Effect Of Wastewater Irrigation And Nitrogen Fertilizer Application On The Bioavailability Of Heavy Metals In Farmland Soil

Water and fertilizer management is the most important agricultural management technology.However,due to water scarcity and growing food demand,wastewater irrigation and excessive fertilization are widely distributed in the North China Plain,which may cause serious damage to the environment.Heavy metals have many chemical speciations in the soil.The bioavailability of heavy metals in different chemical speciations is usually different,and the chemical speciations of different heavy metals will transform each other as the soil environment changes.Wastewater irrigation and nitrogen fertilizer application can significantly change a variety of soil physicochemical processes,affecting soil physical and chemical properties and microbial communities.However,it is still unkonwn whether this change in soil environment will further affect the transformation process between heavy metal chemical speciations and thus the bioavailability of heavy metals.Therefore,it is necessary to evaluate the effects of long-term irrigation with wastewater and long-term nitrogen application on the bioavailability of heavy metals in farmland soils.In this study,wheat rhizosphere soil and wheat root samples under long-term irrigation of tanning wastewater,pharmaceutical wastewater and domestic wastewater were collected in a typical wastewater irrigation area of the North China Plain,as well as a long-term nitrogen fertilizer application test field in Beijing.By analyzing various physical and chemical indicators of soil?pH,redox potential,cation exchange capacity,soil texture and soil organic carbon turnover,etc.?,enzyme activity?Catalase,lignin peroxidase,laccase,etc.?and content of heavy metals?Cu,Cr,Cd,As,Pb and Ni?in different chemical speciations,the effects of long-term irrigation and nitrogen application of wastewater from different sources on the bioavailability of heavy metals in farmland soil and its intrinsic mechanism was explored.The results present here show that:?1?The concentrations of all analyzed heavy metals in the rhizosphere soil of the farmland irrigated with tanning wastewater were increased,and the average concentrations of Cu,Cr,Cd,and Pb exceeded the reference value of soil in Hebei Province.After irrigation with pharmaceutical wastewater,the total amount of heavy metals in the rhizosphere soil almost unchanged except for the increase in Cu content.After irrigation with domestic wastewater,the total amount of heavy metals in the rhizosphere soil remains unchanged except for the increase of Cu and Cr content,indicating that irrigation of domestic and pharmaceutical wastewater will only cause a slight change in the total amount of heavy metals in the soil,while irrigation of tanning wastewater will cause significantly accumulation of heavy metals in the soil,but still does not reach the level of heavy metal pollution.?2?Irrigation with pharmaceutical wastewater could increase the proportion of nobioavilable parts?NB?of heavy metals in rhizosphere soil and reduce the proportion of potentially bioavailable part?PB?and bioavailable parts of heavy metal speciations in rhizosphere soil.Conversely,irrigation with domestic and tanning wastewater could reduce the proportion of nobioavailable parts?NB?of heavy metals in the rhizosphere soil and increase the potentially bioavailable parts?PB?and bioavailable parts?B?of heavy metals in the rhizosphere soil.?3?Irrigation with tanned wastewater and domestic sewage will increase the bioconcentration factor?BAF?of heavy metals in rhizosphere soil.In the farmland irrigated with pharmaceutical wastewater,except for Cr,the BAF values of the remaining heavy metals were reduced.It shows that after the irrigation of tanning wastewater and domestic sewage,the bioavailability of heavy metals in the soil is improved,and after being irrigated by pharmaceutical wastewater,the bioavailability of heavy metals in the soil was reduced except for Cr.?4?Irrigation with tanning and domestic wastewater mainly through increasing the content of soil active organic carbon?KMnO₄-C?content,soil redox potential?Eh?and catalase activity to increase the proportion of potentially bioavailable parts of heavy metal specaitions.KMnO₄-C content and catalase activity have a direct effect on improving the bioavailability of heavy metals,while soil Eh can affect heavy metal bioavailability indirectly by affecting KMnO₄-C content and catalase activity in farmland irrigated with tanning and domestic wastewater.Irrigation with pharmaceutical wastewater reduced the bioavailability of heavy metals in rhizosphere soil,mainly by increasing the soil antibiotic content and reducing the propertions of bioavailable and potentially bioavailable parts of soil heavy metal speciations.?5?Nitrogen application promoted the transformation of heavy metal bioavailable part into organic matter-bound fractions,thereby decreaseing the bioavailability of heavy metals,and this promotion was more significant in smaller particle size of soil aggregate fractions.The results also indicate that the changes in the bioavailable part of heavy metals were negatively correlated with the changes of organic carbon turnover times,while the changes of heavy metal organic matter-bound fractions were positively correlated with the changes of soil organic carbon turnover time.This strong correlation was universal in different soil aggregate fractions.These results indicate that the changes in soil organic carbon turnover time were the root cause of the changes in chemical speciations of heavy metals.After applying nitrogen fertilizer,the soil organic matter turnover time become slower.On the one hand,the interaction time between heavy metals and organic matter can be prolonged.On the other hand,the degree of humification of soil organic matter was increased,the amount of soil organic matter functional groups could be increased,and the adsorption and complexing ability of heavy metals could be enhanced.The enhancement of the adsorption capacity of soil organic matter to heavy metals and the inhibition of desorption capacity ultimately promote the conversion of heavy metal fractions from bioavailable part to organic compounds.