Carbon Dioxide Production in Bedrock Beneath Soils Substantially Contributes to Forest Carbon Cycling. Issue 12 (5th December 2020) - Tune, Alison K.
Abstract: Soils are widely considered the primary terrestrial organic matter pool mediating carbon transactions with the atmosphere and groundwater. Because soils are both a host and a product of rhizosphere activity, they are thought to mark the location where photosynthetic fixation of carbon dioxide (CO2 ) is balanced by the oxidation of organic matter. However, in many terrestrial environments, the rhizosphere extends below soils and into fractured bedrock, and it is unknown if the resulting biological and hydrologic dynamics in bedrock have a significant impact on carbon cycling. Here we show substantial production of CO2 in weathered bedrock at 4–8 m below the thin soils (<0.5 m thick) of a Northern California forest using innovative monitoring technology for sampling gases and water in fractured rock. The deep CO2 production supports a persistent upward flux of CO2(g) year‐round from bedrock to soil, constituting between 2% and 29% of the average daily CO2 efflux from the land surface. When water is rapidly transported across the fractured bedrock vadose zone, nearly 50% of the CO2 produced in the bedrock dissolves into water, promoting water‐rock interaction and export of dissolved inorganic carbon (DIC) from the unsaturated zone to groundwater, constituting as much as 80% of the DIC exiting the hillslope. Such CO2 production in weathered bedrock is subject to unique moisture, temperature, biological, and mineralogical conditions which are currently missing from terrestrial carbon models. Plain Language Summary: In many ecosystems, roots and their associated microbiome extend past the soil into fractured rock. Yet the amount of CO2 production beneath soils is unquantified and it is therefore unclear how important this is to the global carbon cycle. Here, we show CO2 produced in fractured rock contributes significantly to both fluxes of CO2 to the soil (and thereafter the atmosphere) and fluxes of inorganic carbon to groundwater and stream water. These results indicate we should consider carbon cycling within weathered and fractured rock in larger climate and ecosystem models, rather than assume that organic carbon degradation in the upper soil drives CO2 export to the atmosphere and groundwater. Key Points: In an old growth forest, carbon dioxide production ("rock respiration") occurs within fractured bedrock at 4–8 m depth Rock respired CO2 contributes up to 29% of soil efflux and 80% of dissolved inorganic carbon in groundwater Rock respiration is seasonally dynamic and persists during periods when soils are dry