Effect Of Phosphorus Addition On Arsenic Release And Soil Enzyme Activity In Arsenic-Contaminated Soils

Nowadays Arsenic（As）has emerged as one of the most prevalent and highly toxic metalloid pollutants.Phosphorus（P）,as a homologous element with similar characteristics in terms of environmental behavior among them,has also been defined by some scholars as an effective method for altering the toxicity of soil As.Soil enzymes are sensitive to As pollution and,therefore,are often used as bioindicators to assess the toxicity of As.However,the response of soil enzymes to soil arsenic contamination under the application of P has been less reported.Therefore,it is of great theoretical and practical importance to investigate the response and mechanisms of soil enzymes under As contamination with the P addition,and to use soil enzymes to accurately and rapidly evaluate the contamination level.In this study,As-contaminated soils from Shimen,Hunan province and Yunnan province,China were collected.We explored the changes in available As content,soil carbon,nitrogen and P cycle catalytic enzymes including urease（URE）and acid phosphatase（AcP）activities,and microbial activity-related enzymes including dehydrogenase（DHA）and fluorescein diacetate（FDA）hydrolase.The ecological dose of As toxicity to soil was calculated,and the main factors affecting the ecological dose were analyzed using redundancy analysis.Some of the main obtained results are as follows:（1）Maximum P adsorption capacity(Q_max)in 10 lateritic red soils collected from Yunnan and 10 red soils collected from Hunan differed widely.It is ranged from 180.72 to3940.01 mg kg^-1.Compared to red soils,the lateritic red soils defined greater phosphorus adsorption capacity.Except for SOM content for the lateritic red soils,no explained variations reached a significance level.Different P application rates,significantly affected As desorption from the soils.With increasing P concentrations,As desorption increased too.According to redundancy analysis results,total arsenic content played the dominant role in arsenic desorption in the lateritic red and red soils.（2）Soil enzyme activities,such as URE,AcP,FDA and dehydrogenase,in lateritic red and red soils were investigated.Total As content had a significant negative correlation with soil enzymes and functional diversity indexes in red soils.The greatest As contaminated soils(>1000 mg kg^-1)evaluated the lowest soil enzyme activities.The reason for that is higher As pollution and lower SOM content.The ecological doses ED₁₀ and ED₅₀（As concentration of caused 10%and 50%drop in enzyme parameters）were evaluated.URE showed the lowest ED₁₀ and ED₅₀values and can be used as an indicator for soil threshold monitoring studies.The total enzyme activity,geometric mean and weighted mean indicators can better reflect the functional diversity characteristics of long-term As-contaminated soils.（3）To investigate As desorption rate,an aging experiment was conducted with P addition.Different As release trends were observed for analyzed soils during 30 d of the aging experiment.The maximum As desorption after P addition followed the order:H1（60.54%）>H10（22.15%）>H4（6.24%）,with 2895.45 mg kg^-1,38.67 mg kg^-1 and644.96 mg kg^-1.Our results showed that in H1and H4 soils,As release did not reach a steady state after day 30,indicating that 30-day aging experiment was not enough.Meanwhile,in H10 soil,As release was initially increasing followed by decreasing and reaching a steady state at the end of the experiment.The soil physicochemical properties,such as soil organic matter content and total As content,played a significant role in As release efficiency.（4）Different As fraction release trends were observed for analyzed soils during 30 d of the aging experiment.Generally,for H1 and H4 soils,P addition led to an increase in the As content for water-soluble(F_WS,)exchangeable(F_EX),carbonate-bound(F_CB),humus-bound(F_HM),and Fe and Mn oxide-bound(F_FM)fractions,whereas it decreased for residual fraction(F_RES)fraction.In most cases,As fractions on days 1-7 transform from F_RES,F_FM,F_CB to F_WSand F_EX fractions.After that,on days 7-30,F_WS and F_EX gradually transform to later fractions.For H10 soil,on days 1-3,F_WS and F_EX fractions transformed to F_CB,F_HM,F_FM and organic matter bound fraction(F_OM)fractions,on days 7-30 to F_HM,F_FM and F_OM fractions.This indicates that P addition and aging process affected As release.（5）The AcP activity was co-influenced by P application,aging time and released As from the soils.Variation Partitioning Analysis（VPA）revealed that for the H1,added P concentrations were the greatest factor influencing soil acid phosphatase.For H4 and H10soils,it was As desorption and aging time separately,while added P and desorbed As combined had a higher effect,respectively.In summary,results show that the application of chemical phosphorus fertilizers should be controlled,since this can trigger an increase in As release in the soil leading to its toxicity AcP and it can be used as As toxicity evaluation after its release by P application.This research aimed to identify the effect of P application on As（Ⅴ）mobility and its effect on soil enzyme activities.Based on research results,it can be concluded that:As mobility and bioavailability increases with increasing P application rate;soils with higher organic matter content can reduce As（Ⅴ）effect on soil enzymes;acid phosphatase enzyme can be used as As toxicity evaluation after its release by P application.We hope,that our research will be useful theoretically and practically,to use soil enzymes to accurately and rapidly evaluate the As contamination level.