Spatial variation in dissolved organic matter in a vegetation-restored ephemeral gully on the semi-arid region

Dissolved organic matter (DOM) is a crucial fraction of soil organic carbon, playing a key role in assessing soil quality. Nonetheless, the spatial variation characteristics of DOM in typical geomorphic units are not systematically understood. Accordingly, in this study, we selected the widely distributed vegetation-restored ephemeral gullies (EGs) in a typical semi-arid region, and sample points were arranged along the EG top (ET) and EG floor (EF) from the head to the end of the EGs with equal intervals. Soil samples were taken from the top soil layer (TL), middle soil layer (ML) and bottom soil layer (BL) at each sample point. Spatial variation characteristics and driving factors of DOM in the EGs were identified using 3D excitation emission matrix spectral, UV-visible spectral, parallel factor analysis, and principal component analysis. The results showed that the dissolved organic carbon (DOC) contents of the TL varied greatly with the distance from the EG head, but those of the ML and BL were kept stable for both the ET and EF. The DOC contents of the TL at the ET (103.29 mg kg-1) and EF (101.63 mg kg � 1) were 2.65, 4.10 and 2.20, 2.90 times higher than those of the ML and BL, respectively. The DOM of the EGs were primarily composed of humic acid-like substances (i.e., C1 and C2) and a protein-like substance (i.e., C3). The DOM of the TL and ML were organic matter with high aromaticity and molecular weight, while the DOM in the BL was lower in these two indexes for the EGs. The landscape location (i.e., ET and EF) only had a significant effect on the proportion of the C1, C2, and C3 of the ML. The variation in DOM characteristics on the EGs were highly ascribed to the differences in the accumulation or distribution of biocrusts, and plant litter and root.