Forest Recovery Reduces Production and Rising Aridity Diminishes Export of Dissolved Inorganic Carbon

Riverine dissolved inorganic carbon (DIC), essential for understanding terrestrial carbon cycling, is undergoing dramatic changes due to climate change and human disturbances. Quantifying how these changes impact DIC fluxes from land to rivers has remained challenging due to limited long-term data and complex, interacting drivers. Here we ask the question: How and to what extent do climate and land-cover changes distinctively influence long-term seasonal and annual trends of DIC production and export? We developed a reactive transport model, constrained by three decades of streamflow, DIC, and carbon isotope data, for a karst catchment in southwest China simultaneously experiencing a warmer, drier climate (increasing aridity) and forest recovery. Results show that from 1980 to 2010s, precipitation has declined from 1,261 to 1,005 mm/yr, and discharge from 700 to 552 mm/yr, with no significant change in evapotranspiration. DIC production and export have declined at the rates of 2.3 × 10⁵ and 5.4 × 10⁵ mol C/yr/yr, respectively. Drier climate and reduced discharge diminish carbonate weathering but also store more produced DIC, resulting in higher DIC concentrations over time but a twofold decline in DIC export compared to its production. Interestingly, although forest recovery elevates organic carbon content, cooling soils and lower soil moisture reduce rates of soil respiration. Scenario analysis shows that forest recovery accounts for 91% of the production decline, while increasing climate aridity explains 78% of the export reduction. Seasonal analysis further reveals that soil respiration declines most during hot-wet seasons but calcite weathering drops more in cold-dry seasons. These findings underscore the differential impacts of climate and land-cover changes on carbon transport and transformation processes, which are crucial for understanding carbon cycling and budgets under evolving environmental conditions.