Objective The new urea products have good fertiliser-saving and labour-saving characteristics in agricultural production, the purpose of this study was to clarify the effects of stable urea, polyaspartic acid urea, polyurethane coated urea and their blending combinations on soil nitrogen transformation and the growth of winter wheat in the North China Plain.

Methods Soil incubation experiment with different inhibitor addition amounts: a total of 10 treatments were established, including no urea (CK), urea (U), polyurethane-coated urea (CU), polyaspartic acid urea (U_M), 0.4%DMPP+urea, 0.7%DMPP+urea, 1.0% DMPP+urea, 0.09%NBPT+urea, 0.15%NBPT+urea, 0.2%NBPT+urea. Soil incubation experiments with different new types of urea combination ratios: a total of 11 treatments were set up, including CK, U, mixtures of polyaspartic acid urea (U_M) and coated urea (CU) at 1∶3, 1∶1, and 3∶1 ratios; mixtures of 0.7%DMPP+U and CU at 1∶3, 1∶1, and 3∶1 ratios; and mixtures of 0.15% NBPT+U and CU at 1∶3, 1∶1, and 3∶1 ratios. These two incubation experiments were designed to measure soil inorganic nitrogen concentrations and soil urease activities. In the field experiment, ten treatments were set up including CK (no nitrogen fertilizer), U, 75%U, CU, U_M, 0.7%DMPP+U, 0.15%NBPT+U, a 1∶1 mixture of 0.7% DMPP+U and CU (0.7%DMPP+U1-CU1), a 1∶1 mixture of 0.15% NBPT-enhanced urea and CU (0.15%NBPT+U1-CU1), and a 1∶1 mixture of U_M and CU (U_M1-CU1). The indicators included winter wheat yield and its components, and soil inorganic nitrogen contents.

Results The inhibitory effect of DMPP increased with its concentration. On day 15, urease activity in the 0.15% NBPT+U treatment was significantly lower (17.0%) than in the U treatment. When stabilized urea was mixed with coated urea, soil NH₄⁺-N content increased with a higher proportion of stabilized urea, while soil NO₃⁻-N content decreased with increased propertion of coated urea. When the mixing ratio of stabilized urea or polyaspartic acid urea with coated urea was 1∶1, compared with urea treatment, stablilized urea or polyaspartic acid urea increased the soil ammonium nitrogen contents in the early incubation stage, and the coated urea reduced soil nitrate nitrogen content in the later incubation stage. Field results demonstrated that the 0.7% DMPP+U1-CU1 treatment significantly increased the wheat yield and grains per ear, and nitrogen fertilizer partial productivity, nitrogen fertilizer agronomic efficiency, nitrogen uptake efficiency by plant, and nitrogen use efficiency. Also, this treatment showed the highest soil ammonium nitrogen and lowest nitrate nitrogen concentrations. No significant differences of wheat yield, spikes per hectare, thousand-grain weight, nitrogen fertilizer partial productivity, or nitrogen fertilizer agronomic efficiency were observed between 0.15%NBPT+U1-CU1, U_M1-CU1, and 75%U treatments.

Conclusion Urea with 0.7%DMPP or 0.15%NBPT can effectively slow down the soil nitrogen conversion processes. When the ratio of stabilized urea and 0.6% polyaspartic acid urea mixed with coated urea is 1∶1, it can effectively regulate the nitrogen conversion of stabilized urea and polyaspartic acid urea in the early stage, and that of coated urea in the later stage. Under the condition of reducing the total nitrogen input by 25%, this mixed ratio of one-time basal nitrogen fertilizer increased wheat yield and nitrogen uptake compared to other treatments with the same nitrogen fertilizer input, espeacially by mixing 0.7%DMPP urea and coated urea in a 1∶1 ratio.