Optimum application ratio of controlled-release nitrogen fertilizer and urea for high maize yield and nutrient efficiency in black soil

Objectives The efficient application of controlled-release nitrogen fertilizer (CRU) requires a certain ratio of common urea (RU) to meet the whole crop's N requirement across the stages of growth. Here, we studied the optimum blending ratio of CRU and RU for high maize yield and nutrient efficiency in the black soil of Northeast China.

Methods A site-specific field experiment was conducted in Gongzhuling City of Jilin Province from 2017 to 2019, with maize ‘Fumin108’ as the experimental material. Under the same total N input of 210 kg/hm², six application ratio treatments of RU to CRU were designed, including 10∶0 (RU), 8∶2 (CRU20%), 6∶4 (CRU40%), 4∶6 (CRU60%), 2∶8 (CRU80%), 0∶10 (CRU100%), and no N fertilizer application was used as control (N0). The plant biomass, N content, and soil inorganic N content were investigated at the main growth stages of maize, and the yield and yield components were measured at maturity. The crop N accumulation, N use efficiency, and N balance of the soil-crop system were calculated.

Results CRU60% and RU recorded the highest and lowest kernel number per ear and 100-kernel weight of maize, respectively. The yield of CRU60% was 17.3% higher than RU. Compared with RU, all the other fertilization treatments (P<0.05) increased soil inorganic N content from the flowering stage, increased N accumulation at the flowering stage and its proportion in the total N accumulation. This led to an improved contribution of post-flowering N accumulation to grain N in maize. The N recovery efficiency (REN), agronomic efficiency (AEN), and partial factor productivity (PFPN) reached a peak in CRU60%, which were 36.1%, 66.9%, and 17.3% higher than RU, respectively. The apparent N loss decreased first and then increased with an increase in CRU ratio, with the lowest value recorded in CRU60%, 28.8% lower than RU. The CRU ratio equation was fitted with yield, N use efficiency, soil inorganic N, and apparent N loss to obtain a theoretical yield of 11059 kg/hm² at CRU 63.4%. Under this CRU ratio, the REN, AEN, PFPN, soil inorganic N, and accumulated N loss were 42.4%, 17.1 kg/kg, 53.2 kg/kg, 20.8 mg/kg, and 223.1 kg/hm², respectively, with all the values similar to those recorded for CRU 60%. The suitable CRU ratio was calculated as 61%−67% using a 95% theoretical optimum CRU ratio as the confidence interval.

Conclusions In the black soil of Northeast China, the common application of 60% CRU and 40% common urea showed the best result. The combination increased soil inorganic N supply capacity. It promoted maize N uptake at the flowering stage, improving maize yield and N use efficiency and reducing soil apparent N loss.