Enhancement Of Decomposition Of Maize Straw Returned In Soils By Chemical Pretreatments And Sorption Properties Of Straw Residues Toward Pesticides

Northeast China is a major agricultural production base,with an annual straw production of about 200 million tons.Improper disposal of these straw resources may lead to various environmental problems,such as air pollution,soil erosion.The use of pesticides per unit area in the Northeast agricultural region is 1.4 times of the national average level.It is a potential risk of non-point source pollution of pesticides.Straw return as the main method of straw utilization,can supplement the loss of soil organic matter and enhance the soil adsorption toward pesticides,thereby,reducing the risk of non-point source pollution of pesticides.Various residues with significantly different structures and properties were formed in soils with straw decomposition.The effect on the adsorption performance of pesticides in soils by straw residues is unclear.The dense crystalline structure of straw cellulose and the recalcitrant lignin components hinder decomposition of straw in soils.Meanwhile,due to the cold and long winter in the Northeast agricultural region,the effective decomposition time of straw has been shortened,resulting in incomplete decomposition of straw,and the accumulation of vast straw residues in the field.These affect crop production and ecological environment.Biological method to promote straw decomposition in soils,such as inoculants,is easily affected by environmental factors,which lead to unstable effects,and relatively high application costs.These limit its large-scale promotion.In this study,aiming at the problems that the effect on the adsorption performance of pesticides in soils by straw residues is unclear and the straw decomposition is slow in the Northeast agricultural region of China,straw return experiments were carried out,and the straw residues from different decomposition period were characterized,the main structural characteristics of straw residues in the process of straw decomposition and their changing trends were identified.The adsorption performance of straw residues toward 40 commonly detected pesticides in soils was investigated.The model of straw residues adsorption toward pesticides was constructed.Focusing on the two key factors hindering the decomposition of straw in soils,the methods of straw pretreatment with urea and alkali,and sodium persulfate to strengthen straw decomposition were carried out.And the adsorption performance of straw residues on pesticides was evaluated.The main conclusions are as follows:（1）With straw decomposition in the soil,the relative crystallinity of straw followed exponentially decreasing changes,and on the contrary,exponentially increasing changes for specific surface area,water and oil absorption capacity of straw materials(R_adj²>0.748,P<0.007).The proportion of aromatic hydrocarbon was initially increased and subsequently slightly decreased.The lignin contents were fluctuated within a certain range,and displayed no significant change.Through multiple linear regression,a prediction model of the straw adsorption toward pesticides was constructed(n=174,R_adj²=0.838,P<0.0001).Model parameters were included the molar volume,hydrogen bond donor,water solubility of the compound,and the relative crystallinity,water absorption capacity of straw materials.The model can be suitable for adsorption of different types of plant residues(n=148,R_adj²=0.740,P<0.0001),such as crop straw,spent mushroom substrate,branches and leaves.Water absorption capacity of straw residues positively contributes to sorption of pesticides.On the contrary,the relative crystallinity of straw residues negatively contributes to sorption of pesticides.This tradeoff in sorption influences sorption trends of pesticides in soils with straw return practice.Based on the conservation of pesticide mass in soils,a composite linear sorption model of mixed soils and straw materials was conducted（n=950,Radj²=0.891,P<0.0001）.The model can accurately predict the changing trend of K_oc of pesticides after different plant residues input into soils(n=503,R_adj²=0.841,P<0.0001).（2）To solve the problem that dense cellulose crystalline structure hinders straw decomposition in soils,the method of urea and alkali pretreatment of straw in soils was developed.Urea and alkali can destroy the hydrogen bond network of straw cellulose,reduce the relative crystallinity of straw cellulose,and promote the removal of lignin components in straw materials,which was dependent of the application of urea and alkali.The notable destruction of the crystalline regions occurred for the urea and alkali treatments containing Na OH（0.1–1%）and urea（6–8%）,based on straw weight.Urea and alkali had a wide temperature adaptation range（-20^oC to 25^oC）.Under the treatment of low temperature（-20^oC）,the crystalline regions can also be destructed by treatment of urea and alkali.The USH（7%/0.1%）pretreatment resulted in notable enhancement（37%）of straw decomposition in the field within 6 months,superior to current biological-based treatments（6–28%）.Moreover,this pretreatment posed no influence on the adsorption of straw residues collected at the early stage of decomposition（<27 d）toward commonly used herbicides in the cornfield,and resulted in enhanced adsorption capacity of those collected on 67 days and later,indicated by 0.5-to 4-folded increases in K_d values.Additionally,its impacts to soil p H and bacterial/fungal community were insignificant.（3）To solve the problem that recalcitrant lignin component hinders straw decomposition in soils,the method of persulfate pretreatment to enhance the decomposition of straw in soils was developed.Sodium persulfate can break the aromatic hydrocarbon structure of straw,effectively remove the recalcitrant lignin components of straw materials,and promote the disintegration of the crystalline structure of straw cellulose to a certain extent.With the increase of sodium persulfate application rate,the degree of lignin removal was obviously enhanced,when exceeded the degree of destruction of the cellulose crystal structure,resulting in an increase in the relative crystallinity of the straw.Sodium persulfate（0.1%,based on straw weight）pretreatment resulted in enhancement（29%）of straw decomposition in the field within6 months.This pretreatment posed no influence on the adsorption of straw residues collected at the early stage of decomposition（<43 d）toward commonly used herbicides in the cornfield,and resulted in enhanced adsorption capacity of those collected on 181 days and later,indicated by 76–123%increases in K_d values.Additionally,0.1–5%sodium persulfate pretreatment had no significant effect on soil bacterial/fungal relative abundance and diversity.In summary,this study revealed the main structural properties of straw residues with straw decomposition and their changing trends,including the relative crystallinity,specific surface area,water and oil adsorption capacity of straw residues.The model of the straw adsorption toward pesticides was constructed.The effects of straw residues on adsorption and leaching of pesticides in soils were evaluated.The methods of straw pretreatment with urea and alkali,and sodium persulfate to enhance straw decomposition in soils were developed.Meanwhile,the adsorption capacity of straw residues with high decomposition degree toward herbicides was improved.