Effect Of Biochar On Soybean Cadmium Absorption And Rhizosphere N-cycling Microbial Activity

Under the severe situation that the farmland area polluted by heavy metals is expanding year by year and the farm products contains excessive heavy metals,for elucidating the value of remediation of heavy metals with biochar,we systematically analyzed the effects of biochar addition on soybean growth,cadmium uptake,distribution,transport and accumulation under two concentration levels of cadmium stress.The high-throughput sequencing was utilized to help analysis the effects of biochar addition on the structure and composition of soybean rhizosphere microbial community.At the meantime,qPCR technology was used to explain the mechanism of the effect of biochar on soil nitrogen from the perspective of microorganism in order to provide evidence for remediating farmland heavy metals and reducing farmland nitrogen loss.The conclusions are as follow:?1?The addition of biochar significantly improved the nitrogen fixation capacity of soybean in cadmium contaminated soil and promoted the growth of soybean.1%biochar increased the nitrogenase activity of soybean rhizobia by 53.66 times,and increased the nitrogen content in soybean leaves by 13.7%and plant height by 14.6%,respectively,during the flowering period of soil polluted with low concentration.?2?The addition of biochar significantly increased the transport coefficient and enrichment coefficient of cadmium in the inedible part of soybean,significantly increased the accumulation of cadmium in the inedible part,contrary they were significantly reduced in the edible part.And 10%biochar addition reduced the cadmium content in soybean seeds from 1.17mg/kg to 0.58 mg/kg,which is close to the food edible threshold of 0.4 mg/kg.?3?The addition of biochar promoted the growth of soybean rhizosphere nitrification?with gene amoA?and denitrification bacteria?with gene nirS,nirK,nosZ?,but their metabolic activities were inhibited.Gene nirK encodes the key enzyme that produces N₂O,and gene nosZ gene encodes the key enzyme that reduces N₂O.The inhibition effect of biochar addition on the transcription rate of gene nirK is greater than that of gene nosZ,indicating that biochar addition may reduce N₂O emissions mainly by inhibiting the production of N₂O itself.?4?The addition of biochar only had an effect on the structure of soybean rhizosphere microbial community in a short period of time?vegetative growth period?,but with the passage of time,the community structure tended to be stable.In addition,cadmium pollution degree and growth stages of soybean can affect the rhizosphere microbial community.Both the structure and composition of the microbial community were impacted by soybean growth stages while only the microbial community structure was affected by cadmium pollution degree.In conclusion,biochar can not only promote the growth of soybean,but also achieve the purpose of cadmium transfer in farmland soil by increasing the accumulation of cadmium in the inedible part of soybean,and reduce that in the edible part of soybean.The addition of biochar did not break the balance of soybean rhizosphere microbial community and inhibited soil nitrogen loss.Therefore,the application of biochar is a win-win technical measure that can both promote crop growth and remove heavy metals from contaminated farmland.