Effects of humic-acid urea on rice yield, nitrogen utilization, and C:N:P stoichiometric characteristics

This study explored the impacts of humic acid urea(HAU) on rice yield, nitrogen use efficiency and the carbon–nitrogen–phosphorus (C: N: P) stoichiometric characteristics within the plant–soil system in the Chaohu Lake Basin, aiming to provide theoretical support for the application of HAU in green rice production. A field experiment was conducted with three treatments: no nitrogen (CK), conventional urea (U), and humic acid urea (HAU). Rice yield, nitrogen uptake and remobilization after anthesis, as well as the C, N and P contents and stoichiometric ratios of different plant organs and soil were systematically measured. The results showed that, compared with U, HAU significantly increased rice yield, the number of effective panicles, and thousand-grain weight by 8.5%, 3.6%, and 1.0%, respectively (P < 0.05). HAU significantly enhanced nitrogen uptake and remobilization capacity of rice. Specifically, the nitrogen uptake, the contribution of leaf N remobilization, and the contribution of stem-sheath N remobilization were significantly increased by 30.6%, 15.0%, and 25.9% respectively, compared with U (P < 0.05). HAU effectively improved topsoil nitrogen-supplying capacity, which significantly increased alkaline hydrolysis N content by 7.0% in topsoil compared with U (P < 0.05). Due to the positive effects of HAU on N availability in topsoil and rice N uptake and remobilization, apparent nitrogen use efficiency, agronomic nitrogen use efficiency, and partial factor productivity of N fertilizer were significantly increased by 5.6 percentage points, 28.1%, and 10.2%, respectively, compared with U (P < 0.05). In addition, compared with U, HAU significantly decreased the C: N and C: P ratios and increased the N: P ratio in every organs of rice at anthesis. At maturity, the N: P ratios of straw and grain under HAU (4.51–4.62) fell within the range indicative of N–P co-limitation in cereals (4.2–6.7), suggesting a more balanced nutrient status. In contrast, the N: P ratios under U (4.09–4.10) indicated nitrogen limitation. In conclusion, humic acid urea synergistically enhances soil nitrogen supply, promotes nitrogen uptake and remobilization in rice, and optimizes C: N: P stoichiometric balance, thereby increasing grain yield and improving nitrogen use efficiency. HAU is a highly efficient nitrogen fertilizer suitable for green rice production.