Search for: All records

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.