Co-pyrolyzed biochar alters salt mobility and soil chemistry under saline irrigation

Abstract

Salinity is a major constraint to agricultural sustainability, particularly in drylands where irrigation with saline or brackish water is often inevitable. Biochar has been proposed as a strategy to improve soil properties and mitigate salt stress; however, the mechanisms by which co-pyrolyzed biochar rates influence salt mobility and soil chemical dynamics remain poorly understood. This study evaluated the effects of biochar derived from the co-pyrolysis of sewage sludge and cashew pruning residues on salt mobility and soil chemical attributes under saline irrigation in a 30-day soil column experiment. Five biochar rates (0, 5, 10, 20, and 40 Mg ha⁻¹) were applied, and leachate and soil samples were analyzed to assess ion dynamics and chemical changes along the soil profile. Biochar application enhanced salt mobilization and leaching, as indicated by increased electrical conductivity and ion concentrations in the leachate, particularly at higher application rates. In the soil, biochar reduced electrical conductivity and increased pH, especially in the surface layer. However, exchangeable Na⁺ and exchangeable sodium percentage (ESP) were also higher in this layer, indicating surface accumulation of sodium. These results demonstrate that biochar influences salt redistribution within the soil–leachate system rather than promoting a simple reduction in salinity. The findings highlight the dual role of biochar in saline systems, simultaneously enhancing salt leaching while potentially increasing surface sodium contents. Therefore, its application should be carefully managed, particularly under continuous irrigation using saline or brackish water.