Clearing Up Continuous-cropping Obstacles By Reductive Soil Disinfestation(RSD)and Influencing Factors

The development of intensive agriculture with high input and high output has caused China to become one of the countries with the most serious soil-borne diseases in the world.Reductive soil disinfestation（RSD）is an environmentally friendly method of restoring the physical and chemical properties of degraded soil,improving soil microflora,inhibiting soil-borne pathogens,and preventing and controlling soil-borne diseases by adding organic materials into soil followed by mixing,irrigating,and sealing before planting crops.In recent years,extensive in-depth studies have been conducted to explore the types of organic materials available for RSD,the mechanism of action,and the changes in soil microflora.However,the influences of soil moisture content,treatment temperature,duration,and auxiliary additives on the effectiveness of RSD remain unclear.In this study,the optimal combination of the abovementioned parameters and their influence on the treatment effectiveness of RSD were studied through laboratory simulation experiments.In the laboratory simulation experiments,watermelon,tomato,and strawberry continuous-cropping soils were used as the research objects.Four experiments were designed to explore the impact of each factor on the effect of continuous cropping soil treated by RSD.The following factors were included:（1）soil moisture content,including controls（original soils）,80%saturated water,100%saturated water and flooded groups except the control group;1%rice straw was added to all the groups;incubation was performed at 30℃for 3 weeks;（2）treatment temperature,including controls（original soils）,10℃,20℃,30℃,and 40℃except the control group;the soil moisture content of all the other groups was 100%saturated water;culturing was performed for 3 weeks;（3）duration,including controls（original soils）,treatment for3,4,5,and 7 weeks except the control group;all other groups were added with 1%straw and 100%saturated water;incubation was performed at 10°C and 30°C,respectively;（4）auxiliary additive,including 1%rice straw,1%rice straw+0.5%cow dung,and 1%rice straw+0.01%iron powder,1%straw were taken as the basic organic matter in each of them except the control group;the soil moisture contents of other groups were all 100%saturated water;incubation was performed at 10°C and30℃for 3 weeks.The soil physicochemical and microbial indices were analyzed after the end of soil treatment by RSD.To adapt to the actual situation of the window period for RSD-treated watermelon planting in winter in Jianghuai area,the effects of RSD on the prevention and control of continuous cropping obstacles of watermelon were studied by increasing the amount of organic materials and extending the treatment duration.The pot experiment was performed outdoors,including control（original soil,CK）,flood control（FCK）,2%alfalfa meal（AS-RSD）,0.25%ammonia（AW）,0.25%acetic acid（AC-RSD）,2%alfalfa meal+0.25%ammonia（ASAW-RSD）,and 2%alfalfa meal+0.25%acetic acid（ASAC-RSD）.Each treatment was designed with 30 pots each filled soil with 4 kg of soil,and the soil moisture content was 100%saturated water.The reaction vessel was sealed and treated by RSD for 4 months.After soil treatment by RSD,soil samples were collected,watermelon seedlings were planted,and watermelon growth was recorded.Soil samples were collected again after harvesting the watermelons.Soil p H,EC,and the content of NH₄⁺,NO₃^-,and TOC in soils and other physicochemical properties,as well as the abundance and structure of microorganisms such as bacteria,fungi,and Fusarium oxysporum,were analyzed.Results of the laboratory simulation experiment showed that various schemes of RSD for elevating acidic soil p H,decreasing EC,and eliminating the accumulation of NO₃^-had significant effects.The effects were all stable and did not depend on the crops planted in the soil.Among them,the RSD conditions that needed to eliminate soil nitrate accumulation and reduce soil EC were lower than those that needed to increase the p H of acidic soil.Moreover,the contents of available K increased in most soil samples,whereas the contents of available P that decreased in some soils treated by RSD occasionally increased,and the effect was unstable.The number of F.oxysporum in the soils treated by RSD could be reduced effectively,but more rigorous RSD treatment regimens were required and varied among different crops.The effect of RSD on the reduction of F.oxysporum in tomato soil was less stable than that in soil planted with watermelons and strawberries.The stability of efficacy of reducing F.oxysporum improved with increased temperature（>30℃）,prolonged duration（up to5 weeks）,and cow-dung addition into soils during RSD.However,a significant reduction effect on the number of Ralstonia solanacearum in soil treated by RSD was not found because of the small number of R.solanacearum in the tested soil（<5.15 kg copies/g soil）.Conversely,the number of R.solanacearum significantly increased in some samples treated by RSD.Thus,if RSD method was used to eliminate only secondary salinization and a large amount of accumulated NO₃^–in soils,as well as to elevate the p H of acidified soil,the scheme with lower water saturation,shorter duration,and lower temperature should be adopted.If the aim was to reduce soil-borne pathogens,fully saturating the soil water and prolonging the duration at higher temperature would be necessary.Outdoor pot-experiment results showed that even if the daily average temperature was between 0.1°C and 18.7℃during RSD,the killing rate of F.oxysporum remained above 97.7%with increased amount of organic material（2%alfalfa powder added）and prolonged treatment duration to 4 months.The soil microbial activity and community structure also significantly changed.After RSD,the dominant bacteriophyta was Proteobacteria,whereas the dominant eumycota was Ascomycota.Compared with the control,the soil physicochemical properties significantly improved,the biomass of watermelon significantly increased,and the incidence of watermelon decreased from 63.3%to 3.3%–23.3%after RSD.Among them,the highest sterilization rate,the largest biomass of watermelon,and the lowest incidence occurred in ASAC-RSD.Further analysis showed a significant positive correlation existed between the incidence of watermelon and the abundance of F.oxysporum,whereas a significant negative correlation existed between the incidence and microbial activity.Obviously,RSD can improve the yields and reduce the incidence of watermelon by optimizing the soil microbial community structure and reducing the abundance of F.oxysporum,thereby improving the soil physical and chemical properties.After the end of watermelon planting,the abundance of F.oxysporum increased,and the abundance of dominant bacterium and fungus genera produced by RSD treated decreased.Burkholderia became the dominant genus,and the community structure of bacteria and fungi tended to be the community structure of the control.All these findings suggested that RSD can only eliminate the continuous-cropping obstacle factors in soil after watermelon planting but cannot inhibit the formation of continuous-cropping obstacle in the growth process of watermelon.