Evaluation Of Technical Effect Of Pesticide Reduction In Winter Rapeseed Fields Of Middle-lower Reaches Of The Yangtze River And Its Effect On Soil Microbial Diversity

Rapeseed is an important oil crop in China.Sclerotinia rot and weeds are both yield limiting factors in winter rapeseed planting areas in the Yangtze River Basin.At present,pesticide application in rapeseed fields is still large in China.Overuse of pesticides not only increases the pesticide resistance,but also aggravates environmental pollution.Therefore,this study systematically evaluated the application effects of several pesticide reduction techniques in different planting modes of rapeseed in the middle-lower reaches of Yangtze River,and analyzed the effects of different pesticide reduction techniques on soil microbial structure and diversity in rapeseed fields using high-throughput sequencing technology.The main research results are described as follows:Effects of mixed application of herbicide and Coniothyrium minitans on soil microbial diversity in winter rapeseed fields were evaluated.By testing the sensitivity of C.minitans to four different soil-applied herbicides,it was found that high concentration of alachlor,acetochlor,metochlor and s-metolachlor inhibited the growth of mycelia and spore germination of C.minitans at different levels.The EC₅₀for mycelial growth were294.4?g/m L,447.9?g/m L,460.0?g/m L and 500.0?g/m L,separately.EC₅₀on conidial germination were 63.2?g/m L,345.9?g/m L,260.6?g/m L and 223.3?g/m L,separately.Alachlor had the strongest inhibitory effect on conidial germination.In the lab condition,conidia of the four herbicides treated at 1/2 MRAR or 1 MRAR were not lethal,and also could infect sclerotinia of S.sclerotiorum.Therefore,the soil-applied herbicides acetochlor,metochlor and s-metolachlor can be used to mix with C.minitans.In Shayang and Jingzhou of Hubei Province and Hengyang of Hunan Province,rice-rapeseed and rapeseed-rice-rice fields were selected to study mixed application technology of herbicide and C.minitans.The mixed application of C.minitans and herbicide can achieve good results in weed control and sclerotinia disease control in rapeseed field,and reduce labor and chemical pesticide application.The parameters of the technique were listed as follows:(1)10¹⁰spores/g of C.minitans 100 g/667 m²+75%Isosoethylachlor 37.5 m L/667 m²(reduced by 25%)using soil-applied treatment;(2)At the early flowering stage of rapeseed,100 g/667 m²(10¹⁰spores/g)of C.minitans were sprayed.A further study on the herbicide reduction technology was conducted.The 50m L/667 m²of 75%isosoethylachlor was used to treat the soil.At the three leaf stage of weeds,the weeds were then treated with 74%oxadrine·altenone(45 m L/667 m²)+Jijian(15 m L/667 m²).Those treatments had the best control effect on weeds in rapeseed field,and the rapeseed yield was more higher.Thus,the herbicide dosage was reduced by 25%.To evaluate the effects of different soil-applied herbicides on soil microorganism in rapeseed fields,16S r RNA sequencing technology was used to study the changes of soil microbial structure and diversity after treatment with four commonly-used soil-applied herbicides.Soil microbial diversity decreased significantly after 30 days of herbicide treatment,and gradually recovered after 60 days.After herbicide treatment,actinobacteria decreased and acidobacteria increased.After 60 days of soil herbicide treatment,the abundance of functional bacteria related to pollutant degradation,denitrification,photosynthesis,inhibition of pathogen growth and promotion of rhizosphere growth were increased.The absorption of nutrients and excretion of metabolites related to functional bacteria were reduced.Soil-applied herbicides could affect soil microbial diversity,but the effect was not as great as the treatment time.Under different planting modes,control effect of rice-straw returning technique and its effect on soil microbial structure in winter rapeseed field were studied.There are some differences in rice-straw returning technology under different planting modes.The application effect of straw returning technique combined with soil-applied herbicide was much better than other techniques in rice-rapesseed and rapesseed-rice-rice planting patterns.The method of 300 kg/667 m²rotary-tillage/300 kg/667 m²mulching combined with(37.5 m L/667 m²)reduction of isosoethylachlor could not only get better weed control effect,but also reduce the use of herbicide.In the rapessed-rice-ratoon rice planting fields in Wuxue city,the conventional sowing amount was large.The amount of300 kg/667 m²straw returning to the field was directly use to control weeds,and soil-applied and leaf-spraying herbicides were not used.However,too much straw returning and mulching amount would increase the occurrence of sclerotinia rot disease.To evaluate the effects of straw returning technique on soil microbial diversity in rapeseed fields,high-throughput sequencing technology was used to study the changes of soil microbial diversity after straw returning.The results showed that straw returning treatment resulted in a significant increase in soil microbial OTU.There was no significant difference in soil microbial diversity between straw returning and non-returning treatments at seedling stage,but the microbial diversity index of straw returning was significantly higher than that of non-returning at the mature stage.Straw returning treatment could significantly increase the abundance of functional bacteria related to pollutant degradation.In the mature stage,the number of functional bacteria related to denitrification,nitrogen fixation and soil nitrogen transformation was significantly increased.Straw returning showed a greater effect on soil microbial diversity in the mature stage,but the effect on soil microbial diversity in the growth stage was greater than that of straw returning.