Effects And Potential Of Nitrogen Fertilizer Replacement By Green Manure In Spring Rapeseed Area

Spring rapeseed is the main oil crop in northwest China,which occupies an important position in agricultural production.However,excessive fertilization is common in spring rapeseed production.Excessive fertilization will not only affect the yield and quality of rapeseed,lead to an increase in planting costs,but also reduce fertilizer use efficiency,resulting in greater environmental risks.It is not conducive to the green and sustainable development of agriculture in China.As a clean fertilizer source with relatively complete nutrients,can green manure play a role in replacing chemical fertilizer in spring rapeseed production?What is the potential to replace chemical fertilizers?Field experiments were carried out in Qinghai Ping'an and Gansu Minle in this study.The main treatments were different legume green manure,including common vetch,hairy vetch and fallow;The side treatment was different nitrogen application rates in spring rapeseed growing season.The aim of this study was to explore the effects of green manure on yield,nutrient absorption and utilization,quality and soil nutrients of spring rapeseed.The nitrogen application levels of spring rapeseed in Qinghai Ping'an were 0(N0),40(N40),80(N80),120(N120,the lower limit of local recommended nitrogen application),160(N160,the upper limit of local recommended nitrogen application)and 200(N200)kg/hm~2,the tested spring rapeseed was Qingza 5.The nitrogen application levels of spring rapeseed in Gansu Minle were 0(N0),40(N40),80(N80),120(N120),180(N180,the local optimized nitrogen application)and240(N240,the local conventional nitrogen application)kg/hm~2,the tested spring rapeseed was Shengguang 402.The main results are as follows:(1)The effects of legume green manure and nitrogen application rate on the yield of spring rapeseed were significantly different in the two places,which may be related to the soil mineral nitrogen content.The mineral nitrogen content in soil profile of Qinghai Ping'an spring rapeseed field was lower,and planting leguminous green manure significantly increased grain yield.Under different legume green manure,the optimal nitrogen reduction treatments were common vetch+N80,hairy vetch+N40 and fallow+N160,respectively.Compared with the local recommended nitrogen application(120?160 kg/hm~2),the amount of nitrogen application can be reduced by 33%?50%when turning over common vetch and 67%?75%when turning hairy vetch.Green manure planting had no significant effect on the grain yield of spring rapeseed in Gansu Minle.The optimal nitrogen reduction treatments under different green manure conditions were common vetch+N80,hairy vetch+N80 and fallow+N120,respectively.Compared with the local optimized nitrogen application(N180),the amount of nitrogen application can be reduced by 56%when turning over green manure.(2)Compared with fallow,planting leguminous green manure significantly increased the protein content and decreased the oil content of spring rapeseed grain.With the increase of nitrogen application rate,the oil content of rapeseed grain decreased,and reducing nitrogen application rate is helpful to improve the quality of rapeseed.(3)The analysis of soil nitrogen balance in spring rapeseed field showed that when leguminous green manure was turned over,the apparent surplus of soil nitrogen in spring rapeseed field was significantly higher than that in fallow.In Qinghai Ping'an,leguminous green manure improved the soil nitrogen deficit under the condition of low nitrogen application(0?120 kg/hm~2),while in Gansu Minle leguminous green manure increased the soil nitrogen surplus.(4)More than 50%of the nitrogen absorbed by spring rapeseed comes from soil,indicating that soil is the main source of nitrogen absorption and accumulation of aboveground nitrogen in spring rapeseed.In Qinghai Ping'an,the mineral fertilizer equivalent value of leguminous green manure is 74.96%?99.62%,with an average of 87.29%.In Gansu Minle,the mineral fertilizer equivalent value of leguminous green manure is 67.23%?88.45%,with an average of 77.89%.(5)In Qinghai Ping'an,the increase effect of green manure on the economic benefit of spring rapeseed was better than that of fallow,and the increases of common vetch and hairy vetch were 18.55%and 24.57%,respectively.In Gansu Minle,the economic benefit of green manure was lower than that of fallow.Under fallow conditions,the net income of N120 was significantly higher than that of N240,and there was no significant difference with N180,and33%and 50%nitrogen fertilizer could be saved at this time.To sum up,the effect of leguminous green manure instead of chemical nitrogen fertilizer in spring rapeseed area is related to the content of mineral nitrogen in soil profile before sowing.In Qinghai Ping'an,the soil mineral nitrogen content is low,planting overturned common vetch and hairy vetch can replace 33%?75%inorganic nitrogen fertilizer,with a potential of 75%?100%.According to grain yield and soil nitrogen apparent balance of spring rapeseed,it is suggested that hairy vetch should be combined with 40 kg/hm~2 nitrogen fertilizer.In Gansu Minle,the soil mineral nitrogen content is high,planting leguminous green manure does not increase the grain yield of spring rapeseed and reduces the economic benefit.Therefore,it is recommended that leguminous green manure should not be planted in that year,and the amount of nitrogen application should be directly reduced to 120 kg/hm~2.