Effects Of Soil Aggregates On Kinetics Of Glyphosate Degradation In Different Soil

Intensive glyphosate pesticides application leads to high glyphosate residues and accumulation risks,especially in the soil,which would affect soil health and ecological environment safety.Soil aggregates play an essential role in soil physiochemical properties which directly influence pollutant behavior and fate in soil environment.Previous studies mainly focused on the glyphosate behavior in different soil layers while very few studies work on glyphosate degradation in soil aggregates,as well as their interactions on soil phosphorus(P)fraction,which particularly adsorb and bound glyphosate in soil particles for a long term.Based on this limit contributions and knowledge,the experimental study was conducted under lab conditions.There typical farmland soil(loess soil from Yangling,black soil from Gongzhuling,red soil from Qiyang)was selected and sieved into different sizes group of aggregates(loess soil: <10 mm,2-10 mm,1-2 mm,0.25-1 mm,0.05-0.25 mm and<0.05 mm;black and red soil aggregate: <2 mm,1-2 mm,0.25-1 mm and <0.25 mm).The degradation kinetics of glyphosate in different types of soil aggregates variations of phosphorus fraction and soil properties were considered in this study.Glyphosate and its main metabolite aminomethylphosphonic acid(AMPA)were analyzed by high performance liquid chromatography tandem mass spectrometry;P fractions were separated and detected by colorimetry method;and soil properties were determined by traditional methods.The Main findings are shown as following:(1)The degradation characteristics of glyphosate in different soil aggregates were similar.To be specifically,the initial degradation rate was faster,and slowed down according to the incubation days.The degradation data was fitting the first-order kinetic degradation curve and the half-life of glyphosate were 1.17-2.20 d,2.92-3.18 d,and 15.8-20.6 d in loess soil,black soil,and red soil,respectively.The particle sizes of aggregates had no significant impact on the degradation of glyphosate,but had a greater impact on the initial degradation rate.During the same observation period,the glyphosate residues were significantly different in the different size of loess soil aggregates(P<0.05),and the lowest content of glyphosate in the smallest aggregates,while the black soil and red soil had no significant among different sizes of aggregates.AMPA was significantly different in soils(P<0.05),but the AMPA content varied among different aggregate sizes.(2)P was significantly different in different soil aggregates after glyphosate application.Residual P and HCl-P were mainly detected in loess soil and black soil,while the moderately active phosphorus NaOH-Pt was dominant in red soil,and the proportion of HCl-P and residual P was much lower than those in loess soil and black soil.Significant differences of P fractions in the aggregates of different soils,and the changing trends of the same phosphorus components in the aggregates of the loess soil were different,while the changing trends were the same in the black soil and red soil.(3)After glyphosate application,the chemical properties of organic matter(OM),total nitrogen(TN)and available phosphorus(AP)in soil aggregates showed responses.OM(except black soil),TN and AP contents in different aggregates differed significantly in the same type of soil(P<0.05).OM,TN and AP contents were highest in the aggregates with0.05-0.25 mm size in loess soil,while the highest content of them in aggregates with <0.25 mm size in black and red soil.In addition,soil enzyme activities were affected obviously after glyphosate application.The activity of phosphatase in loess soil and red soil was obviously inhibited while it was stimulated in black soil.The activity of β-glucosidase and alaninase fluctuated greatly in the three soils.In terms of aggregate size,the activities ofβ-glucosidase,alaninase and phosphatase of aggregates with <0.25 mm size in different soils were higher than those in others aggregate groups.The principal component and correlation analysis showed that the glyphosate content was significantly negatively correlated with AMPA,TN and AP content(P<0.05)in loess soil.The glyphosate content was significantly positively correlated with AMPA,OM,TN and AP content in black soil(P<0.05),while it was significantly negatively correlated with β-glucosidase,alaninase and alkaline phosphatase activities(P<0.05).The glyphosate content in red soil was significantly positively correlated with AP content and alaninase activity(P<0.05).In summary,the degradation of glyphosate in different soil aggregates were similar,following the first-order kinetic model.However,the degradation rate of glyphosate was significantly different among different aggregates,and it was faster in loess soil and black soil than in red soil,indicating that the environmental risk of glyphosate in red soil might be relatively high.In addition,the chemical properties and enzyme activities were different in soils and aggregates during glyphosate degradation.In general,the content of OM,TN,AP,and enzyme activities were higher in small-size aggregates than those in large-size aggregates.With the degradation of glyphosate pesticides,the content of AP was relatively stable in black soil,while the content of AP in red soil decreased and increased in loess soil.This may be related to the degradation rate of glyphosate pesticide and the content of AP in the soil itself.Therefore,the fate of glyphosate under conditions of phosphorus deficiency or abundant soil should be explored in order to provide detailed information on glyphosate risk assessment in red soil.