Study On Nitrogen Balance Characteristics Under The Reduction Of Controlled-release Urea In Double-rice Cropping Field Of Hunan

Nitrogen?N?application in large quantity will result in N loss in paddy field.The systematic study of N input and output in the rice field ecosystem,and the non-point source pollution control technology based on N management of farmland will play a positive role in promoting the level of agricultural production and improving the ecological quality of the environment,and have a long-term guiding significance for the sustainable development of regional agricultural economy and ecological environment.A long-term experiment was conducted to compare the input,output and input-output characteristics of N under the reduction of urea?U?and controlled release urea?CRU?during 2016-2017 in a typical double-cropping paddy field of Hunan,and the amount of N surplus was calculated.Fertilization treatments included zero-N(CK,control,N 0 kg·ha^-1),conventional urea(U,N180 kg·ha^-1),and four polymer-coated urea fertilization levels,i.e.,1.0 CRU,0.9 CRU,0.8CRU,0.7 CRU,which represented 0,10%,20%and 30%reduction of fertilizer inputs relative to U,respectively.Results showed that:1.Controlled-release urea reduced application on N absorption characters in double-rice cropping field:U treatment obtained average grain yield of 11.52 t hm^-2 for double-rice season,and average nitrogen recovery use efficiency?NRE?20.93%for single-rice season.The N release of CRU coincided with the N period required for rice production,and the application of CRU significantly increased N accumulation and rice yield at the mature stage of double cropping rice.CRU treatments?1.0CRU,0.9CRU,0.8CRU,0.7CRU?of double cropping rice produced 12.45-13.80 t·hm^-2 and 12.14-12.94t·hm^-2 in 2016 and 2017,respectively.The average annual yield was highest at 0.9CRU treatment(13.28 t·hm^-2),significantly higher than 1.0CRU,0.7CRU and U treatments.0.8CRU treatment took second place,not significantly different from that of 0.9 CRU treatment.There was a significant positive correlation between effective spike and yield of rice,and CRU in increasing the yield of late rice was superior to early rice.The NRE,N agronomic efficiency,physiological efficiency,partial productivity and harvest index under CRU treatment decreased with the increase of N rate.The average NRE in single season was34.40%-40.05%in average,which was significantly higher than that under U treatment?64.36%-91.36%?.There was no significant difference between 0.9cru,0.8cru and 0.7cru treatments.Therefore,considering the yield and N fertilizer utilization rate,the author considered 0.9CRU and 0.8CRU as a more appropriate N management mode under the conditions of this study.2.Controlled-release urea reduced application on the characteristics of gaseous loss of N in double-rice cropping field:NH₃ volatilization in early rice after basal and tillering fertilization occurred within 8-9 days after fertilization,and peaked at 1-4 days,respectively.NH₃ volatilization in late rice occurred within 6-9 days after fertilization,and peaked at 2-3 days and 3-4 days,respectively.Under the influence of fertilization,NH₃volatilization loss of tillering fertilizer was higher than that of base fertilizer.Under the influence of temperature and rainfall,high temperature and low rainfall were beneficial to NH₃ volatilization emissions from rice fields in late rice growing season.In the growing season of double-cropping rice,the total volatile loss?rate?of NH₃ under U treatment was the highest,with a two-year average of N 110.25 kg·hm^-2?26.82%?.The total NH₃ volatiles loss?rate?under CRU treatments,with a two-year average of N 58.10-77.69kg·hm^-2?15.74%-16.95%?,increased with the increase of fertilizer application.Compared with U treatment,the total NH₃ volatilization loss in 1.0CRU,0.9CRU,0.8CRU and0.7CRU treatments was significantly reduced by 29.53%,37.03%,42.25%and 43.45%,respectively.Sunning period is the most important period of N₂O emission in rice field.The total N₂O emission in double cropping rice growing season is the highest under U treatment(3.01 kg·hm^-2-3.23kg·hm^-2),and the N₂O loss rate is 0.49%-0.52%.CRU application resulted in a significant reduction in N₂O emission,which decreased with the decrease of CRU rate.Compared with U treatment,N₂O emission during the double cropping rice growing seasons under 0.9CRU,0.8CRU and 0.7CRU treatment were reduced with the average emission by 15.71%,20.99%and 25.48%,respectively.At the same time,the equation fitting showed that 0.8CRU and 0.9CRU treatments within the range set for this experiment were the relatively ideal N rate of CRU treatment.3.Controlled-release urea reduced application on the characteristics of liquid N loss in double cropping paddy field:The peak of N runoff and leakage appeared in the early stage of fertilization in paddy field,which was the critical period for prevention and control of N liquid loss.Under the influence of rainfall,runoff events tend to occur in the early-rice season,and the rainfall intensity will increase the liquid loss load of TN.With the decrease of CRU-N applied,its liquid loss?rate?decreased gradually.The main form of runoff water treated with N was NH₄⁺-N,accounting for more than 60%of TN.In the early and late rice growing seasons,the TN loss?rate?of runoff water treated by U treatement were 5.99 kg·hm^-2?2.40%?and 5.81 kg·hm^-2?2.11%?in average,respectively.The TN losses of runoff water under CRU treatments were 13.86%-35.39%and 14.54%-34.34%lower than that under U treatment,respectively.The leaching loss?rate?of TN in U treatment was the highest in the early-,late-rice seasons and double-cropping rice seasons,with the two-year average of 25.62 kg·hm^-2?13.12%?,25.99 kg·hm^-2?12.67%?and 51.61 kg·hm^-2?12.33%?,respectively.TN leaching loss in 0.8CRU and 0.7CRU treatments in double cropping rice growing seasons in average was 40.75 and 36.22 kg·hm^-2,respectively,which was significantly lower than that in U treatment by 21.04%and 29.83%,respectively.Taking the yield and emission reduction effect of double-cropping rice into consideration,we believe that 0.8CRU and 0.9CRU treatments under this experimental condition can obtain higher double-cropping rice yield and significantly reduce TN runoff loss and leakage loss risk of double-cropping rice fields.4.Controlled-release urea reduced application on the characteristics of soil N residue from double-rice cropping field:with the increase of soil depth,total N,NO₃^--N content in soil was in decline.Rational application of CRU can significantly improve the content of total N and NO₃^--N in 0-20 cm soil of paddy field.Inorganic N residues in the soil increased with the N rate of CRU increased.A moderate amount of CRU was effective at increasing inorganic N residues in the soil,and there was a risk of depleting the land with excessive reduction of CRU?e.g.0.7CRU?.Thus,under the conditions of this experiment,treatment with 0.8CRU and 0.9CRU treatments can obtain higher double-cropping rice yield and better soil inorganic N residues in the paddy field.5.Characteristics of annual N budget of double-cropping rice field under controlled release urea reduced application:in the study area,the N surplus of the whole system of double-cropping rice field treated with conventional fertilization was N 53.26kg·hm^-2·a^-1 due to high fertilization input.Fertilizer input is the main source of N in farmland.Although crop harvest is the main mode of N output of farmland,the loss of N caused by denitrification,NH₃ volatilization and leaching loss is also considerable.The reduced application of CRU can effectively improve plant nutrient uptake,reduce nutrient loss in the field,and effectively reduce soil N nutrient surplus,which is reduced with the reduction of N dosage,especially in 0.8CRU and 0.7CRU treatment.