Grassland woody encroachment alters subsurface mineral weathering and groundwater composition in a carbonate system

Displacement of grasses by woody plants (woody encroachment) is occurring in grasslands worldwide. Previous studies indicate that encroachment can alter subsurface carbon dioxide (CO2) concentrations and mineral weathering, though these impacts are still poorly understood. To address this knowledge gap, we sampled groundwater and stream water every three weeks during the 2022 water year from two watersheds at Konza Prairie Biological Station, a native tallgrass prairie underlain by limestone and mudrock units in Kansas, USA. Amounts of woody encroachment differ between the watersheds primarily because of differences in fire frequency. One watershed is burned annually and contains 6 % and 45 % woody plant coverage in its upland and riparian areas, respectively, whereas the other is burned every four years and contains 28 % and 74 % woody plant coverage, respectively. We expected to find higher CO2 levels in the more encroached watershed, assuming the deep roots of woody plants increase inputs of CO2 to bedrock. However, we found the opposite. Our results indicate that groundwater from a single limestone aquifer contained an average of 1.4 mM CO2 in the less encroached watershed and 1.0 mM CO2 in the more encroached watershed. Similarly, stream water CO2 concentrations at the outlet of the less encroached watershed (0.25 mM) were more than twice that of the more encroached watershed (0.12 mM) on average. Despite these differences in CO2 concentration, amounts of mineral weathering per liter of groundwater differed little between watersheds. We hypothesize that encroachment is causing differences in CO2 concentrations between watersheds by decreasing the proportion of mineral weathering that occurs under conditions that are open with respect to CO2 exchange. During open-system weathering, dissolved CO2 consumed by weathering reactions can be replaced from an adjacent gas phase, allowing CO2 concentrations to remain elevated as weathering progresses. In contrast, during closed-system weathering, CO2 is not replaced and decreases in concentration as weathering progresses. If weathering primarily occurs under open-system conditions within the study area soils, which are unsaturated, and closed-system conditions within the underlying bedrock, where pores are more commonly saturated, then woody encroachment has the potential to decrease the proportion of open-system weathering by increasing soil permeability and thus decreasing soil water residence times. This hypothesis is consistent with our findings and implies that a shortening of soil water residence time with woody encroachment lowers the proportion of CO2 delivered from the soil to the subsurface and creates a more aggressive weathering engine at depth and along deeper flow paths. Encroachment may also be altering soil CO2 production and/or venting, though these possibilities require further investigation.