Biochar-based phosphate fertilizer improve phosphorus bioavailability, microbial functioning, and citrus seedling growth

•MRS8-P, a biochar-based phosphate fertilizer, significantly enhances phosphorus availability and soil phosphorus fixation.•Application of MRS8-P enhances soil pH, EC, and alkaline hydrolytic nitrogen (AHN), promoting plant growth conditions.•Treatment with MRS8-P enriches soil microbial communities, boosting beneficial taxa essential for effective nutrient cycling.•Citrus seedlings treated with MRS8-P exhibit significant improvements in height, stem coarseness, and chlorophyll content.•MRS8-P facilitates greater NPK accumulation in citrus seedlings, improving overall nutrient uptake. Phosphorus is essential for plant growth but often becomes immobilized in soils, reducing its availability to plants. This study explored the enhancement of phosphorus bioavailability through the co-pyrolysis of rice straw with magnesium oxide (MgO) at 800 °C, resulting in a novel biochar-based phosphate fertilizer, termed MRS8-P. The treatment involved enriching biochar with MgO to increase phosphorus loading and evaluating its effects on soil physicochemical properties and citrus seedling growth. Soil analysis revealed improved physicochemical properties, including increased available phosphorus (AP), enhanced soil pH, higher electrical conductivity (EC), and rapidly increased available potassium (RAK); however, a temporary immobilization of alkaline hydrolytic nitrogen (AHN) was noticed by biochar application. Notably, soil microbiota dynamics shifted with an increase in beneficial taxa, such as Proteobacteria and Actinobacteria, which enhanced phosphorus solubilization and organic matter decomposition, and a decrease in taxa, such as Methylomirabilota and Desulfobacterota. These microbial changes significantly contribute to soil fertility and nutrient availability. The application of MRS8-P significantly enhanced nitrogen, phosphorus, and potassium (NPK) uptake in citrus seedlings, as demonstrated by increased growth metrics, such as plant height, stem coarseness, and chlorophyll content measured by the SPAD index. These findings underscore the potential of MRS8-P in reducing reliance on traditional phosphate fertilizers, which are linked to environmental degradation through runoff and pollution. The study confirms that biochar-based phosphate fertilizers, such as MRS8-P, can promote more sustainable agricultural practices and improve crop health and soil management while minimizing environmental impacts.