Influences of land use change on dissolved carbon in karst headwater streams

Land use/land cover effects are key driver of biogeochemical cycling in fluvial systems, but its impact on dissolved carbon dynamics in headwater streams-critical zones for carbon transformation and export-remains underexplored. This study investigates spatial-temporal patterns and sources of dissolved carbon across three karst headwater streams in southwestern China with similar geological backgrounds but contrasting land use types. Using multi-method approaches including hydrochemical analysis, parallel factor analysis, Bayesian stable isotope mixing models, and structural equation modeling (SEM), we reveal that catchments with higher levels of urbanization and agricultural land exhibited obviously higher concentrations of dissolved inorganic carbon, driven by enhanced rock weathering from anthropogenic acid inputs such as wastewater treatment plant effluents and agricultural irrigation. Although differences in dissolved organic carbon levels among the catchments were minor, fluorescence analysis indicated mixed allochthonous and autochthonous sources, with stronger autochthonous characteristics in urban and agricultural streams. Isotope mixing models further confirmed shifts in carbon source contributions associated with land use type. SEM results showed that biological metabolism mediated the transformation between inorganic and organic carbon, with stronger coupling and higher degradability of dissolved organic matter components in urban and agricultural streams compared to forested streams. Our findings demonstrate that land use exerts a strong influence not only on the concentrations and sources of dissolved carbon, but also on its transformation pathways. These insights enhance our understanding of land use-carbon interactions in karst critical zones and have important implications for regional carbon budgeting and watershed management under intensifying land use change.