Characterization of Biochars from Different Origins and Their Effects on the Physical-Hydric Properties and Water Use...

Biochar has been widely investigated as a soil conditioner due to its potential to improve the relationship between volumetric moisture and water mass, as well as to enhance water use efficiency in agricultural systems. However, there is still limited information regarding its influence on the physical-hydraulic properties of commercial substrates used for seedling production, particularly concerning water retention behavior and practical application in irrigation management. In this study, biochars produced from lemongrass biomass (BCL) and municipal solid waste (MSW) were characterized for physicochemical properties, nutritional composition, and levels of potentially toxic metals. The effect of adding these biochars in different proportions (zero, 5.0%, 7.5%, and 10% m·m^-1) on the physico-hydric properties and water retention behavior of a commercial substrate used for seedling production was evaluated. The experiment was conducted in pots under controlled conditions, with volumetric moisture and substrate mass measured at successive time points throughout saturation and gravitational drainage. The physical-hydric parameters of the substrate were evaluated, including volumetric moisture, electrical conductivity, water content at saturation, and water content at container capacity, and regression equations were adjusted to estimate available water. The materials exhibited an alkaline character, with pH values of 9.58 for BMSW and 8.83 for BCL, and differences in ash content, ranging from 15.12% m·m^-1 in BCL to 36.45% m·m^-1 in BMSW. The nutritional composition indicated the presence of macronutrients, including Ca, Mg, N, K, and P, with higher levels of Ca, N, and P in BMSW and higher levels of Mg and K in BCL. The levels of potentially toxic metals remained below the established limits for agricultural use, indicating environmental safety for application in the substrate. Although overall changes in most physical-hydraulic parameters were limited, biochar incorporation increased container capacity at the highest rates evaluated and enabled the development of a predictive operational model for estimating substrate water retention. This model allows for the estimation of substrate water availability based on volumetric moisture measurements, serving as a practical tool for irrigation management in seedling production systems. Specific differences were observed mainly for volumetric moisture, with values ranging from approximately 0.43 m³·m^-3 in the control substrate to 0.53 m³·m^-3 in the biochar treatments, especially at doses of 7.5% and 10.0% m·m^-1 of BMSW and at a dose of 10% m·m^-1 of BCL. The retention curves showed high linearity (R² between 0.962 and 0.998), allowing estimation of the operationally available water in the substrate for water management purposes. It is concluded that biochar can act as a substrate conditioner contributing to the sustainable use of waste and offering a practical approach to irrigation management and increased water use efficiency in container-grown crops, without imposing environmental constraints on the metals evaluated.