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

Dissolved organic and inorganic carbon (DOC and DIC) influence water quality, ecosystem health, and carbon cycling. Dissolved carbon species are produced by biogeochemical reactions and laterally exported to streams via distinct shallow and deep subsurface flow paths. These processes are arduous to measure and challenge the quantification of global carbon cycles. Here we ask: when, where, and how much is dissolved carbon produced in and laterally exported from the subsurface to streams? We used a catchment‐scale reactive transport model, BioRT‐HBV, with hydrometeorology and stream carbon data to illuminate the "invisible" subsurface processes at Sleepers River, a carbonate‐based catchment in Vermont, United States. Results depict a conceptual model where DOC is produced mostly in shallow soils (3.7 ± 0.6 g/m2/yr) and in summer at peak root and microbial respiration. DOC is flushed from soils to the stream (1.0 ± 0.2 g/m2/yr) especially during snowmelt and storms. A large fraction of DOC (2.5 ± 0.2 g/m2/yr) percolates to the deeper subsurface, fueling deep respiration to generate DIC. DIC is exported predominantly from the deeper subsurface (7.1 ± 0.4 g/m2/yr, compared to 1.3 ± 0.3 g/m2/yr from shallow soils). Deep respiration reduces DOC and increases DIC concentrations at depth, leading to commonly observed DOC flushing (increasing concentrations with discharge) and DIC dilution patterns (decreasing concentrations with discharge). Surprisingly, respiration processes generate more DIC than weathering in this carbonate‐based catchment. These findings underscore the importance of vertical connectivity between the shallow and deep subsurface, highlighting the overlooked role of deep carbon processing and export. Plain Language Summary: Dissolved organic and inorganic carbon (DOC and DIC) are important chemical species that affect water quality, ecosystem health, and carbon dioxide emissions from streams. DOC and DIC are produced through different reactions at and below the ground surface before they are transported to streams through underground flow paths. However, it is difficult to measure and observe these reactions and transport pathways, limiting our understanding of when, where, and how much dissolved carbon species are produced and exported from distinct subsurface depths. Here we used a computational model, BioRT‐HBV, to simulate reactions and transport processes and to better understand the production and export of dissolved carbon at Sleepers River, a small catchment in Vermont, United States. Results show that DOC was primarily produced through shallow subsurface reactions and exported through shallow flow paths. DIC was produced in both the shallow and deep subsurface but primarily exported through deep flow paths. Reactions that produced DOC and DIC occurred faster under warm and wet conditions (summer and spring), while export of DOC and DIC increased under wet conditions (spring, snowmelt, storms). Results suggest that climate change may affect the production and export of dissolved carbon species through increased temperatures and intense storm events. Key Points: Dissolved organic carbon (DOC) was mainly produced in warm summer and exported in wet spring from shallow subsurfaceDissolved inorganic carbon (DIC) was comparably produced in the shallow and deep subsurface but exported mostly from the deep subsurface in wet springDIC originated more from biogenic (soil respiration and deep respiration) than geogenic (carbonate weathering) sources in a carbonate‐based catchment [ABSTRACT FROM AUTHOR]