Effect of Biochar on Soil Shrinkage and Cracks and Its Evolution Under Dry and Wet Cycles: A Meta-Analysis

Biochar has been widely investigated as an amendment for mitigating soil desiccation cracking. However, variation in its effects across soil textures, application rates, and long-term drying–wetting (DW) cycles remains insufficiently resolved. Based on a global dataset comprising 278 paired observations from 44 peer-reviewed articles, this study employed a random-effects meta-analysis with article-clustered robust variance estimation to quantify the impacts of biochar on soil crack intensity factor (CIF), crack length density, and mean crack width. The pooled estimates indicated average reductions of 39.12%, 35.17%, and 22.20% in CIF, crack width, and crack length density, respectively; however, substantial heterogeneity (>98%) and prediction intervals crossing zero indicated considerable variation in both the magnitude and direction of responses among experimental conditions. Subgroup analyses revealed the following: (1) Soil-group differences were statistically supported for CIF, while the 14.20% increase in crack length density and 25.86% reduction in mean width observed in non-clay soils were not statistically significant. (2) Application-rate groups differed significantly across all three indices. The <2% group was associated with an 18.75% increase in crack length density, whereas the >5% group showed the largest average reductions. However, application rate explained only 12.60–15.80% of the observed variation. (3) Mean crack suppression was generally greater during the early DW cycles, but evidence beyond 6 cycles was too limited to determine whether the effect persisted under prolonged cycling. Overall, the evidence supported average reductions in CIF and mean crack width, whereas the effect on crack length density and the persistence of crack suppression under prolonged drying–wetting exposure remained uncertain. Field application should balance crack mitigation with agronomic feasibility, soil quality, and long-term performance under repeated drying–wetting conditions.