Assessing the impacts of wetting and drying cycles on soil aggregate structure. A global meta-analysis study

Wetting and drying cycles (WDCs) impose critical environmental concerns through their direct impacts on soil structure degradation, which further increases soil erosion and greenhouse gas emission. Several studies have evaluated the effects of WDCs on soil structural properties. However, the findings presented inconsistent conclusions. A meta-analysis of 659 paired observations from 26 individual studies was conducted to provide the unified understanding of WDCs impacts on soil structure and determine how the effects vary under different soil properties and climatic conditions. Overall, the WDCs significantly decreased soil aggregate stability, with the effect sizes of -28.45 %, -17.13 %, and -13.43 % for the mean weight diameter (MWD), geometric mean diameter (GMD), and water stable aggregate (WSA), respectively. Moreover, the WDCs induced a significant decrease in the proportion of macroaggregates (>0.25 mm) and increase in microaggregates (<0.25 mm) by -24.57 % and 17.32 % (p < 0.001), respectively. The increase in WDCs increased aggregate degradation under high mean annual precipitation (MAP) and mean annual temperature (MAT), with the highest decrease of -52.87 % and -64.1 % for [10-15] compared to a marginal decrease of -16.48 % and -15.04 % for [1-3] WDCs under MAP >1000 mm/year and MAT >15 °C, respectively. The decrease in the MWD and macroaggregates under WDCs was relatively low in soil with SOC >10gkg-1 compared to <10gkg-1. Clay content showed a positive and significant (p < 0.05) correlation with the MWD and macroaggregates effect sizes. The overall effect of WDCs on the total pore numbers (TPoN) and total porosity (TPo) was positive, suggesting that the WDCs increase porosity and pore numbers. This study provides scientific evidence that WDCs degrade soil structure, which could have a negative implication to surrounding environment due to possible increase in soil erosion. Thus, sustainable soil conservation measures (e.g., mulching, and agroforestry) should consistently be applied to control degradation and promote soil aggregates resilience.