Effects Of Rice-Rape Rotation And Rice-Wheat Rotation On Nitrogen Use Of Crop And Nitrogen Transformation Of Soil

The Jianghuai region is an important base of grain production in China.Rice-rape rotation is one of the important planting patterns in this region.Recently,rice-rape pattern has rapidly changed to the rice-wheat pattern,attributed to the effects of economic and climatic.There are large differences in management and crops combination between the two patterns,which may have impact on the nitrogen conversion of soils and annual nitrogen uptake of crops.But related studies have not been reported.The effects of conversion from rice-rape pattern to rice-wheat pattern on yield of crops,utilization of nitrogen,nitrification and denitrification of soils were studied with method that combining with filed experiment and indoor incubation.In order to provide a theoretical basis for improving the nitrogen utilization rate of farmland and maintaining the safety of atmosphere and water in the Jianghuai area.The experiment set two treatments containing the rice-rape pattern and rice-wheat pattern.The main findings are as follows:1)The transformation of planting pattern is conducive to increase yield.The annual average yield is increased by 4018kg/hm²,an increase of 37.4%.However,the convertion also caused a significant decrease in nitrogen absorption efficiency,with an average reduction of 7.5%.2)The convertion of planting pattern reduce nitrification potential,because the reduction of nitrification potential in 0-10 cm soil is greater than that of nitrification potential in 10-20 cm soil.The main factors causing the decrease of nitrification potential in 0-10 cm layer as following:the conversion of planting pattern resulted in a significant decrease in soil pH and a significant increase in nitrate nitrogen content,both which inhibited the activity of nitrifying bacteria.In addition,wheat roots can also reduce the nitrification potential of rhizosphere soil.3)After the convertion of the planting pattern,the denitrification capacity（DC）and denitrification potential（DP）of 0-20 cm soil increased significantly.The reduction of pH promotes the activity of denitrifying bacteria during the conversion.The increase of total C and ammonium nitrogen and the returning of straw to the fields provide energy and substrate for denitrifying bacteria.Both of these factors enhanced soil denitrification in the rice-wheat pattern.Environmental factors such as soil pH,total carbon and total nitrogen are important factors influencing the change of denitrifying microbial community structure.This structural change may play an important role in the process of denitrification enhancement.In summary,although the conversion of the rice-rape pattern to rice-wheat pattern is beneficial to increase yield,it will reduce the rate of nitrogen absorption and thus face greater environmental risks.The increasement of nitrification potential in 10-20 cm soil due to the conversion may accelerate the rate of nitrate leaching,which not only reduces nitrogen utilization,but also promotes denitrification.The pattern change significantly increases N₂O/（N₂O+N₂）,so that the N₂O accumulated by denitrification cannot be reduced,increasing the risk of greenhouse gas N₂O emissions from rice-wheat pattern.Therefore,after the conversion of the rice-rape pattern to rice-wheat pattern in the Jianghuai region,the application of nitrogen fertilizer can be appropriately reduced and the straw returning method can be improved.As a result,the environmental hazards that may result from the conversion of the planting pattern can be effectively reduced.