| Reasonable application of nitrogen fertilizers can improve crop yield and quality.However,with the rapid development of the intensive agriculture in China,the phenomenon of excessive input of nitrogen fertilizer in various planting systems was common.At the same time,the nitrogen that was not absorded by crops discharged in the form of ammonia volatilization,nitrate nitrogen leaching,nitrous oxide emissions and resulted in a series environment problems.It is urgent to optimize nitrogen management and improve nitrogen utilization efficiency,in order to reduce nitrogen emissions and its environmental risks.Therefore,understanding the nitrogen flow characteristics of different cropping systems and identifying the dominant factors of nitrogen emissions are important fundamental.This study takes winter wheat-summer maize,winter wheat-soybean,peach,greenhouse tomato cropping system in the Central Hebei Plain as the research object.By adopting the method of tracing investigation and nutrient flow model,the different planting systems's nitrogen emission characteristics and its main influencing factors were analysised.On the basis of these work,the mitigation measures were put forward.The main research results are as follows:(1)The total nitrogen emissions,ammonia volatilization,nitrate nitrogen leaching and nitrous oxide emissions of unit area of the four planting systems(winter wheat-summer maize,winter wheat-soybean,peach and greenhouse tomato)are significantly different,which show certain temporal and spatial characteristics.In the system of "winter wheat-summer maize",the 2 years average total nitrogen emissions amount was 84.7 kg N/hm2.Among which,the ammonia volatilization,nitrate nitrogen leaching and nitrous oxide emission amount were 55.1,26.0 and 3.7 kg N/hm2,respectively(showing significant differences).Compared with the first planting rotation cycle,the total nitrogen emissions,ammonia volatilization and nitrate nitrogen leaching amount in the second cycle were increased by 24.7%,33.0%and 54.7%,respectively.Seasonally,the total nitrogen emissions per unit area were significantly higher in autumn,winter and spring than in summer.The total annual average nitrogen emissions per unit area of winter wheat-soybean system were 44.8 kg N/hm2,which ammonia volatilization,nitrate nitrogen leaching and nitrous oxide emissions were 30.1,13.0 and 1.8 kg N/hm2,respectively(showing significant differences).In the peach planting system,the annual average total nitrogen emissions per unit area were 101.2 kg N/hm2,of which ammonia volatilization,nitrate nitrogen leaching and nitrous oxide emissions were 67.3,29.2 and 4.7 kg N/hm2,respectively(showing significant differences).In the second year of peach planting,the total nitrogen emissions,ammonia volatilization and nitrate nitrogen leaching amount per unit area were significantly reduced by 32.9%,33.1%and 35.6%,respectively.The total nitrogen emissions per unit area of the greenhouse tomato planting system were 201.9 kg N/hm2,among which the nitrate nitrogen leaching,ammonia volatilization and nitrous oxide emissions amount showed significant differences and which were 107.8,88.8 and 5.2 kg N/hm2,respectively(showing significant differences).The total nitrogen emissions per unit area of the four planting systems were significantly different.(2)Nitrogen emissions of the four planting systems are closely related to management factors.The total nitrogen emissions increased significantly with the increase of nitrogen application amount or irrigation frequency.The correlation between planting density of peach and tomato and total nitrogen emissions were y=0.0943x-130.75,R2=0.3881**,y=0.3059x-26.271,R2=0.4369**.The nitrogen fertilizer inputs per unit area of the four planting systems(winter wheat-summer maize,winter-soybean,peach and greenhouse tomato)were 424.4,246.8,763.9,and 1084.4 kg N/hm2,respectively.Among the four planting systems,the irrigation freguency of summer maize and soybean were significantly different from other planting systems.(3)The total nitrogen emissions per unit area was affected significantly by the soil texture in the four cropping systems(winter wheat-summer maize,winter wheat-soybean,peach and greenhouse tomato).With the decrease of soil clay,the total nitrogen emissions per unit area were increased.The total nitrogen emissions per unit area of winter wheat-summer maize system in the loam,sand loam and sandy soil were 69.4,87.2 and 95.9 kg N/hm2,respectively,and in the winter wheat-soybean system were 32.0,49.5 and 53.4 kg N/hm2,respectively,and 89.0,124.4 and 135.8 kg N/hm2 of peach system.The total nitrogen emissions per unit area of tomato system in the loam,sand loam were 184.9 kg and 209.0 kg N/hm2.Soil nitrate accumulation is also related to planting system,the nitrogen leaching risk in peach planting system was much higher than that in winter wheat-summer maize system.The nitrogen emissions of the four planting systems were also affected by the farmers' education level.(4)Under the conventional management,the per unit total nitrogen emissions amount of winter wheat,summer maize,soybean,peach,greenhouse tomato were 47.2,36.4,1.7,120.0,201.9 kg N/hm2.By optimizing the application amount of nitrogen fertilizer,irrigation frequency,sowing date and sowing quantity,the winter wheat,summer maize,soybean,peach,greenhouse tomato can achieve the highest yield,and nitrogen emissions can be reduced by 50.5%,15.1%,28.2%,7.0%and 12.4%.On the basis of the most high-yield management measures,if the nitrogen application and irrigation times are reduced appropriately,the yield is still at the high yield level,but the nitrogen emissions can effectively reduced.The optimal nitrogen application amounts were 222?242 kg/hm2,165?185 kg/hm2,2.5?22.5 kg/hm2,556?656 kg/hm2 and 775?875 kg/hm2 of the winter wheat,summer maize,soybean,peach and greenhouse tomato.The total nitrogen emissions of winter wheat-summer,winter wheat-soybean,peach and greenhouse tomato per unit area of farmland are about 81.4,43.1,71.8 and 191.8 kg N/hm2,respectively.The irrigation frequency were in the range of 2?4 times,1?2 times,6?8 times and 4?6 times of the winter wheat,summer maize,peach and greenhouse tomato,respectively.The total nitrogen emissions of winter wheat-summer maize,peach and gerrenhouse tomato per unit area of farmland were about 45.2,112.7 and 190.1 kg N/hm2,respectively.In each planting system,the total nitrogen emissions per unit area of farmers who believed that fertilization would cause agricultural environment pollution were significantly lower than those who did not believe.To sum up,the nitrogen emissions from farmland can be reduced by adjusting crop layout and crop management measures,in a certain extent reduce nitrogen application amount,decrease irrigation frequency and planting density,improve the farmers' awareness about agricultural environment. |
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