gradients were set:p H=2.5,p H=4.5,p H=5.6 and p H=6.5（CK）;650°C fruit wood charcoal was selected and three application rates（0%,1%and 5%）were set for indoor constant temperature incubation trials;seven fertilization treatments were set:water irrigation（CK）,100%P inorganic liquid fertilizer（T1）,70%P inorganic liquid fertilizer with 30%P organic liquid fertilizer（T2）,40%P inorganic liquid fertilizer with 60%P organic liquid fertilizer（T3）,100%P inorganic liquid fertilizer+5%fruit charcoal（T4）,70%P inorganic liquid fertilizer with 30%P organic liquid fertilizer+5%fruit charcoal（T5）and 40%P inorganic liquid fertilizer with 60%P organic liquid fertilizer+5%fruit charcoal（T6）were used for indoor constant temperature incubation.At the end of the experiment,the active phosphorus（Olsen-P）,total phosphorus（Total-P）,phosphorus activation coefficient（PAC）,physicochemical properties,soluble phosphorus（Ca Cl₂-P）,phosphorus sorption index（PSI）and degree of phosphorus saturation（DPS）to investigate the changes of phosphorus availability and loss risk of red soils in navel orange orchards under different rates of fruit charcoal application and different liquid fertilizer irrigation.The main results of the study are as follows:（1）Indoor soil column leaching tests showed that the Olsen-P and Total-P contents of the soil were greater in the 0-10 cm than in the 10-30 cm layer,and greater in the drip line than outside the drip line;the PAC values of the soil were lower in the drip line than outside the drip line,and compared to the p H=6.5 treatment,the PAC values of the soil layers outside the drip line increased in the p H=2.5,p H=4.5 and p H=5.6 acid rain The PAC values increased for all soil layers outside the drip line at p H=2.5,p H=4.5 and p H=5.6 compared to the p H=6.5 treatment,and were the highest for the p H=5.6 treatment.Total phosphorus（TP）,total dissolved phosphorus（TDP）and soluble reactive phosphorus（SRP）were measured in the leachate collected in the soil column test,indicating that the total phosphorus loss in the drip line was greater than the total phosphorus loss in the drip line.Outside the drip line,the ratio of SRP to TP was greater in all treatments than inside the drip line,except for the p H=5.6 treatment,and soils outside the drip line were of greater environmental risk.The Ca Cl₂-P content of soils outside the drip line increased with decreasing acid rain p H,while the PSI and DPS values of all treatments severely exceeded the environmental threshold.Correlation analysis showed that soil physicochemical properties were one of the main factors influencing the environmental risk parameters for phosphorus,with soil Ca Cl₂-P content positively correlated with soil p H,SOM,Fe₀,Al₀,Olsen-P and Total-P.PSI values were negatively correlated with soil p H,SOM,CEC,Olsen-P and Total-P.DPS values were negatively correlated with both soil Fe₀ and Al₀,and DPS values were significantly positively correlated with p H and CEC.（2）Incubation trials with different rates of fruit wood charcoal showed that both soil Olsen-P content and Total-P content were greater inside the drip line than outside the drip line,and that the difference in Olsen-P content was smaller after the application of 1%rate of fruit wood charcoal to the soil;compared to the p H=6.5 acid rain treatment,the PAC values of the soil decreased after the application of 0%and 5%rate of fruit wood charcoal to the p H=5.6 treatment,and the PAC values of the soil increased after the application of p H=2.5 acid rain treatments both showed an increase in PAC values.Soil sorption of phosphorus was best and desorption was lowest when 1%fruitwood charcoal was applied to soils inside the drip line;soil Ca Cl₂-P content was lower when 5%fruitwood charcoal was applied inside the drip line than other application rates;application of fruitwood charcoal significantly increased soil PSI values,with 5%fruitwood charcoal applied to soils inside the drip line having the highest PSI values,while 1%fruitwood charcoal applied to soils outside the drip line had the The maximum PSI values were achieved by applying 5%fruit charcoal in the drip line and 1%fruit charcoal outside the drip line.Soil DPS values were within the environmental thresholds for soils within the drip line with 1%fruit charcoal application.Correlation analysis showed that soil PAC values were positively correlated with p H and SOM,Ca Cl₂-P was positively correlated with CEC,and Ca Cl₂-P was negatively correlated with p H,SOM and Fe₀.DPS values Fe₀ and Al₀ were negatively correlated.Redundancy analysis showed that soil p H,SOM,Fe₀ and Al₀ were the most significant factors influencing soil phosphorus environmental risk parameters.Compared to the soil column leaching tests described above,the application of fruit charcoal significantly reduced the risk of soil erosion by increasing soil PSI and decreasing soil DPS values,although it reduced soil Olsen-P content and PAC values.（3）The experiments with the application of fruit charcoal and organic liquid fertilizer incubation showed that the Olsen-P content of soils outside the drip line was increased compared with CK,and the Olsen-P content of soils in treatments T4,T5 and T6 under the application of fruit charcoal was higher than that of soils in treatments T1,T2 and T3 under the application of liquid fertilizer alone;the PAC values of soils in each treatment,except for soils under the p H=5.6 acid rain treatment in treatment T2 The PAC values of soils in the drip line were higher than those outside the drip line,and the highest PAC values were found in soils under the p H=5.6 acid rain treatment in the T6 treatment inside the drip line and in soils under the p H=5.6 acid rain treatment in the T2treatment outside the drip line.Soil sorption was less in the T1 treatment than in the T2 and T3treatments within the drip line;soil sorption was less in the T4,T5 and T6 treatments than in the T1,T2 and T3 treatments alone outside the drip line.Soil PSI exceeded the environmental threshold in all treatments,and the PSI values of soils with fruit charcoal and liquid fertilizer outside the drip line（T4,T5 and T6）were lower than those of the treatments with liquid fertilizer alone（T1,T2 and T3）.Correlation analysis showed that soil Ca Cl₂-P content was positively correlated with Olsen-P and Total-P content,Ca Cl₂-P and DPS were positively correlated with p H and SOM,and PSI values were positively correlated with Fe₀ and Al₀.Redundancy analysis showed that soil p H,SOM,Olsen-P and Total-P content were the main factors influencing phosphorus loss risk parameters.Compared to the soil column leaching test described above,the application of fruit charcoal and organic liquid fertilizer not only increased the soil Olsen-P content,but also increased the PSI and decreased the DPS of the soil.Compared to the biochar incubation tests with different rates,the application of fruit wood charcoal and organic liquid fertilizer resulted in higher soil Olsen-P content and PAC values,but both soil risk parameters increased and were of greater environmental risk.