Developing slow-release fertilizer based on pyrolysis of domestic biogas digestate cake: A circular economy approach for rural farmers

The scientific community is becoming more interested in biochar-based slow-release fertiliser (BSRF) as a solution to the problems with ready-to-use fertilisers, such as limited plant uptake and nutrient leaching when applied to soils. Dependency on chemical fertilizer can also be subsidized through BSRFs under waste-to-wealth approaches. In this study, we prepared the BSRFs using anaerobic digestate (AD) and for that, the solid residues and press liquid were separated from AD and the solid part was pyrolysed at 350 °C (ADB350) and 500 °C (ADB500) after blending with Ca-bentonite (25%). Charred biomass was impregnated with press liquid to prepare respective modified BSRFs. SEM−EDS, XRD, and FT-IR analysis suggested significant changes in surface morphology and chemical characteristics of ADBs. ADBs were examined for their slow-release behaviour and plant nutrient (N, P and K) uptakes by wheat (Triticum aestivum) seedlings using a pot setup. To be specific, the respective release ratio for N, P, and K in the initial 10 d of the ADBs incubation study was 78.90%, 57.55%, and 80.50% for ADB350 and 59.80%, 42.50%, and 55.60% for ADB500. Slow-release performance exhibited synchronization well with the growth and biomass yield in wheat seedlings. Plant uptake results revealed significant utility (mg/pot) of N (63.0 ± 2.04), P (1.58 ± 0.37) and K (64.0 ± 5.8) suggesting the slow-release behaviour of ADBs. In summary, pyrolysed AD could be utilized efficiently as an environmental-friendly and low-cost fertilizer for sustainable crop production. [Display omitted] •Biochar-based slow-release fertilizer (BSRFs) was prepared using biogas digestate.•Pyrolysed digestate (500 °C) blended with Ca-bentonite showed a relatively low release of NPK.•Wheat growth showed synchronization with the slow-release behaviour of BSRFs.•The plant uptake rate was significantly higher in BSRFs.