Optimizing nitrogen fertilizer application in ground cover rice production system using multi-objective synergies among yield, environment, and economy

Ground cover rice production system (GCRPS) can be expected for the water-conservation and temperature-raising impacts, in order to prevent the seasonal drought and early low-temperature stress during rice cultivation in the hilly and mountainous areas. Plastic film or mulching materials can be used to effectively reduce the soil water evaporation for the soil moisture and root-zone temperature. Prevalent fertilizer practices under GCRPS—often relying on a one-time, basal application of chemical nitrogen—frequently result in the crop growth patterns. Specifically, the excessive growth can be caused in the initial stages, while the nitrogen (N) deficiency is found at a later stage. This study aims to systematically investigate and then optimize N strategies for GCRPS. A field experiment was conducted from 2021 to 2022 in the hilly area of Ziyang City, the central Sichuan Province, China. Two water treatments (W1, conventional flooding paddy; W2, GCRPS) and three N treatments (N1, zero-N fertilizer; N2, 135 kg /hm² as a urea-based fertilizer; and N3, 135 kg /hm² of urea with a 3:2 base-topdressing ratio for the W1 or a 67.5 kg /hm² each of urea and manure as basic fertilizer for W2) were designed. A holistic, multi-objective optimization framework was adopted to maximize the economic benefits. The constraints of yield and environment were combined with the various scenario simulations of N application and organic fertilizer substitution rate from the WHCNS (Soil Water Heat Carbon Nitrogen Simulator) model. An N fertilizer model for GCRPS was constructed using the multi-objective synergies of yield, environmental, and economic goals. The optimal ratio and quantity of combined organic-inorganic fertilizer application were determined for the GCRPS. The results showed that the coefficients of determination between simulated and measured values were all above 0.95 and significant at the 0.001 level for the N fertilizer model. The crop yield and N losses were excellently simulated to verify the robust model under GCRPS, providing a reliable tool for scenario optimization. Taking 2022 as an example, the N loss of the 64% organic fertilizer substitution strategy (with a total N application rate of 157.9 kg/hm²) remained the same, compared with the original strategy of 50% organic fertilizer substitution (with a total N application rate of 135 kg /hm²). Notably, the yield and net profit increased by 41 kg/hm² and 45 CNY/hm², respectively. The production and efficiency were obtained to reduce pollution in sustainable agriculture. In conclusion, the optimal approach was recommended to integrate organic-inorganic fertilization in GCRPS. The N fertilizer optimization was identified to maximize the net economic benefits for the high and stable yields, particularly with the environment to prevent additional nitrogen pollution. Therefore, a robust technical reference was provided for the precise, efficient, and environmentally sound application of nitrogen fertilizers in the ground cover rice production system. A great contribution was made to advance green and sustainable rice cultivation in vulnerable mountainous regions.