Improving crop yields, nitrogen use efficiencies, and profits by using mixtures of coated controlled-released and uncoated urea in a wheat-maize system

The economic benefits of using controlled-release urea (CRU) on wheat and maize depend on costs and grain yields. Delays in the release of nitrogen (N) from CRUs can limit the availability of early season N and crop growth. Because of its high cost, the use of CRUs for wheat and maize production by commercial growers is very limited. We investigated the effects of a mixture of CRU and uncoated urea (N ratios 7:3, respectively) on crop yields, economic benefits, N budgets, and soil fertility in a wheat-maize rotation system over an 8-year period in Shandong Province, China. Annually, the mixture was applied at 270 kg N ha−1 (Mix 1), 540 kg N ha−1 (Mix 2), and 810 kg N ha−1 (Mix 3). Uncoated urea treatments were simultaneously applied at 540 kg N ha−1 (Urea 2) and 810 kg N ha−1 (Urea 3). The N release characteristics of these mixtures under field condition were synchronized with N requirements of the wheat and maize crops during their entire growth periods. Applying the mixture increased grain yields of wheat and maize by 8.2-11.9% and 6.8-9.8%, respectively, compared with treatments of monotypic, uncoated urea. Mean N use efficiencies were increased by 35.7-37.6% and 13.2-14.3% for wheat and maize, respectively, and annual net profits increased 15.4-21.8%, compared with uncoated urea treatments at the same rates of N application. Considering the N budget, apparent N losses were increased with higher N application rates, and applying the mixture significantly reduced N losses compared with monotypic urea. Regardless of application rate, the mixed urea treatments reduced the leaching of N in the soil profile by augmenting NO3−-N and NH4+-N contents at the 0-40 cm soil depth and reduced the leaching of NO3−-N into deeper soil layers. The pH of the surface soil was also lowered and exchangeable Ca2+ and Mg2+ were reduced by the long-term application of monotypic urea. In conclusion, applying the mixture of CRU and uncoated urea is recommended for sustainable increases in crops yields, maintaining soil fertility, reduction of N leaching, and increased net profits.