Study Of Environmental Benefits Of Winter Oilseed Rape In Yangtze River Basin Based On Nitrogen Benchmark And Emergy Analysis

With the increasing population and people’s desire for a better standard of living,China’s resource consumption is increasing.Vegetable oil accounts for the main part of China’s consumption of oil,but China’s edible vegetable oil largely depends on imports,with the self-sufficiency rate less than 40%.Oilseed rape is the largest oil crop in China.With the national attention to edible oil security,oilseed rape planting area has been increasing year by year in recent years.However,in order to obtain higher yield,some farmers still applied large amounts of nitrogen(N)fertilizer,which brought potential risks to the environment and unreasonable management measures,resulting in low N use efficiency of oilseed rape.Therefore,based on field test data,farmer survey data,meteorological data and soil property data,this study estimated the N surplus benchamrk and N leaching benchmark of winter oilseed rape producing areas in the Yangtze River Basin,and evaluated the sustainability of oilseed rape product-processing process based on emergy analysis.The main conclusions are as follows:(1)The response of oilseed rape relative yield to N application rate was linear plus platform model.Taking 90%to 95%of the relative yield as the target yield,the agricultural threshold of N application rate(N input)was 146.8 to 164.9 kg N/hm~2,and the corresponding N surplus was 42.3 to 57.0 kg N/hm~2.With the increase of N application rate and N surplus,ammonia volatilization increased linearly,while direct nitrous oxide emission increased exponentially.Taking 57.0 kg N/hm~2 as the N surplus threshold,45.7%～65.5%of farmers in different regions of the Yangtze River Basin exceeded the benchmark,and the active N loss per unit area was higher in the southwest and lower reaches of the Upper Yangtze River,while the active N loss per unit yield was higher in the Triple cropping of Southern.(2)The evapotranspiration of winter oilseed rape in the Yangtze River Basin was calculated by collecting oilseed rape seasonal meteorological data from 1991 to 2020 and Penman-Monteith model.The results showed that the temperature was above 10℃during the growth period of winter oilseed rape in the Yangtze River Basin,and the precipitation was sufficient.The evapotranspiration gradually increased from north to south,and the evapotranspiration showed a decreasing trend from west to east.Based on the water balance model and eu groundwater nitrate limit,we estimated the seasonal N leaching benchmark of winter oilseed rape in 931 counties in the Yangtze River Basin.The regional variation characteristics of N leaching threshold of oilseed rape were highly similar to that of precipitation.The N leaching benchmark was the highest in the southern region of the lower reaches of the Yangtze River with abundant precipitation and low evapotranspiration,and the lowest in the upper reaches of the Yangtze River.The average benchmark of N leaching in the Yangtze River Basin was 34.0 kg N/hm~2(7.9～75.1 kg N/hm~2),and the average threshold of N leaching in the Yangtze River Basin was 345.4 kg N/hm~2(201.6～490.3 kg N/hm~2).Only 10.0%of farmers exceeded the benchmark,and the upper reaches of the Yangtze River and the northern reaches of the Middle reaches of the Yangtze River with lower precipitation had higher risk of N leaching.(3)Based on the household survey and food processing enterprises and combined with emergy analysis,the emergy input composition and the sustainability of winter oilseed rape production system in the Yangtze River Basin were evaluated.According to the results,straw returning,electric power,geothermal energy,N fertilizer and rainwater chemical energy were the main inputs of energy,while rape straw,meal and rapeseed oil were the main outputs of energy.According to emergy analysis,a variety of emergy indexes were provided.In different N application levels,the economic benefit and environmental benefit reached a balance when the N application rate was 150～210 kg N/hm~2,and the production efficiency of the system was high while the pressure on the environment was also at a low level.In the whole region,the winter oilseed rape production system had low environmental load rate and low environmental pressure,but low production efficiency.In conclusion,the agronomic thresholds of N fertilizer input for winter oilseed rape production in the Yangtze River Basin were 146.8～164.9 kg N/hm~2,and the environmental thresholds of N leaching were 201.6～490.3 kg N/hm~2.Under the farming conditions,the economic and environmental benefits can be taken into account when the N application rate is 150～210 kg N/hm~2.In terms of energy input,straw,electricity and N fertilizer accounted for the main parts,and the main output was oilseed rape straw,cake and rapeseed oil.Based on the current evaluation,winter oilseed rape planting system has low environmental pressure,but low production efficiency.