Longitudinal heterogeneity dominates dissolved organic matter dynamics in sediments of an arid inland river

Sedimentary DOM in the Niya River showed weak vertical but strong longitudinal heterogeneity. Six fluorescent DOM components revealed shifting terrestrial, hydrological, and microbial influences. Physicochemical properties of sediments jointly controlled DOM composition. Sedimentary dissolved organic matter (DOM) plays a critical role in carbon cycling in arid inland river systems, yet its spatial variability and associated environmental factors remain poorly understood. Herein, we investigated the spatial heterogeneity of sedimentary DOM along the Niya River Basin (northwestern China) using optical spectroscopy coupled with PARAFAC modeling and multivariate statistical analyses. Results showed that DOM exhibited weak vertical differentiation but pronounced longitudinal heterogeneity along the upstream to downstream gradient. Six fluorescence components were identified, including humic-like and protein-like substances derived from terrestrial inputs and microbial processing. Upstream sediments were dominated by terrestrially derived DOM, whereas midstream and downstream regions reflected combined influences of hydrological transport and microbial transformation. Statistical analyses showed that grain-size parameters, including sand content and mean grain size, together with sediment physicochemical indicators such as TOC, TN, NH₄⁺, pH, and electrical conductivity, were strongly associated with variations in DOM composition. Hydrological conditions may also be indirectly linked to DOM variability through their covariance with sediment properties. Overall, our findings highlight that hydrological conditions and multi-factor associations are closely related to sedimentary DOM variability in arid inland rivers, providing new insights into carbon cycling and ecosystem functioning in water-limited environments.