Carbon Exports from Terrestrial Ecosystems: A Critical-Zone Framework

Influences of carbonate weathering and hyporheic exchange on carbon fluxes in Pearl River Basin, China

Deciphering riverine dissolved carbon dynamics is pivotal for a comprehensive picture of the global carbon cycle. Through rigorous in-situ sampling across the Pearl River Basin (PRB), our investigation reveals the Pearl River networks function as a significant carbon source, with the annual carbon dioxide (CO2) emission of 2.57 ± 1.94 Tg C, which offsets 10 ± 8 % of the forest carbon sequestration or 65 ± 49 % carbon sink via chemical weathering in the PRB. Based on the mass balance of 222Rn, we initially reveal that the contributions of water flux from the hyporheic zone increased with the river orders (Hack Order) across both dry and wet seasons. Conversely, the evasion rates of dissolved CO2 (CO2*) and dissolved inorganic carbon (DIC) from the hyporheic zone into river channels exhibited a decline with the increasing river orders. The hyporheic exchange contributes 4 - 11 % of the lateral and vertical DIC losses, thereby is a key mechanism in the riverine carbon cycle. Furthermore, CO2* derived from the hyporheic zone was ∼4 times of riverine CO2 emissions and this CO2* flux from the hyporheic zone was buffered into carbonates/bicarbonates in river channels, due to the high riverine pH resulted from carbonate weathering in the basin. These results not only highlight the substantial role of carbonates and hyporheic processes in modulating riverine carbon fluxes but also signify their broader implications on understanding riverine carbon dynamics at both regional and global scales.

Climate forcing controls on carbon terrestrial fluxes during shale weathering

Climate influences near-surface biogeochemical processes and thereby determines the partitioning of carbon dioxide (CO2) in shale, and yet the controls on carbon (C) weathering fluxes remain poorly constrained. Using a dataset that characterizes biogeochemical responses to climate forcing in shale regolith, we implement a numerical model that describes the effects of water infiltration events, gas exchange, and temperature fluctuations on soil respiration and mineral weathering at a seasonal timescale. Our modeling approach allows us to quantitatively disentangle the controls of transient climate forcing and biogeochemical mechanisms on C partitioning. We find that ~3% of soil CO2 (1.02 mol C/m2/y) is exported to the subsurface during large infiltration events. Here, net atmospheric CO2 drawdown primarily occurs during spring snowmelt, governs the aqueous C exports (61%), and exceeds the CO2 flux generated by pyrite and petrogenic organic matter oxidation (~0.2 mol C/m2/y). We show that shale CO2 consumption results from the temporal coupling between soil microbial respiration and carbonate weathering. This coupling is driven by the impacts of hydrologic fluctuations on fresh organic matter availability and CO2 transport to the weathering front. Diffusion-limited transport of gases under transient hydrological conditions exerts an important control on CO2(g) egress patterns and thus must be considered when inferring soil CO2 drawdown from the gas phase composition. Our findings emphasize the importance of seasonal climate forcing in shaping the net contribution of shale weathering to terrestrial C fluxes and suggest that warmer conditions could reduce the potential for shale weathering to act as a CO2 sink.

High Dissolved Carbon Concentration in Arid Rocky Mountain Streams

Warming in mountains is known to intensify aridity and threaten water availability globally. Its impacts on water quality, however, have remained poorly understood. Here we collate long-term (multi-year to decadal mean), baseline stream concentrations and fluxes of dissolved organic and inorganic carbon, two essential indicators of water quality and soil carbon response to warming, across more than 100 streams in the United States Rocky Mountains. Results show a universal pattern of higher mean concentrations in more arid mountain streams with lower mean discharge, a long-term climate measure. A watershed reactor model revealed less lateral export of dissolved carbon (via less water flow) out of the watersheds in more arid sites, leading to more accumulation and higher concentrations. Lower concentrations typically occur in cold, steep, and compact mountains with higher snow fraction and lower vegetation cover, which generally have higher discharge and carbon fluxes. Inferring from a space-for-time perspective, the results indicate that as warming intensifies, lateral fluxes of dissolved carbon will decrease but concentrations will increase in these mountain streams. This indicates deteriorating water quality and potentially elevated CO₂ emission directly from the land (instead of streams) in the Rockies and other mountain areas in the future climate.