Unraveling the spatial heterogeneity of dissolved organic matter characteristics from the optical level in slope-gully systems

Dissolved organic matter (DOM) is a crucial component of soil organic matter, and it significantly affects a range of environmental processes. However, few studies have focused on the spatial heterogeneity of DOM characteristics in typical geomorphic units. Therefore, in this study, a southern slope-gully system (SGS) and a northern slope-gully system (NGS) widely distributed in semiarid regions were selected as the study objects. Soil samples were collected from the three soil horizons (i.e., top horizon (0–2 cm), middle horizon (3–20 cm), bottom horizon (21–50 cm)) at each landscape locations (i.e., summit (SU), middle ridge slope (MS), lower ridge slope (LS), loess shoulder-line (SL), gully slope (GS), and gully floor (GF)) of the SGS and NGS. Spatial heterogeneity and controlling factors of DOM characteristics in the slope-gully systems were identified systematically using the optical techniques and principal component analysis. Results indicated that the dissolved organic carbon (DOC) content in both SGS and NGS followed the modes of top horizon (261.39 mg kg⁻¹, 347.31 mg kg⁻¹) > middle horizon (178.47 mg kg⁻¹, 164.83 mg kg⁻¹) > bottom horizon (83.19 mg kg⁻¹, 100.42 mg kg⁻¹). Three fluorescence components (i.e., two humic-like substances and one protein-like substance) were identified from all soil samples. The proportion of protein-like component of the NGS was higher than that of the SGS for the bottom horizon. DOM of the top and middle horizons were mainly from plant source with high aromaticity and molecular weight, whereas DOM of the bottom horizon was mostly dominated by microbial source with low aromaticity and molecular weight. Landscape location significantly influenced the aromaticity, humification degree, and source of DOM in the top horizon. The variation in DOM characteristics of slope-gully system was primarily attributed to the difference in characteristics of biocrusts, roots, and litter driven by topographic conditions.