Vegetation Types Shape the Vertical Distribution of Dissolved Organic Matter in a Mountainous Soil

Field studies have explored the potential effects of different land use types on the vertical characteristics of dissolved organic matter (DOM) content and quality. However, owing to spatial and temporal variations, potential effects of different vegetation types on the vertical distribution of DOM content and quality remains unclear. Our study investigated the vertical characteristics of DOM in soil profiles of Cambisols across land-use types (grassland and woodland) in Longmenshan fault zone. Chemical characterization of soil DOM and its source identification across land use types inferred by fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC) and principal component analysis (PCA). The results indicated that soil organic carbon (SOC), readily oxidizable carbon (ROC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) content in the different landscape decreased with soil depth. Also, DOC content in soils of woodland was higher than in grassland. Three fluorescence components were identified in all soil samples by EEMs-PARAFAC analysis, including two humic acid-like substances (C1 and C2) and a tryptophan-like substance (C3). Overall, the relative distribution of the three DOM components mainly showed that the proportion of C1 and C2 increased with soil depth, but neither increased significantly; and the proportion of C1 was greater than that of C2 in all samples. The variability in optical indices also indicated that soil DOM was dominated by both microbial and terrestrial sources. PCA further analyze the differences in DOM in soils of different land use types in the region. The first two principal components (PC1 and PC2) across soil layers explained more than 80% of the total data variance. From the scoring plots, the variation in DOM scores across vegetation types and soil layers indicated that DOM components were not only affected by vegetation depended on soil layers, but also by other factors. These findings provided insights into the characteristics of DOC changes in soil profiles of different land use types.