Dissolved organic matter characteristics of soil aggregates on slopes covered by natural successional vegetations with different root structures

Vegetation restoration is a critical measure to control soil erosion and improve soil quality. However, limited research has examined how dissolved organic matter (DOM) within soil aggregates changes during natural vegetation succession. Therefore, soil samples were collected from two series of slopes covered by natural successional herbaceous vegetation with tap roots (TR) or fibrous roots (FR) in a semiarid region. The effects of succession year, root structure, and soil aggregate size on the DOM characteristics within soil aggregates were assessed by the optical techniques. Overall, the dissolved organic carbon (DOC) content of soil aggregates generally increased with successional age for both TR and FR. The DOC content of FR was larger than that of TR, and the DOC content of soil aggregates on the TR and FR followed the order of small macro-aggregates (SMA, 0.25–2 mm) > large macro-aggregates (LMA, >2 mm) > soil micro-aggregates (MIA, <0.25 mm). The DOM of soil aggregates on the TR and FR were constituted by two humic-like fractions and a tryptophan-like protein fraction. The proportion of autochthonous DOM increased with succession years for FR. The aromaticity of DOM in MIA and SMA were higher than that in LMA for TR and FR. The aromaticity and degree of humification of DOM in the FR was highly larger than that of TR for most sizes of soil aggregates. The dynamics of DOM within soil aggregates were directly or indirectly controlled by the variations in the vegetation root and litter densities, soil texture, and soil bulk density.