Nitrogen Regulation And Environmental Emission Reduction Of Hybrid Maize Seed Production In Northwest Of China

Increasing crop yield and reducing environmental cost are two challenges for agricultural green development nowadays.The application of nitrogen fertilizer is of great significance to global food security,but the irrational application of nitrogen fertilizer has caused serious environmental problems.Maize is an important grain,feed and biological energy crop globally,and hybrid maize seed production is the cornerstone to ensure maize production and national food security.In this paper,life cycle assessment（LCA）and soil nitrogen balance model were firstly used to evaluate the agronomic and environmental effects of hybrid maize seed production.Two years field positioning experiment were carried out to study the potential of increasing yield,enhancing efficiency and reducing emissions potential of hybrid maize seed with different nitrogen levels and different types of nitrogen fertilizer.The new approaches and methods of nitrogen management of hybrid maize seed production were explored.The main results were as follows:（1）The reactive nitrogen loss and greenhouse gas emissions of 150 farmers from 6companies in the hybrid maize seed production region of northwest China were quantified by life cycle assessment（LCA）.The results indicated that the average yield was 7.0 Mg ha^-1,the average reactive nitrogen loss was 53 kg N ha^-1,and greenhouse gas（GHG）emissions were 8077 kg CO₂ eq ha^-1.The average nitrogen and carbon footprints of the process were 12.2 kg N Mg^-1 and 1495 kg CO₂ eq Mg^-1,respectively.Nitrogen fertilizer and electricity consumption for irrigation were the main contributors to high GHG emissions,accounting for 60%and 30%of the total,The GHG emissions from seed production for different companies varied greatly with their resource input.Based on an analysis of the yield group,we found that the carbon footprint of the first group（the one with the highest yield）was 27%lower than the overall average.Scenario analysis suggests that a combined reduction of N input rate,optimizing irrigation,and increasing yield can eventually mitigate the carbon footprint of hybrid maize seed production by 37%.An integrated systematic approach（e.g.,ISSM:integrated soil-crop system management）can reduce the GHG emissions involved in producing hybrid maize seeds.This study provides quantitative evidence and a potential strategy for GHG emissions reduction of hybrid maize seed production.（2）Field plot experiments on nitrogen level of hybrid maize seed production for two consecutive years were carried on.The results showed that the accumulation of parental biomass of hybrid maize seed production increased with the increase of nitrogen gradients.The yield of maize seed production increased with the nitrogen gradients as linear plus platform trend.N240 treatment achieved higher yield,nitrogen use efficiency and grain nitrogen concentration at the same time,which were consistent in the two years.The yield of N168treatment was higher than that of N240 treatment in the second year,but the nitrogen concentration was lower than that of N240 treatment.The nitrogen concentration of the whole plant of female parent straw and male parent was higher in high nitrogen gradient treatment than that in low nitrogen gradient treatment;the critical nitrogen concentration of maximum biomass of female parent at filling stage was 15.08 g kg^-1,and there was a linear correlation between female parent biomass and nitrogen concentration at harvest stage.The post-silking biomass of each topdressing nitrogen fertilizer treatment was higher than that of pre-silking stages in two years,and increased with the increase of nitrogen gradients.The change rule of post-silking nitrogen uptake ratio with nitrogen gradients was consistent with the biomass.There was no significant difference in yield level,biomass accumulation,and nitrogen uptake between N320treatment and N240 treatment.N240 could be regarded as the recommended nitrogen application rate for hybrid maize seed production in this region.Optimized nitrogen application increases yield and improve nitrogen use efficiency by regulating the biomass accumulation,nitrogen concentration,and nitrogen absorption ratio pre-and post-silking of hybrid maize seed production.In this study,the critical nitrogen concentration to achieve the maximum biomass in the key growth period of hybrid maize seed production was identified for the first time,and it was revealed that the key to high yield and efficiency of maize seed production was to stabilize the accumulation of pre-silking biomass and increase nitrogen supply post-silking by optimizing the gradient of applied nitrogen.（3）Based on the field localization experiment of different nitrogen supply levels of hybrid maize seed production,the environmental effects of different treatments were compared by using life cycle assessment（LCA）combined with nitrogen balance model.The results showed that the optimized nitrogen level（N240）increased nitrogen uptake of maize plant above the ground and without excessive nitrogen surplus or nitrogen deficit.The average nitrogen surplus of optimized nitrogen treatment（N240）was 12 kg N ha^-1,which was 83%lower than that of excessive nitrogen application（N320）,and the average apparent nitrogen loss of N240 was 75 kg N ha^-1,which was 46%lower than that of excessive nitrogen application（N320）.There was no significant difference in yield between N240 and N320 treatments.The reactive nitrogen loss（Nr）and nitrogen footprint（NF）of N240 were 24.4 kg N ha^-1 and 8.1 kg N Mg^-1,respectively,which were 24%Nr and 10%CF lower than those of N320,respectively.The greenhouse gas emissions（GHG）and carbon footprint（CF）of N240 are 5783 kg CO₂eq ha^-1 and 1108 kg CO₂ eq mg^-1,which are 14%GHG and 11%CF lower than that of N320,respectively.Electricity emissions for seed corn irrigation and emissions due to nitrogen application account for 43%and 45%of total GHG emissions,respectively.Under the comprehensive consideration of yield,quality and environmental effects,N240 ensured nitrogen accumulation,increased grain yield,and was the best choice for high yield and low environmental cost of hybrid maize seed production.（4）Field experiments were carried out for two consecutive years to study the effects of different new types of nitrogen fertilizer on agronomic effects of seed maize.In the first year,the yield of UAN+UI and controlled release fertilizer（CRU）treatment was 85%and 80%higher than that of control（CK）,respectively.The yield of ammonium urea nitrate solution（UAN）and ammonium urea nitrate combined with nitrification inhibitor（UAN+NI）were lower than that of ammonium urea nitrate solution combined with urease inhibitor（UAN+UI）by 24%and 28%.In the second year,UAN+UI had the highest production(8.8 Mg ha^-1).There was no significant difference in the production of Urea,CRU and UAN,while UAN+NI had the lowest production(6.9 Mg ha^-1).Grain number per ear had the most significant effect on yield,but 100 grain weight had no significant difference among treatments.The variation range of maternal biomass was2.8-18.4 Mg ha^-1,and the highest UAN+UI was 20.6 Mg ha^-1 at harvest,which was7.3%,12.6%,18.4%and 24.8%higher than CRU,Urea,UAN and UAN+NI,respectively.Compared with Urea,all the new nitrogen fertilizers except UAN+NI increased grain nitrogen concentration.UAN+UI treatment grain nitrogen concentration was 15.2 g·kg^-1,followed by UAN 14.7 g·kg^-1,CRU grain nitrogen concentration was not significantly different from Urea.UAN+NI was the lowest,which was 12.2%and9.5%lower than UAN and CRU treatment,respectively.The variation range of female parent biomass was 2.8-18.4 Mg ha^-1.In the 0-30 cm soil layer,the nitrate concentration of CRU was 35.4 mg kg^-1,which was significantly higher than that of other treatments,and the nitrate concentration of UAN+NI was 13.3 mg kg^-1.The analysis of the agronomic effects of different nitrogen forms will help us to screen more suitable new nitrogen fertilizers,and provide a scientific basis for improving the yield and fertilizer utilization efficiency of seed maize,and reducing nitrogen surplus and apparent loss.（5）The environmental effects,economic benefits and ecological economic benefits of hybrid maize seed production were analyzed based on two consecutive field experiments with different new nitrogen fertilizers.Compared with Urea treatment,CRU,UAN,UAN+NI and UAN+UI reduced the reactive nitrogen loss by 43%,9%,37%and 55%,respectively,and CRU reduced the N₂O emission by 39%compared with Urea treatment.UAN reduced N₂O emission by 38%compared with Urea treatment,and UAN+NI and UAN+UI reduced N2O emission by 60%and 50%compared with UAN,respectively.The greenhouse gas emission per unit area of the new nitrogen fertilizer was 2%-10%lower than that of Urea treatment.In the field application stage,the emission of CRU,UAN+NI and UAN+UI treatment was 39%,56%and 57%lower than that of Urea treatment,respectively.The acidification potential of urea treatment was 132 kg SO₂ eq ha^-1,which was 50%,56%,24%and 86%higher than CRU,UAN,UAN+NI and UAN+UI,respectively.The field application of nitrogen fertilizer accounted for 75%of the total eutrophication potential.The eutrophication potential of urea treatment was 36.5 kg PO₄ eq ha^-1,59%,44%and 87%higher than that of CRU,UAN+NI and UAN+UI,respectively.The energy loss of irrigation power accounted for66%of the total consumption.The highest environmental damage cost was Urea treatment,which was 22%,18%,16%and 36%higher than the new nitrogen fertilizers CRU,UAN,UAN+NI and UAN+UI,respectively.UAN+UI has the highest ecological and economic benefit,with an average value of 25.7×10³￥ha^-1 in two years,followed by CRU treatment with an average value of 22.6×10³￥ha^-1.UAN+NI has the lowest ecological and economic benefit,27%lower than UAN treatment.UAN+UI has the highest ecological and economic benefits,followed by CRU.Both of them can be used as new nitrogen fertilizer in hybrid maize seed production.Three scientific research prospects for the application of new fertilizers:（i）the cost of new fertilizers can be reduced by government subsidies to farmers;（ii）new nitrogen fertilizer manufacturers can be encouraged to carry out product research and development and reduce prices by means of tax exemption;（iii）guideline standards for crop planting fertilizer input can be issued to control the input amount of nitrogen fertilizer in each crop system.