Recovery of slow-release multi-nutrient matrix from dairy manure through acid-mediated mild hydrothermal treatment

Global agriculture increasingly demands sustainable soil fertilization strategies that balance nutrient efficiency with the preservation of environmental safety. Here, we demonstrate a novel integrated approach coupling acid mediated mild hydrothermal treatment (HT) of dairy manure at 120, 140, 160 °C with alkaline precipitation to synthesize a slow-release multi-nutrient matrix (SRMNM) that achieves pH responsive controlled nutrient availability through inherent material properties. SEM-EDX, XRD, and FTIR analyses confirmed an amorphous, mineral-rich matrix of the produced SRMNM, achieving nitrogen content of 1.10-1.31%, phosphorus of 4.36-4.79%, and potassium of 6.74-7.09%. Micronutrient profiles included zinc (39.6-48.0 mg kg⁻¹), copper (71.4-76.3 mg kg⁻¹), and manganese (209-226 mg kg⁻¹), all within nutritional ranges for plant growth. The contrasting nutrient release profiles; near-complete retention (99.5-99.8%) in deionized water versus near-complete mobilization (70.8-98.6%) under citric acid extraction, supporting the slow-release potential of the recovered matrix, confirming that SRMNM resists nutrient loss in bulk soil while remaining bioavailable in the rhizosphere. All heavy metal concentrations remained below U.S. EPA biosolids standards (40 CFR 503.13) with limited bioavailability. This integrated approach uniquely valorizes dairy waste into a multi-nutrient matrix with slow-release potential combining amorphous mineral architecture with inherent pH-responsive nutrient bioavailability, advancing sustainable nutrient management in circular agricultural systems.