Combining time-variable controlled release urea formulations to improve spring maize yield and reduce nitrogen losses in northeastern China

The application of controlled-release urea (CRU) has become an important practice to increase maize yield and nitrogen (N) use efficiency (NUE) with one-time fertilization management. The appropriate ratios and application effects of combinations of regular urea and CRU with different N-release periods (U–CRU) may vary because of regional climate and crop specificity. A three-year (2019–2021) field experiment was conducted in the northeastern region of China to investigate the effects of U–CRU on maize yield, NUE, changes in soil inorganic N content, and apparent N loss. With N fertilizer applied at 210 kg ha^–1, two types of CRU with release periods of 60 days (CRUa) and 90 days (CRUb) were mixed with regular urea in four different combinations: CRU1 (Urea 40 % + CRUa 60 %), CRU2 (Urea 40 % + CRUa 40 % + CRUb 20 %), CRU3 (Urea 40 % + CRUa 20 % + CRUb 40 %), CRU4 (Urea 40 % + CRUb 60 %). The controls were Urea (U) only and no N fertilizer application (N0). Compared with U, all U–CRU treatments significantly increased soil inorganic N contents (NO₃^––N, 1.4–7.0 mg kg^–1; NH₄⁺–N, 0.3–1.3 mg kg^–1) in the 0–20 cm soil layer from twelfth-leaf (V12) to physiological maturity (PM) stages of maize. Furthermore, U–CRU treatments increased N uptake from silking (R1) to PM stages and therefore contributed to grain N accumulation which increased 17.1–30.9 % and improved N recovery efficiency (REN), which increased by 8.1–13.1 %. Associated with increases in quantity of ears and 100-grain weight, maize yield increased by 12.2–18.9 % in CRU3, with the increases the highest among treatments. Additionally, compared with U, all U–CRU treatments significantly reduced the inorganic N content in the deep soil layer (40–100 cm) and the lowest apparent N loss was in CRU3, decreasing by 93.4 kg ha^–1 compared with that in U. Regression relations between the mixture ratios of CRUs and maize yield, apparent soil N loss, and REN were used to determine that the appropriate ratio of U:CRUa:CRUb to attain high maize yield and NUE was 40 %:18.6–24.6 %:35.4–41.4 %. Therefore, determining the optimum mixture ratios of CRUs can increase maize yield and NUE while reducing the risk of N loss in continuous maize cropping systems. The results provide a scientific basis for N fertilizer management in spring maize production in northeastern China.