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Earth's Critical Zone (CZ), the near‐surface layer where rock is weathered and landscapes co‐evolve with life, is profoundly influenced by the type of underlying bedrock. Previous studies employing the CZ framework have focused primarily on landscapes dominated by silicate rocks. However, carbonate rocks crop out on approximately 15% of Earth's ice‐free continental surface and provide important water resources and ecosystem services to ∼1.2 billion people. Unlike silicates, carbonate minerals weather congruently and have high solubilities and rapid dissolution kinetics, enabling the development of large, interconnected pore spaces and preferential flow paths that restructure the CZ. Here we review the state of knowledge of the carbonate CZ, exploring parameters that produce contrasts in the CZ in different carbonate settings and identifying important open questions about carbonate CZ processes. We introduce the concept of a carbonate‐silicate CZ spectrum and examine whether current conceptual models of the CZ, such as the conveyor model, can be applied to carbonate landscapes. We argue that, to advance beyond site‐specific understanding and develop a more general conceptual framework for the role of carbonates in the CZ, we need integrative studies spanning both the carbonate‐silicate spectrum and a range of carbonate settings.

 

Soil sampling pits across three hillslope positions - toeslope, backslope, and summit - were dug in 2020 in watershed N4D (burned every 4 years) and N1D (burned annually) to characterize the impacts of woody encroachment on subsurface soil physical, chemical, and biological properties. Pits were hand-dug to 120 cm in the toeslope position and to 60 cm deep at the backslope and summit positions. Soil pits in N4D were dug directly under dogwood shrubs (Cornus drumondii) while pits in N1B were dug under grasses and forbs. Soil pit faces were photographed to determine root fractions with depth,  soil monoliths were take to charaterize soil macroporosity with depth while soil cores were taken in each horizon for water retention analysis.  Soil sensors were also installed at four soil depths at the toeslope position and 3 soil depths at the backslope and summit positions to record half hourly soil moisture, soil temperature, soil water potential, soil electrical conductivity, and soil carbon dioxide, and soil oxygen. In addition, geophysical measurements were taken in N4D using time-lapse electrical resistivity. 

Abstract. Carbonate weathering is essential in regulating atmosphericCO2 and carbon cycle at the century timescale. Plant roots accelerateweathering by elevating soil CO2 via respiration. It however remainspoorly understood how and how much rooting characteristics (e.g., depth anddensity distribution) modify flow paths and weathering. We address thisknowledge gap using field data from and reactive transport numericalexperiments at the Konza Prairie Biological Station (Konza), Kansas (USA), asite where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots can enhance weathering in two ways.First, deepening roots can control thermodynamic limits of carbonatedissolution by regulating how much CO2 transports vertical downward tothe deeper carbonate-rich zone. The base-case data and model from Konzareveal that concentrations of Ca and dissolved inorganic carbon (DIC) areregulated by soil pCO2 driven by the seasonal soil respiration. Thisrelationship can be encapsulated in equations derived in this workdescribing the dependence of Ca and DIC on temperature and soil CO2. The relationship can explain spring water Ca and DIC concentrations from multiple carbonate-dominated catchments. Second, numericalexperiments show that roots control weathering rates by regulating recharge(or vertical water fluxes) into the deeper carbonate zone and exportreaction products at dissolution equilibrium. The numerical experimentsexplored the potential effects of partitioning 40 % of infiltrated waterto depth in woodlands compared to 5 % in grasslands. Soil CO2 datasuggest relatively similar soil CO2distribution over depth, which in woodlands and grasslands leads only to 1 % to∼ 12 % difference inweathering rates if flow partitioning was kept the same between the two landcovers. In contrast, deepening roots can enhance weathering by ∼ 17 % to200 % as infiltration rates increased from 3.7 × 10−2 to 3.7 m/a. Weathering rates in these cases however are more than an order of magnitude higher than a case without roots atall, underscoring the essential role of roots in general. Numericalexperiments also indicate that weathering fronts in woodlands propagated> 2 times deeper compared to grasslands after 300 years at aninfiltration rate of 0.37 m/a. These differences in weathering fronts areultimately caused by the differences in the contact times of CO2-charged water with carbonate in the deep subsurface. Within the limitation of modeling exercises, these data and numerical experiments prompt the hypothesis that (1) deepening roots in woodlands can enhance carbonate weathering by promotingrecharge and CO2–carbonate contact in the deepsubsurface and (2) the hydrological impacts of rooting characteristics canbe more influential than those of soil CO2 distribution in modulatingweathering rates. We call for colocated characterizations of roots,subsurface structure, and soil CO2 levels, as well as their linkage to waterand water chemistry. These measurements will be essential to illuminatefeedback mechanisms of land cover changes, chemical weathering, globalcarbon cycle, and climate.