Impact of coupling biochar and clay minerals on physicochemical properties of sandy soils and CH4/CO2 flux for agriculture

Sandy soils present key limitations to sustainable agriculture due to low water retention, poor aggregation, and limited nutrient-holding capacity, motivating the use of amendments to overcome these constraints. This study evaluated zeolite, bentonite, and aluminum hydroxide (Al(OH)₃) at 2, 5, and 10 wt%, with and without biochar, on the physical, chemical, and gas-exchange properties of a sandy soil in addition to an agronomy implication under greenhouse conditions. Results showed that all clay minerals reduced soil bulk density, improved aggregation, and increased available water content (AWC), particularly with biochar (e.g., 10% bentonite + biochar gave the highest AWC, +172%; 10% Al(OH)₃ + biochar produced the lowest bulk density, -12%). Biochar's effects were not always additive, as it moderated chemical shifts by buffering pH and reducing EC. In gas exchange, Al(OH)₃ markedly increased CO₂ emissions (+1833%), and this effect further amplified with biochar (+2346%), while CH₄ fluxes remained negative (uptake). To verify agricultural relevance, millet was cultivated under identical greenhouse conditions. In soil only treatments, all minerals exceeded the control; the best response occurred at 5% Al(OH)₃ (+278%), followed by 5% bentonite (+160%). Increasing to 10% did not add benefit because EC with Al(OH)₃ and wilting point with bentonite rose significantly that inversely affect the plants growth. With biochar, responses increased overall, peaking at 5% bentonite + biochar (+315%) and 5% Al(OH)₃ + biochar (+312%). These results provide practical guidance for selecting amendment combinations to balance water retention, fertility, aeration, and greenhouse gas outcomes.