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1. Hydrology, rather than wildfire burn extent, determines post‐fire organic and black carbon export from mountain rivers in central coastal California

   https://doi.org/10.1002/lol2.10360  

   Barton, Riley; Richardson, Christina M.; Pae, Evelyn; Montalvo, Maya S.; Redmond, Michael; Zimmer, Margaret A.; Wagner, Sasha (October 2023, Limnology and Oceanography Letters)

   Abstract Coastal mountain rivers export disproportionately high quantities of terrestrial organic carbon (OC) directly to the ocean, feeding microbial communities and altering coastal ecology. To better predict and mitigate the effects of wildfires on aquatic ecosystems and resources, we must evaluate the relationships between fire, hydrology, and carbon export, particularly in the fire‐prone western United States. This study examined the spatiotemporal export of particulate and dissolved OC (POC and DOC, respectively) and particulate and dissolved black carbon (PBC and DBC, respectively) from five coastal mountain watersheds following the 2020 CZU Lightning Complex Fires (California, USA). Despite high variability in watershed burn extent (20–98%), annual POC, DOC, PBC, and DBC concentrations remained relatively stable among the different watersheds. Instead, they correlated significantly with watershed discharge. Our findings indicate that hydrology, rather than burn extent, is a primary driver of post‐fire carbon export in coastal mountain watersheds. 

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

   https://doi.org/10.1029/2023WR035940  

   Stewart, Bryn; Shanley, James_B; Matt, Serena; Seybold, Erin_C; Kincaid, Dustin_W; Vierbicher, Andrew; Cable, Bren; Hicks, Niara; Perdrial, Julia_N; Li, Li (June 2024, Water Resources Research)

   Abstract 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. 

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3. Du Feu à l'Eau: Source and Flux of Dissolved Black Carbon From the Congo River

   https://doi.org/10.1029/2020GB006560  

   Drake, Travis W.; Wagner, Sasha; Stubbins, Aron; Wabakanghanzi, José N.; Dinga, Jean Bienvenu; Six, Johan; Spencer, Robert G. M. (August 2020, Global Biogeochemical Cycles)

   Abstract Pyrogenic organic residues from wildfires and anthropogenic combustion are ubiquitous in the environment and susceptible to leaching from soils into rivers, where they are known as dissolved black carbon (DBC). Here we quantified and isotopically characterized DBC from the second largest river on Earth, the Congo, using 12 samples collected across three annual hydrographs from 2010 to 2012. We find that the Congo River exports an average of 803 ± 84 Gg‐C as DBC per year, comprising 7.5% of the river's average annual dissolved organic carbon (DOC) flux (10.7 ± 1.2 Tg‐C yr⁻¹). Concentrations of DBC were strongly correlated with discharge and DOC concentration, indicating transport limitation for DBC flux from the Congo River Basin. Stable carbon isotopic signatures of DBC revealed a seasonal shift in pyrogenic source from forest dominant to an increasing contribution from savannah biomass, which derives from the North‐South bimodal hydrologic regime within the basin. Our results also indicate that black carbon produced within the Congo Basin is exported by the river on relatively short time scales and that total DBC export will increase with climate change predictions for the central African region. 

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

   https://doi.org/10.1029/2023WR036077  

   Kerins, Devon; Sadayappan, Kayalvizhi; Zhi, Wei; Sullivan, Pamela_L; Williams, Kenneth_H; Carroll, Rosemary_W_H; Barnard, Holly_R; Sprenger, Matthias; Dong, Wenming; Perdrial, Julia; et al (July 2024, Water Resources Research)

   Abstract Terrestrial production and export of dissolved organic and inorganic carbon (DOC and DIC) to streams depends on water flow and biogeochemical processes in and beneath soils. Yet, understanding of these processes in a rapidly changing climate is limited. Using the watershed‐scale reactive‐transport model BioRT‐HBV and stream data from a snow‐dominated catchment in the Rockies, we show deeper groundwater flow averaged about 20% of annual discharge, rising to ∼35% in drier years. DOC and DIC production and export peaked during snowmelt and wet years, driven more by hydrology than temperature. DOC was primarily produced in shallow soils (1.94 ± 1.45 gC/m²/year), stored via sorption, and flushed out during snowmelt. Some DOC was recharged to and further consumed in the deeper subsurface via respiration (−0.27 ± 0.02 gC/m²/year), therefore reducing concentrations in deeper groundwater and stream DOC concentrations at low discharge. Consequently, DOC was primarily exported from the shallow zone (1.62 ± 0.96 gC/m²/year, compared to 0.12 ± 0.02 gC/m²/year from the deeper zone). DIC was produced in both zones but at higher rates in shallow soils (1.34 ± 1.00 gC/m²/year) than in the deep subsurface (0.36 ± 0.02 gC/m²/year). Deep respiration elevated DIC concentrations in the deep zone and stream DIC concentrations at low discharge. In other words, deep respiration is responsible for the commonly‐observed increasing DOC concentrations (flushing) and decreasing DIC concentrations (dilution) with increasing discharge.  DIC export from the shallow zone was ~66% of annual export but can drop to ∼53% in drier years. Numerical experiments suggest lower carbon production and export in a warmer, drier future, and a higher proportion from deeper flow and respiration processes. These results underscore the often‐overlooked but growing importance of deeper processes in a warming climate. 

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5. Annual Net Community Production of Particulate and Dissolved Organic Carbon From a Decade of Biogeochemical Profiling Float Observations in the Northeast Pacific

   https://doi.org/10.1029/2020GB006599  

   Haskell, II, W. Z.; Fassbender, A. J.; Long, J. S.; Plant, J. N. (October 2020, Global Biogeochemical Cycles)

   Abstract Carbon export out of the surface ocean via the biological pump is a critical sink for atmospheric carbon dioxide. This process transports organic carbon to the deep ocean through sinking particulate organic carbon (POC) and the downward transport of suspended POC and dissolved organic carbon (DOC). Changes in the relative contribution of each pathway can significantly affect the magnitude and efficiency of carbon export to depth. Net community production (NCP), an analog of carbon export under steady state assumptions, is typically estimated using budgets of biologically important chemical tracers in the upper ocean constrained by ship‐board or autonomous platform observations. In this study, we use measurements from biogeochemical profiling floats, the Ocean Station Papa mooring, and recently developed algorithms for carbonate system parameters to constrain budgets for three tracers (nitrate, dissolved inorganic carbon, and total alkalinity) and estimate NCP in the Northeast Pacific from 2009 to 2017. Using our multiple‐tracer approach, and constraining end‐member nutrient ratios of the POC and DOC produced, we not only calculate regional NCP throughout the annual cycle and across multiple depth horizons, but also partition this quantity into particulate and dissolved portions. We also use a particle backscatter‐based approach to estimate POC attenuation with depth and present a new method to constrain particle export across deeper horizons and estimate in situ export efficiency. Our results agree well with previously published estimates of regional carbon export annually and suggest that the approaches presented here could be used to assess the magnitude and efficiency of carbon export in other regions of the world's oceans. 

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