Spectroscopic insights into the mechanisms driving spatial heterogeneity of dissolved organic matter in complex slope systems with patchy habitats in arid and semiarid regions

Soil erosion profoundly influences dissolved organic matter (DOM) dynamics and regional carbon cycling in arid and semiarid regions. Complex slope systems, consisting of ridge slopes and gully slopes with abrupt topography and patchy habitats, may regulate DOM spatial distribution, yet the underlying mechanisms remain unclear. This study examined the spatial heterogeneity and drivers of soil DOM in a typical complex slope system by optical techniques. Overall, DOM composition varied with landscape position. Stable and humified DOM dominated at summit and gully shoulder line, while less-decomposed DOM accumulated at middle ridge slope and gully slope, the active erosion zones. Among habitats (i.e., Bothriochloa ischaemum (L.) Keng + biological soil crusts (BSCs) (BI BSCs ), Artemisia sacrorum Ledeb + BSCs (AS BSCs ), and BSCs), BSCs showed the highest dissolved organic carbon (DOC) content, while BI BSCs contained the largest fraction of humic-like DOM. DOC was highest in the top soil horizon across all three habitats. The top soil horizon contained higher proportions of humic-like and plant-derived DOM, whereas the bottom soil horizon was enriched in protein-like and microbial-derived DOM. DOM molecular weight, aromaticity, and humification degree declined with soil depth. The spatial heterogeneity of DOM was primarily controlled by habitat type, landscape position, and soil depth, which together influenced litter input, root density, and BSCs traits. These findings elucidate the coupling between topography, habitat, and DOM dynamics, providing evidence for improving regional carbon cycle models in arid and semiarid landscapes.