Effect And Inhibition Mechanism Of Arsenic Contamination To Soil Enzyme On Aggregate Scale

Arsenic(As)is considered as a key polluting metalloid and its contamination could be assessed by the soil enzyme activity.However,when As entered into soil,various reactions(oxidation-reduction,absorption-desorption,etc.)will occur.These reactions could cause the redistribute of arsenic.Then,the biotoxicity of each fraction of arsenic would be distinguished.Meanwhile,As distribution would show various characteristics in different size aggregates due to its role as the basic structural unit and major index of soil fertility.The enzyme activities in different scales of soil aggregates are also varied.Thus,it is necessary and meaningful to evaluate the impacts of arsenic on nutrient contents,enzyme activities and inhibition mechanization among different size aggregates.The results would reveal the relationships between arsenic contamination and soil enzyme and supply more excise evidence to evaluate the arsenic pollution level.In this study,the contents of different arsenic fraction and other soil properties like organic matter(OM)were tested in soil aggregates under simulation As(?)contamination condition.The different size aggregates were obtained by dry sieving method.The buffer capacity of soil on instantaneous pollution of As(?)was analyzed,as well as the relationships between soil enzyme and arsenic fractions and kinetics mechanism on aggregates scales in simulated long-term pollution.Besides,the biological dose of As(?)in different size aggregates were calculated.Afterwards,the bioavailability of different arsenic fractions in different aggregates size fractions were evaluated.The conclusions would provide useful information on the enzymology evaluation of As(?)polluted soil.The conclusions are as follows:(1)The buffer capacities of 18 kinds of soil in China on As(?)pollution and maximum specific absorption quantity were studied using adsorption-desorption analysis.The fit results using Langmuir model showed that the maximum absorption quantity Qmax ranged from 50.25 to 263.16 mg kg-1.The maximum specific absorption quantity Qmax*ranged from 12.03 to 285.71 mg kg-1.Different soil owned different buffer capacity.Acidic and neutral soil have higher absorption capacity compared to alkaline soil.The adsorption capacity was significantly influenced by amorphous iron(Feox)and clay content.This result may due to As(?)was mainly bound with Fe and humic via inner-sphere complexation.(2)The impacts of dry and wet sieving on the distribution of aggregates scales,nutrient content and soil enzyme activity were studied.The proportions of macroaggregates after wet sieving decreased,while those of microaggregates increased.Wet sieving could decline the content of total nitrogen(TN),available nitrogen(AN),available phosphorus(AP)and available potassium.In addition,wet sieving decreased the activities of phosphatase,sucrose,and aryl sulfatase in aggregates.In conclusion,wet sieving was more suitable for physical stability study,since it has significant effect on the nutrient contents and enzyme activities in aggregates.While dry sieving could be utilized to analyze chemical stability due to its insignificant effects.(3)The distributions of OM,AP and Feox in different pH soil aggregates(dry sieving)under long-term As(?)pollution were evaluated.It was demonstrated that after As(?)aging,macroaggregate was the predominant part in soil occupied over 90%.Long-term pollution by As(?)had few effects on organic matters and amorphous iron.However,with the rise of As(?)content,available phosphorus content in all the aggregates of three tested soils were significantly increased which indicates the competitive adsorption between As and phosphorus.(4)The distribution of different As fractions after long-term As(?)pollution aggregates was analyzed using sequence extraction.Water-soluble As(F1),exchangeable As(F2)and carbonate As(F3)occupied lower proportion in total arsenic content.Humus bound(F4)and iron-manganese oxides bound As(F5)became the major forms of arsenic in soil,and the proportion raised as the increased of arsenic pollution degree.The proportion of residual form(F7)decrease with the increase of arsenic pollution degree.Especially,the decrease ratio in coastal soil(alkaline)was higher than others,which means that the deactivation rate was lower than others.After the correlation analysis,it was observed that iron-manganese oxide was the maj or agent to bound arsenic in red and paddy soil.While,OM was the maj or agent to bound arsenic in coastal soil.There was a significant positive correlation between the content of AP and the content of available arsenic in soil.Therefore,it is necessary to take available phosphorus content into consideration when evaluating the biotoxicity of arsenic.(5)The effect of long term As(?)contamination on aggregates alkaline phosphatase(ALP)has been studied.The inhibition mechanism of As(?)on ALP has been tested through kinetics method.As(?)could inhibited ALP strongly and the inhibition degree of different aggregates ALP were varied.The ecological dose 10[ED10,the concentration of As(?)as the ALP activities declined 10%]of three soil samples were 37.04,55.56 and 18.52 mg kg-1.It showed that the ecotoxicity of As was higher in coastal soil than that in red and paddy soil.The Vmax of ALP were remained or slightly increased as the increase of As(V)concentration,while the Km increased significantly.Thus,the type of inhibition was competitive or the decrease of affinity between enzyme and substract as well as the secret of isoenzyme.The catalytic efficiency(kcat)of 3 tested soils were varied between 0.18 to 23.95 h-1.Besides,it showed negative related with the As(V)concentration.For acid soil,each As fraction except residual has inhibition effect on ALP.F1-F3 could strongly inhibited the ALP in neural and alkaline soil.In this study,the relationship between enzyme activities,enzyme kinetics parameters and the distribution of As fractions on aggregate scale has been supplemented.The environmental factors that could influence the bioavailability and mobility of As has been revealed.This study enriched the theoretical basis for As(V)risk assessment and remediation,provided scientific information for soil enzymology in As(V)assessment.