Co-recycling wheat straw and eggshells for sustainable agriculture: A composite coated urea synergistically mitigating nitrogen loss and salt stress.

The inefficient use of nitrogen fertilizers in saline-alkali soils severely limits crop productivity and contributes to environmental nitrogen losses. To address this issue, we developed three types of bio-based coated fertilizers including wheat straw coated urea (WSU), eggshell coated urea (EGU), and composite wheat straw-eggshell coated urea (WS-EGU) via a bentonite-assisted coating technique using low-temperature pyrolyzed wheat straw and eggshells. The low-temperature (250 °C) wheat straw was near-neutral (pH 7.04), unlike conventional alkaline high-temperature biochar, while the eggshell component (CaCO3) enhanced the coating's compactness and mechanical durability. Soil leaching experiments demonstrated that the coated fertilizers reduced NH4+-N and NO3--N leaching losses by up to 17.82% and 19.97%, respectively, compared to uncoated urea. In pot experiments, all coated urea promoted pakchoi growth and maintained higher soil NH4+-N and NO3--N levels. Moreover, they effectively reduced exchangeable Na+ and the exchangeable sodium percentage (ESP), thereby improving the ionic environment and nutrient retention capacity of the soil. Among them, WS-EGU composite exhibited the most balanced performance. Its effectiveness was attributed to a synergistic mechanism involving diffusion control, ion exchange, and adsorption regulation, facilitated by a Ca2+-Na+ exchange process and the dense composite coating formed as eggshell particles filled the straw-derived pores. This work provides a green and sustainable strategy for the resource utilization of agricultural waste, enhanced nitrogen management, and the mitigation of saline-alkali stress.