The Study On Nitrogen Fate Of Double Cropping Rice In Hunan Province

With the continuous increase of N fertilizer input in rice fields,it is a major challenge to reduce the amount of N fertilizer,the loss of N fertilizer and the negative impact on the environment while realizing the stable increase of rice yield in China.It is of great significance for the sustainable development of paddy field and agricultural ecological environment to systematically understand the source and direction of N in paddy ecosystem,that is,to explore the basic laws of N input and output pathways,and then to propose and formulate scientific and reasonable N fertilizer application and management measures according to these laws.This study conducted a field plot experiment and a ¹⁵N micro-plot experiment in Hunan double cropping rice area in2017-2018,set four treatments N0（no nitrogen fertilizer）,N1（with nitrogen fertilizer90 kg/hm² in both early and late rice）,N2（120 kg/hm² of nitrogen fertilizer in early rice,135 kg/hm² of nitrogen fertilizer in late rice）and N3（150 kg/hm² of nitrogen fertilizer in early rice,180 kg/hm² of nitrogen fertilizer in late rice）according to the N application level（based on pure N）.The field plot experiment was to study the nitrogen absorption and the loss of nitrogen in various ways of double-cropping rice for two consecutive years in 2017-2018,calculating the annual nitrogen surplus,preliminarily revealing the characteristics of nitrogen budget in double cropping rice fields.¹⁵N labeling-urea was applied in the microzone in 2017,the absorption and utilization of ¹⁵N,the residue of ¹⁵N in the soil and the loss rate of ¹⁵N in each treatment were studied,and the different directions and proportions of fertilizer ¹⁵N were determined,the same amount of unlabeled urea was applied in 2018,analyzing the absorption and utilization of residual ¹⁵N,the total loss rate of residual ¹⁵N in each treatment.The results of the study are as following:1.On the nitrogen absorption,rice yield increased significantly with the increase of nitrogen application,N2 and N3 were significantly higher than N1,while N3 was higher than N2 without significant difference,the grain yield of double cropping rice was 8 869.6～11 002.1 kg/hm²,straw yield was 8 666.2～10 744.2 kg/hm².Nitrogen accumulation in rice also increased significantly with nitrogen application,the average nitrogen uptake of single cropping rice was 70.6～112.5 kg/hm²,and the nitrogen uptake of double cropping rice was 140.88～226.5 kg/hm².The average nitrogen uptake efficiency was 25.6%～28.7%,6.5～8.3 kg/kg in agronomic,23.8～27.0kg/kg in physiological,and 33.5～56.1 kg/kg in partial productivity,the nitrogen uptake efficiency of N2 treatment was highest.2.On the nitrogen loss,the loss of N3 treatment was the highest,N2 was slightly higher than N1,but the difference was not significant,the annual ammonia volatization loss of each treatment was 20.04～111.97 kg/hm²and the loss rate was22.33%～26.68%,the N₂O emission loss was 1.38～3.15 kg/hm²and the loss rate was0.49%～0.72%,the N leaching loss was 5.10～40.97 kg/hm²and the loss rate was8.63%～10.87%,the N runoff loss was 3.78～12.98 kg/hm² and the loss rate was1.67%～3.38%,the single annual soil inorganic nitrogen residue was-5.70～41.53kg/hm²,and the total nitrogen residue was-15.18～53.02 kg/hm².3.On the nitrogen budget,the nitrogen surplus in each treatment was increased with the increase of nitrogen application,the nitrogen surplus of N3 treatment was the highest,N2 was slightly higher than N1,the nitrogen surplus was 13.05～32.20 kg/hm²in each nitrogen treatment in 2017,it was 29.18～39.90 kg/hm²in 2018,and the annual nitrogen surplus showed an upward trend.It showed that fertilization was the most important source of nitrogen in double cropping rice field N cycle,the nitrogen output was dominated by crop absorption,ammonia volatilization and nitrogen leaching loss were also important N output pathways.N2 treatment is an appropriate level of nitrogen application,that is to reduce nitrogen by 20%～25%on the basis of farmers’customary application of fertilizer（N3）,which can not only ensure the nitrogen absorption and use efficiency of double cropping rice,but also reduce the nitrogen loss and systematic nitrogen surplus.4.¹⁵N experiment results showed,the results of difference subtraction method showed that the nitrogen absorption use efficiency decreased significantly with the increase of nitrogen application in the microzone.In 2017,the nitrogen absorption use efficiency of early and late rice was 42.14%～46.62%and 35.45%～43.08%,respectively,37.93%～42.56%and 37.20%～44.51%in 2018.The ¹⁵N tracer method results showed that in 2017,the ¹⁵N recovery rate of early rice under different treatments was 24.49%～24.53%,and late rice was 25.32%～26.59%,late rice was slightly higher than early rice.The ¹⁵N recovery rate of all treatments was similar,with no significant difference.Nitrogen accumulation in rice mainly comes from soil,and the contribution rate of soil N is more than 71.16%.The direction of ¹⁵N fertilizer in each treatment was basically the same,the crop absorption,soil residues and total losses accounted for about 25%,23%and 52%,respectively.Fertilizer ¹⁵N mainly remained in the 0～20 cm soil layer,accounting for about 77%of the total residual amount,about 19%in the 20～40 cm soil layer,and about 4%in the 40～60 cm soil layer.The residual fertilizer N from the last rice season can be absorbed and utilized by the next rice season,which becomes the supplement of soil nitrogen pool.The residual N in the soil at 0～20 cm has the best residual effect.In 2018,the cumulative absorption rate of the residual ¹⁵N in early and late rice is 8.13%～9.28%,the cumulative loss rate is 38.68%～52.97%,and the final residual rate is38.90%～52.05%.