Eluviation of dissolved organic carbon under wetting and drying and its influence on water infiltration in degraded soils restored with vegetation

One mechanism in the restoration of severely degraded soil by vegetation might be the movement of dissolved organic carbon (DOC) to macropore and aggregate surfaces. We propose that this lowers the soil wetting rate and subsequently its slaking resistance by creating a partially hydrophobic surface. In this study, we determined how wetting and drying (w/d) cycles redistribute DOC to soil surfaces, and how DOC affects hydrophobicity where it accumulates, in relation to the soil surface area to volume ratio and to different types of vegetation planted to restore a severely degraded soil. Repacked soil cores that simulate different soil aggregate sizes were tested. The results showed that w/d cycles increase surface DOC concentration through a depletion of DOC in the interior of the soil. Correspondingly, w/d cycles enhanced hydrophobicity, measured as a water repellency index, R, from 1.5-2.3 to 3.6-7.6, the values affected significantly by the type of vegetation. This index (R) did not change for a control soil with no vegetation. The link between the amount of DOC and water repellency was weak (coefficient of determination r(2) = 0.06-0.26), indicating that DOC quality was probably more important than its quantity. Although increasing the core size resulted in a greater accumulation of DOC on the drying surface of the core, the impact of this on water repellency was minimal. Incubation caused a decrease in the amount of DOC, but had minimal influence on water repellency. This work improves the understanding of changes in soil wetting and soil stabilization under processes of natural weathering and vegetation restoration.