How to Minimize the Impact of Biochar on Soil Salinity in Drylands? Lessons from a Data Synthesis

Biochar application in dry regions holds promise for improving soil properties, but its impact on soil salinity remains controversial. To evaluate the short-term effect of biochar on soil salinity under dry conditions, we conducted a meta-analysis of 149 observations from 40 peer-reviewed publications conducted in Mediterranean, arid, and semi-arid climates, or under simulated dry/saline conditions. Overall, biochar addition significantly increased soil electrical conductivity (EC) by 34.63% compared to controls. However, this effect was highly dependent on pedoclimatic conditions, soil pH, biochar feedstock types, pH and EC, irrigation practices, and management factors. The most substantial increases in salinity occurred when applying biochar produced from high-ash feedstocks (e.g., seafood shell powder, peanut shell), at high application rates (>20 t ha−1), to soils with low initial organic carbon content, or in the absence of a leaching fraction. In contrast, the use of biochar made from low-ash ligneous materials at rates ≤ 20 t ha−1 did not significantly increase soil EC. Random forest analysis identified biochar EC, initial soil EC, and biochar pH as the most influential factors. We conclude that the risk of biochar-induced salinization in drylands can be effectively minimized by selecting appropriate lower-EC biochar, applying it at moderate application rates, and implementing irrigation with a leaching fraction. These findings provide critical guidelines for the sustainable implementation of biochar technology in water-scarce environments.