Effect Of Different Preceding Cover Crops On The Abundance Of Soil Microorganisms And The Growth Of Winter Oilseed Rape(Brassica Napus L.)

The legacy effect of preceding cover crops refers to the effects of preceding cover crops on the soil physicochemical properties,soil microorganisms,and subsequent crop growth through a series of biological and abiotic factors.Different crops have different characteristics and cultivation management measures.Based on a 2 years field experiment that involved four treatments,soybean as preceding cover crop?SW?,hairy vetch as preceding cover crop?HW?,maize as preceding cover crop?MW?,fallow as preceding cover crop?FW??as control?,this study described the effects of different treatments on soil nutrients,soil enzyme activities,soil microbial functional gene abundance as well as growth,yield quality of subsequent winter oilseed rape to determine suitable preceding crops for winter oilseed rape production.The main results were as follows:?1?The different treatments have significant effect on rhizosphere and bulk soil nutrients and enzyme activities.In contrast to FW,the SW and HW treatments significantly improved rhizosphere and bulk soil alkali hydrolyzed nitrogen,nitrate nitrogen,ammonium nitrogen and available phosphorous content,decreased p H value at the key growth stages of winter oilseed rape.In addition,the SW and HW treatments can also increase rhizosphere and bulk soil invertase,urease,and alkaline phosphatase activities,compared to FW.Among the leguminous treatments,SW treatment has the most obvious effect of increasing soil available nutrient content and soil enzyme activities.The available soil nutrients,and soil enzyme activities of MW treatment on winter oilseed rape during the key growth period were lower than those of FW treatment.However,different treatments had no obvious effect on rhizosphere and bulk soil total nitrogen,total phosphorous and organic matter content.?2?By analyzing the key microbial functional gene abundance in the rhizosphere and bulk soil of winter oilseed rape at flowering and harvest periods,it can be concluded that the SW and HW treatments improved rhizosphere soil abundance of nif H,Bacterial-amo A?Archeal-amo A gene and decreased abundance of nir S,nir K gene of winter oilseed rape at flowering and harvest periods.There was no significant difference between MW and FW on rhizosphere soil nif H gene,Bacterial-amo A gene,Archeal-amo A gene,nir S gene and nir K gene at flowering and harvest periods of winter oilseed rape.The response of bulk soil microbial functional gene abundance of nif H,Bacterial-amo A,archeal-amo A,nir S and nir K gene to different treatments was different.In addition,there was no significant difference in rhizosphere and bulk soil abundance of 16Sr RNA and 18Sr RNA gene among different treatments at flowering and harvest periods of winter oilseed rape.?3?Compared to FW,the SW and HW treatments can significantly increase the plant height,stem diameter,root length,and aboveground dry matter accumulation,nitrogen and phosphorus accumulation of winter rape at harvest period.Also,The SW and HW treatments exhibited significant effect of improving SPAD value,net photosynthetic rate,stomatal conductance,intercellular CO₂,and transpiration rate of winter oilseed rape at full flowering stage.Among the leguminous treatments,the FW has the most obvious effect of promoting growth of winter oilseed rape.However,the growth indexes of winter oilseed rape in MW treatment are not significantly different from those in FW treatment.?4?The yield and quality of winter oilseed rape varied significantly among different treatments.In contrast to FW,the SW and HW treatments significantly improved seed yield,number of seeds per pod,oil production,protein content.Especially,SW had more improvement than HW in yield and quality of winter rape.However,the yield and quality of winter oilseed rape in MW treatment did not change significantly compared with FW treatment.To sum up,the use of leguminous crops?soybean,hairy vetch?as the preceding crops can increase the soil available nutrient supply,the abundance of rhizosphere soil nitrogen fixing microorganism,ammoxidation microorganism functional genes,improve the growth and quality of winter oilseed rape,and increase winter oilseed rape production.The improvement effect of soybean was the most obvious among leguminous crops.Therefore,soybean and hairy vetch are the reasonable choice of preceding crop for winter oilseed rape during summer fallow.More importantly,soybean is recommended as the best feasible preceding crop to improve the soil environment and optimize seed yield for winter oilseed rape in the Loess Plateau.