Effect of combined application of controlled-release urea and conventional urea under reduced N rate on yield and N utilization efficiency of rice

The effects of different blending rates of controlled-release urea (CRU) and conventional urea (CU) at different nitrogen (N) application rates on the growth, yield, N uptake and N use efficiency of rice were studied to provide a reference base for the application and dissemination of CRU in rice production. Field experiments of rice were carried out in Taishan City and Wengyuan County in Guangdong Province during the 2015 rice growing season. Ten treatments were set in the experiment—conventional fertilization with 4 split applications (CF), single basal application of 25% CRU plus 75% CU (25%CRU) and single basal application of 50% CRU plus 50% CU (50%CRU), respectively, under 100%, 80% and 60% of conventional N rate195 kg(N)·hm^-2, 156 kg(N)·hm^-2 and 117 kg(N)·hm^-2, and no nitrogen fertilization (CK). Tiller number and leaf SPAD values were measured at vegetative stages of rice. Straw and grain yield were collected at maturity for N analysis. Then yield and yield components were recorded after harvest. The results showed that there was no difference in tiller number at vegetative growth stages under different N fertilization treatments, and that leaf SPAD increased with increasing N application rate. With increasing N application rate, rice grain yield increased initially and then decreased. Also the highest rice grain yield was obtained under 156 kg(N)·hm^-2 treatment. N uptake of rice increased with increasing N application. At the same N rate, no differences was noted in grain yield, N uptake of rice grain and rice straw among CF, 25%CRU and 50%CRU treatments. With increasing N application rate, N partial factor productivity and N harvest index decreased gradually. For the same N rate, agronomic N use efficiency, N physiology efficiency under 25%CRU and 50%CRU treatments were higher than that under CF treatment (P < 0.05), with average increases of 14.99%, 17.23% and 98.22%, 57.44%, respectively. N harvest indices under 25%CRU and 50%CRU treatments were averagely higher by 6.99% and 6.69%, respectively, than that under CF treatment at conventional application rate195 kg(N)·hm^-2, especially in Taishan City. Soil available N under 60% conventional N rate117 kg(N)·hm^-2 treatment after rice harvest was significantly lower than that under 100% and 80% conventional N rate treatments. In conclusion, rice grain yield and N use efficiency under 25%CRU at 80% conventional N rate in both experiment sites were superior over those under other treatments. Therefore, single basal application of 25%CRU plus 75%CU with N rate of 156 kg(N)hm^-2 increased rice yield and N use efficiency, and maintained soil N fertility in Guangdong. This was a potential beneficial and rational N application mode for the region.