Transport And Transformation Behavior Of As/P In Biochar Applied Soils In Tidal Flat Reclamation Areas

The enclosure and reclamation of coastal tidal flat are important measures to increase land resources globally,and enclosure and reclamation activities further aggravate the pollution level of heavy metals such as arsenic（As）and affect the safety of subsequent agricultural use in the enclosure area.Biochar application is an effective method to reduce As accumulation in rice and to promote nutrient contents in soils,but will the competitive adsorption of As and phosphorus（P）,both of which exist as oxygen anions in soils,further promote the fixation of As and release of P,and how will biochar addition affect the adsorption and fixation or release of As and P?In this study,four different types of biochar（reed straw biochar,corn straw biochar,rice straw biochar and rice husk biochar）were added to three different types of soils（intertidal sediments,enclosure soils and rice soils）to study the effects of biochar addition in soils on As adsorption and P release through isothermal adsorption experiments.The release characteristics of As and P from soils under anaerobic conditions were investigated by short-term reduction experiments.And the behavior of As and P in the soil-rice system after biochar addition were elucidated by rice pot experiments and DOM spectroscopy.The main findings of the paper are as follows:（1）The nature of intertidal sediments,enclosure soils and rice soils differed significantly,and the contents of As,Al,Ca,Fe and Mn in enclosure soils and rice soils decreased due to the natural leaching effect.The pH value,total organic carbon（TOC）content,electrical conductivity（EC）and total P content in rice soils were significantly higher than those of enclosure soils and intertidal sediments due to the influence of anthropogenic tillage.The physicochemical properties of different biochar also differed significantly in pH value,EC,P,Al,Ca,Fe and Mn contents,which were mainly influenced by raw materials.（2）The effects of biochar addition in soils on As adsorption and P release were clarified.Compared with the control group,the adsorption capacity and sorption strength of As decreased after the addition of biochar.the Freundlich adsorption model and Langmuir adsorption model fitted the isothermal adsorption results well,and the fitted R²ranged from 0.961 to 0.998.Both biochar addition and the increase of exogenous As content significantly enhanced the P contents in soil solution,and the most significant enhancement（99.4%～2599.5%）was achieved by reed straw biochar due to the difference of biochar phosphorus content and competitive adsorption of As.（3）The effect of biochar addition on soil As and P behavior under short-term reducing conditions was revealed.After adding exogenous As and conducting reduction incubation experiments,the rice straw biochar addition group showed better performance on As adsorption.With the increase of As concent,P release amount in most soil treatment groups increased first and then decreased.The P release amount of the reed straw biochar treatment group was higher and showed a sudden increasing in P release amount with As increasing.Eh value of the soil solution had a great effect on the mobility of P and Fe and the adsorption of As in soils.Soil Fe release showed a significant positive correlation with both As and P contents in the soil solution,indicating that biochar addition may promote As and P release amount by enhancing soil reducibility and Fe oxide dissolution.Biochar addition changed the morphological proportions of As and P in soils,promoting the increase of the unstable As proportion and the content of active P in soils,among which rice straw biochar had the greatest increase in soil unstable As and active P.The addition of exogenous As tended to activate soil P morphology,and biochar addition made soil P morphology more sensitive to the response of exogenous As.（4）The effects of biochar addition on the behavior of As and P in the soil-rice system were discussed.The results of rice pots experiment showed that biochar addition reduced As contents in brown rice,and As reduction in each treatment group of paddy soil was:rice husk biochar（35.9%）>reed straw biochar（32.5%）>rice straw biochar（26.7%）>corn straw biochar（20.5%）;the reduction in each treatment group of enclosure soil was:reed straw biochar（15.6%）>rice husk biochar（15.1%）>rice straw biochar（11.1%）>corn straw biochar（9.6%）.The contents of As,Fe,and P in pore water increased significantly after biochar addition into the soils,which might be related to biochar addition promoting the dissolution of Fe oxides in soils.However,As contents in brown rice did not increase as a result,which was mainly related to the competitive uptake of P and As in plants and the fact that biochar application enhanced the fixation of As by the iron film on the root surface and weakened the transport process of As from straw to brown rice.（5）P is an important factor influencing As transport in soil-rice system.Biochar addition significantly elevated the P contents in pore water.Since both As and P exist as oxygen anions in soils,As was competitively adsorbed with P and released into pore water,which increased As mobility.On the other hand,pore water P contents showed a significant negative correlation with straw As contents,indicating that phosphate could inhibit As uptake by plant through competing with Asate transporter protein on the plasma membrane.Fe is also an element that affects As and P migration in the soil-rice system.Correlation analysis results showed that Fe contents in the around iron film root surface showed a significant positive correlation with both As and P contents,indicating that the rhizosphere oxidative environment of rice promoted the formation of Fe oxide,and Fe oxide had a strong adsorption and fixation effect on both As and P.（6）Biochar DOM affected the transport and transformation of As and P in the soil-rice system.Compared with the control,biochar addition increased the molecular mass and structural complexity of soil DOM and increased the proportion of fulvic acid-like substances in DOM.Rice straw biochar and corn straw biochar addition increased the soil DOM contents and increased the degree of humification and hydrophobic organic fraction of soil DOM.Reed straw biochar and rice husk biochar had the opposite performance.Correlation analysis results showed that P contents in pore water showed significant positive correlation with authigenic index（BIX）（r=0.831,p<0.01）and freshness index（β:α）（r=0.826,p<0.01）,indicating that DOM might promote soil P release by enhancing soil biological activity.As contents in pore water showed a significant positive correlation with DOM spectral characteristic parameters(SUVA₂₅₄,Fn（355）),and Fe contents in pore water also showed a significant positive correlation with DOM contents（r=0.729,p<0.05）,indicating that DOM might induce reductive dissolution of Fe oxides through redox reactions,which in turn promoted As migration in soils.These results suggested that the transport transformation of As in soils was influenced by the ternary system of P-Fe-DOM.