Nitrogen release characteristic of polymer coated urea in paddy soil and its relationship with cumulative temperature

Abstract: Single application of fertilizer is more than necessary for rice (Oryza sativa L. subsp. Japonica) production in southern China, which is in shortage of agricultural labor and keeps high potential to mechanized production. However, split applications of conventional urea play a key role on rice production in this area. To detect nitrogen (N) supply from controlled release fertilizer by single application, a field experiment was conducted during 3 years (from 2012 to 2014) in Mashan Station of Agricultural Bureau of Shaoxing, Zhejiang Province. In the study, a kind of polymer coated urea (PCU) for single application was investigated. The N release characteristics of PCU were studied in the rice field with buried bag method. Treatment plot of PCU was divided into fertilization area and buried bag area. A total of 150 PCU bags were buried each year. According to rice growth stage, 5 bags each time were taken out for one plot. Then the coated urea samples were slightly washed by distilled water and ground using a hand mill. The residual urea was dissolved into distilled water, and the N release from residue PCU was measured using the spectrophotometric method. The result of three-year field experiment suggested that the released period was about 5 months under field condition, which was longer than the designation of PCU controlled release period (4 months). Duration from transplanting to harvesting was 152 d in 2012, 158 d in 2013, and 142 d in 2014. Corresponding cumulative N release rates of PCU in the whole growth stage were 73%, 83% and 69%, respectively. The relationship between cumulative N release rate and number of days after transplanting was fitted by a quadratic equation, and values of R2 were 0.991 in 2012, 0.997 in 2013, and 0.984 in 2014. In our study, no lag period was determined for this product in the field. When the coated urea was applied into paddy soil, it started to release N. The N preliminary solubility rate was 0.36%-3.75%. It was 3.75% in 2014, which was much higher than the other 2 years. There was only 0.36% N released out in 24 h in 2012 and less than 0.9% in 2013. Its release process could be divided into 2 phases: quick release phase and slow release phase. Daily N release rate of PCU was around 0.5% in the whole rice growth stage, which was 0.48% in 2012, 0.49% in 2013, and 0.5% in 2014, respectively. In the first 2 months, the daily release rate was around 1% in most of time, and gradually slowed down after then. Soil relative moisture had differences among 3 years, which was 72%-80% in 2012, 60%-77% in 2013 and 80%-99% in 2014, respectively, and all of these were over 60%. When soil moisture is sufficient, temperature is another and even only important factor to influence N release rate of PCU in the field. The study showed that cumulative N release rate was closely correlated with cumulative temperature. Cumulative soil temperature and cumulative air temperature showed similar regulations. Cumulative N release rate of PCU reached about 50% when cumulative temperature reached about 2 000℃·d. According to the experiment model above, people can hold the release regulation of PCU when this kind of urea is applied. Nitrogen release characteristics of PCU suggest that it can match the N need of rice and decrease the N loss to surrounding environment. Single application of polymer coated urea need to be concerned for famers in future.