Study On The Environmental Behaviour Of Metam Sodium Producing Methyl Isothiocyanate In Relation To Soil-Borne Pest

The use of soil disinfection is currently the most effective and stable key technology for preventing and controlling soil-borne disease.However,after the most effective fumigant,Bromomethane,was eliminated due to the destruction of the ozone layer,the prohibition of the highly toxic pesticide,chloropicrin,has also been put on the agenda.This has resulted in a shortage of fumigant products.At present,only Dazomet and Metam sodium are widely used in domestic agricultural production,and the active ingredients of both are the degradation product methyl isothiocyanate（MITC）.Among them,the application cost of Metam sodium is easier to be accepted by farmers than that of Dazomet.However,due to the lack of application standards for Metam sodium according to local conditions,the effect of controlling soil-borne diseases is unstable in its application.A series of studies have been carried out in this thesis to solve the above problems,and the research results are as follows:1.In order to reduce losses the dilution process before the application of Metam sodium,we studied the effects of dilution rations and p H aqueous solutions on the MITC produced by Metam sodium,as well as its digestion dynamics.Our research results indicate that,under neutral conditions,the amount of MITC produced by the decomposition of Metam sodium and its digestion rate increase with increasing dilution rations.Under acidic conditions,the largest amount of MITC produced and the highest rate of digestion were observed at a dilution of 100times,while the lowest amount of MITC produced was observed at a dilution of 50 times and the MITC digestion was the slowest when it was diluted 400 times.Under alkaline conditions,the amount of MITC produced increased with the dilution factor,the amount of MITC produced was the lowest at 1000 times,and the MITC digestion was slowest when it was diluted 100times.It is recommended to use drip irrigation or watering to apply Metam sodium,and the dilution rations should not be higher than 50 times.2.The area under the concentration-time curve（AUC）of total MITC in gas-solid-liquid in soil can better reflect the MITC activity effect than the AUC of gas-phase MITC.In order to develop application standards for metam sodium for local conditions,the headspace bottle method was used to study the changes in the AUC of total MITC in gas-solid-liquid and its relationship with the efficacy of soil-borne pathogens under different soil types,moisture content,temperature,application dose and organic matter content.The results of the study showed that the highest MITC concentration was detected after 1.5-1.75 hours after metham sodium entered the soil and then decreased with time.Among the ten soils,the AUC of total MITC in air-solid-liquid was the lowest in Beijing soil at 474.10 mg kg^-1h and the highest in Zhejiang soil at 3190.67 mg kg^-1h.There were significant positive correlations with the half-life of MITC degradation and the applied dose,negative correlations with the content of available kalium,effective phosphorus and soil temperature,and positive correlations with the effect of MITC on Fusarium and Phytophthora.The half-life of MITC degradation was significantly negatively correlated with the relative abundance of Phylum Firmicutes,Bacillus,Myxomycota and Actinomycetes in the soil.3.In order to reduce the application cost of metham sodium,we innovated the layer injection of metam sodium,and compared the single layer injection+PE film treatment,layer injection+PE film treatment,layer injection+compaction treatment and layer injection+water sealing treatment through indoor soil columns and field trials.The distribution of MITC into the atmosphere,the distribution of diffusion,the law of residue in soil and the control effect on soil-borne pathogens were different between solid treatment and layered injection+water seal treatment.The results of the study showed that after the layer injection of metam sodium,there was no significant difference between the soil surface compaction treatment and the PE film treatment in terms of the reduction of atmospheric emission of MITC,the AUC value of total MITC in the soil air-solid-liquid,and the control effect on Fusarium and nematodes,but the prevention effect on Phytophthora the compaction treatment was lower than that of the PE film treatment;the compaction treatment could reduce the application cost of metam sodium by 500RMB/ha.Therefore,the application method of metham sodium by layer injection+compaction treatment can be promoted in soils with low Phytophthora counts.