Integrated effects of biochar and treated wastewater applications on soil carbon, salinity and hydro-physical properties in a Semiarid hillslope

Water scarcity and soil degradation are major constraints for sustainable land management in semiarid regions. This is of particular importance on hillslopes of alluvial environments that are highly susceptible to erosion and carbon losses, both in rural and urban areas. The reuse of treated domestic wastewater for irrigation has emerged as an alternative water source in these regions, and as a sanitation solution; however, its long-term sustainability is often limited by salt accumulation and changes in soil physical and hydraulic functioning under high evaporative demand. This study evaluates the integrated effects of biochar application combined with treated wastewater irrigation on soil carbon stocks, salinity dynamics and hydro-physical properties in a hot semiarid environment.

Field experiments were conducted on shallow, steep sandy loam soils developed on hillslopes of alluvial deposits, characterized by low water storage capacity and strong hydrological connectivity along slopes. Soil surface management strategies included bare soil, organic mulching, and the combined application of mulch and biochar produced from agricultural wood residues, representing contrasting conditions of surface protection and organic input. The system was irrigated using treated domestic effluent with moderate to high electrical conductivity through a localized drip irrigation scheme, reflecting realistic water reuse practices in water-scarce regions. The assessment focused on soil electrical conductivity, total organic carbon and key physical and hydraulic attributes controlling infiltration, water retention and solute transport, monitored over successive field campaigns and soil depths. This integrated approach allowed the evaluation of responses of soil–water–carbon interactions under combined water reuse and soil amendment practices. Results indicate that the integration of biochar with organic surface cover promotes higher soil carbon accumulation and greater temporal stability compared to bare soil conditions. Organic amendments also attenuated salinity buildup under wastewater irrigation, reducing variability in soil electrical conductivity and buffering salt accumulation in the surface layer. These effects are associated with improvements in soil structure and porosity, which enhance water retention and infiltration capacity, reduce surface runoff and limit salt concentration in the root zone, particularly following rainfall events. These processes are especially relevant in sloping alluvial semiarid landscapes, where soil physical degradation and hydrological processes strongly influence carbon redistribution and salinity risks.

Overall, the findings highlight the potential of integrating biochar with treated wastewater irrigation as an innovative and scalable Nature-based Solution strategy for improving soil–water–carbon interactions in semiarid environments. This approach explicitly supports the United Nations Sustainable Development Goals by contributing to SDG 2 (Zero Hunger) through improved soil productivity, SDG 6 (Clean Water and Sanitation) by promoting safe wastewater reuse, SDG 13 (Climate Action) via soil carbon sequestration, and SDG 15 (Life on Land) by mitigating land degradation, while offering practical insights for climate-resilient land use planning and the implementation of Nature-based Solutions in vulnerable dryland regions.