Insight into the vertical characteristics of dissolved organic matter in 5-m soil profiles under different land-use types on the Loess Plateau.

Dissolved organic matter (DOM) is a natural chemical component of all soils and influences soil organic pollutant migration, nutrient cycling, and global climate change. Previous field studies have focused on a single ecosystem, such as cropland, grassland, or forestland. However, the potential effect of different land-use types on the vertical distribution of soil DOM quantity and quality remains unclear. This study investigated the vertical characteristics of DOM in 5-m soil profiles under different land-use types (cropland, grassland, and forestland) on the Loess Plateau. The data from ultraviolet-visible spectral and parallel factor analysis of fluorescence excitation-emission matrix spectrophotometry were combined. These results indicated that the mean content of dissolved organic carbon (DOC) in the 30-yr forestland (203.33 mg kg soil) was the highest, and the lowest was observed in the cropland (83.70 mg kg soil). Meanwhile, the mean DOC content of the forestland increased through time, particularly after 20 years. In other words, afforestation activities only significantly affected soil DOM after a long time (over 20 years). The DOC content of the cropland initially increased and then decreased with soil depth in the 1-m soil profiles, which may be related to agricultural activities. Three fluorescence components, including two humic acid-like substances (C1 and C3) and a tryptophan-like substance (C2), were identified from all samples. The humic acid-like components significantly decreased by 51% with soil depth, while the tryptophan-like component increased by 49%, particularly in the cropland. The variation in ultraviolet-visible spectral and optical indexes also indicated that soil DOM was dominated by both microbial and terrestrial sources. These findings help to understand the dynamics of DOC in deep soil profiles and the biogeochemical effects of DOM in the natural environment.