Coconut husk biochar as a climate-smart soil amendment: optimizing pyrolysis temperature and feedstock cut size for a carbon-rich, waste-derived agricultural input
Climate change is steadily eroding agricultural productivity through soil degradation, water and salinity stress, and declining nutrient-use efficiency, while the heavy reliance on conventional mineral fertilizers contributes to greenhouse-gas emissions, nutrient losses, and, in some contexts, reduced soil biological activity. Biochar from lignocellulosic agro-residues can convert low-value waste into a carbon-rich material that has been reported to carry and slowly release mineral and organic nutrients, aligning with the goals of organo-mineral fertilization. This study evaluated the feasibility of coconut ( Cocos nucifera L.) husk, a major underutilized by-product of the coconut industry-as a feedstock for such an amendment by optimizing two production variables: pyrolysis temperature and feedstock cut size. A laboratory experiment combined four cut sizes (1/8, 1/4, 1/2, and chips) with five temperatures (275 °C, 325 °C, 375 °C, 425 °C, and 475 °C) in a completely randomized design (60 samples), followed by a field-scale trial using a double-chamber pyrolyzer comparing compacted versus loosely packed loading densities. Conversion efficiency and proximate composition (moisture, volatile matter, ash, and fixed carbon) were determined and analyzed by two-way factorial ANOVA to test the temperature × cut-size interaction, followed by one-way ANOVA to characterize the main effects once the interaction proved non-significant. Conversion efficiency ranged from 36.6% to 68.3% and declined with increasing temperature, whereas volatile matter decreased, and both ash (3.6%–12.3%) and fixed carbon (0.27%–5.54%) increased significantly with temperature ( p < 0.05); fixed carbon is an operationally defined proximate fraction and was not corroborated by elemental (H/C, O/C) ratios, aromaticity or stability indices, or mineralization assays. The 1/4 cut size offered the best balance of yield, carbonization, and operability and was selected for field trials, where compacted loading achieved a significantly higher conversion efficiency (46.95%) than loose packing (40.90%; p = 0.03). A resource-utilization analysis over a 60-year plantation lifespan indicated that biochar amendment could reduce husk demand by approximately 94% relative to direct mulching and 81% relative to husk burial. Moderate pyrolysis (325 °C–375 °C) with a 1/4 cut size produced a carbon-rich biochar with potential use as a component or carrier of an organo-mineral soil amendment, suggesting a practical route to valorize coconut husk waste for resilient tropical cropping systems. Agronomic performance, nutrient release, and carbon persistence were not assessed in the present study.