Use of biochar to manage soil salts and water: Effects and mechanisms

Soil salinization is a widespread land degredation, especially in water-stressed regions, jeopardizing agriculture sustainability. Current desalinization methodology involves excessive water consumption. Biochar has the potential to mitigate soil salinization while increasing water holding capacity. As a saline and sodic material, however, how it works and whether it can be used to sustain the agriculture at reduced water resource remain to be studied. Here, by monitoring transport of water, salts and nutrients in the profile of irrigation-silt soil during watering and evaporation in both laboratory and field in Kashgar oasis, Xinjiang, China, we find biochar exacerbates salinization upon application. This is changed, however, after several cycles of irrigation-evaporation due to strengthened salt leaching in irrigation and salt removal out of the depth through intensified top accumulation by evaporation, both resulting from increased capillary effect and thereby the enhanced movement of salts despite the competing electrical adsorption to the cations. The resulted salt distribution facilitates desalinization by removing the top 2 cm soil. Biochar also promotes evaporation after irrigation due to inceased water content and capillary suction. This is reversed once the soil cracks, a common phemomenon in irrigated land. Biochar counteracts the cracking through alleviation of soil compaction, saving tillage while lowering water evaporation, e.g., by 43% at 10% biochar. Our findings indicate that application of biochar changes salt distribution, enabling desalinization with little water consumption. Together with the effect of anti-fracturing and enhanced salt leaching, it lowers water demand substantially, providing a novel solution for agricultural sustainability in salt-affected regions.