Interplay of soil water repellency, soil aggregation and organic carbon. A meta-analysis

Soil water repellency (SWR) is a widely observed phenomenon where water infiltration is delayed on the soil surface for a time before soaking in. High SWR not only causes problems in agriculture for prolonged irrigation and waste of water, but also might benefit soil by enhancing aggregate stability in post-fire, arid and seasonally dry ecosystems. For a better understanding of the mechanisms behind SWR, we performed a random-effects meta-analysis of 119 experiments from 27 publications, measuring aggregate stability (AS), the content of soil organic carbon (SOC) and soil water repellency. We explored how SWR was moderated by AS, SOC, soil pH, soil sand content, treatments (fire, microbe and organic matter), experimental setting (laboratory or field), plant type, sampling depth, sample drying methods, and SWR measurement (contact angle, water drop penetration time, repellency index and molarity of ethanol droplet). We found that SWR, AS and SOC were correlated positively; soil pH (4-8), soil sand content and experimental duration affected SWR significantly. We also found that in burned soil the interaction between SWR and AS was different from other treatments, suggesting the presence of a different underlying mechanism. Two data issues were identified: (1) the effects of soil microbes and soil fauna were generally less well studied; and (2) the measuring conditions of SWR, especially soil water content, were not reported which caused problems in study comparisons. Our results emphasize the importance of integrating SWR and AS in soil research as interacting soil processes that influence soil stability and functions. (C) 2016 Elsevier B.V. All rights reserved.