Amplified Production and Export of Dissolved Inorganic Carbon During Hot and Wet Subtropical Monsoon

Abstract

Understanding the origins and processes of riverine dissolved inorganic carbon (DIC) is crucial for predicting the global carbon cycle with projected, more frequent climate extremes yet our knowledge has remained fragmented. Here we ask: How and how much do DIC production and export vary across space (shallow vs. deep, uphill vs. depression) and time (daily, seasonal, and annual)? How do the relative contributions of biogenic (soil respiration) and geogenic (carbonate weathering) sources differ under different temperature and hydrological conditions? We answer these questions using a catchment‐scale reactive transport model constrained by stream flow, stable water isotopes, stream DIC, and carbon isotope data from a headwater karstic catchment in southwest China in a subtropical monsoon climate. Results show climate seasonality regulates the timing of DIC production and export. In hot‐wet seasons, high temperature accelerates soil respiration and carbonate weathering (up to a factor of three) via elevating soil CO2 and carbonate solubility, whereas high discharge enhances export by two orders of magnitude compared to cold‐dry seasons. Carbonate weathering is driven more by soil CO2 than water flow. At the annual scale, 92.9% and 7.1% of DIC was produced in shallow and deep zone, respectively, whereas 64.5% and 35.5% of DIC was exported from shallow and deep zone, respectively. These results highlight the uniqueness of subtropical karst areas as synchronous reactors and transporters of DIC during the hot‐wet monsoon, contrasting the asynchronous production and export in other climate regions. A future hotter and wetter climate with more intensive storms in the region may further intensify DIC production and export, accentuating the potential of subtropical karst regions as global hot spots for carbon cycling.

Illuminating the “Invisible”: Substantial Deep Respiration and Lateral Export of Dissolved Carbon From Beneath Soil

“…Results here show that ∼70% of DIC was biogenic, consistent with a previous study at W-9 suggesting that the average contribution of soil CO 2 to groundwater DIC was higher than 60% (Doctor et al, 2008). A recent modeling study in a catchment with karst geology also found that ∼60% of riverine DIC was produced from biogenic sources (Wen et al, 2024). Similarly, a stable carbon isotope analysis that differentiated sources of biogenic and geogenic DIC in a catchment with dolomite geology showed that 83%-94% of the total DIC flux was biogenic (Duvert et al, 2020).…”

“…Production rates for DIC in the SZ were sensitive to both air temperature and water storage (Figure 6). Surprisingly, carbonate weathering rates were higher in the SZ than the DZ (Figure 8), potentially reflecting the faster shallow soil water flow that, when present, often drives carbonate dissolution to occur (Wen et al, 2024).…”

Hydrology Outweighs Temperature in Driving Production and Export of Dissolved Carbon in a Snowy Mountain Catchment

“…rates are often strongly dependent on temperature (Dawson et al, 2008;Perdrial et al, 2014;Wen et al, 2020Wen et al, , 2024Winterdahl et al, 2016). This supports the idea that soil moisture influences production more in seasonally defined catchments with large fluctuations in soil moisture (Li et al, 2017).…”

“…Carbon respiration within bedrock fractures has been shown to exhibit rates comparable to those observed in soils (Hasenmueller et al, 2017;Tune et al, 2020). Deep respiration beneath soils have been shown to contribute to 40% of DIC production and >80% of DIC export (Stewart et al, 2024), and >7% of DIC production and >35% of DIC export (Wen et al, 2024). These deep processes however are typically not considered or quantified in Earth System models (Harper & Tibbett, 2013).…”