Deepening roots can enhance carbonate weathering by amplifying CO2-rich recharge
Authors: Wen Hang , Sullivan Pamela L. , Macpherson Gwendolyn L. , Billings Sharon A. , Li Li 
Published: January 2021

Abstract. Carbonate weathering is essential in regulating atmospheric
CO2 and carbon cycle at the century timescale. Plant roots accelerate
weathering by elevating soil CO2 via respiration. It however remains
poorly understood how and how much rooting characteristics (e.g., depth and
density distribution) modify flow paths and weathering. We address this
knowledge gap using field data from and reactive transport numerical
experiments at the Konza Prairie Biological Station (Konza), Kansas (USA), a
site 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 carbonate
dissolution by regulating how much CO2 transports vertical downward to
the deeper carbonate-rich zone. The base-case data and model from Konza
reveal that concentrations of Ca and dissolved inorganic carbon (DIC) are
regulated by soil pCO2 driven by the seasonal soil respiration. This
relationship can be encapsulated in equations derived in this work
describing 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, numerical
experiments show that roots control weathering rates by regulating recharge
(or vertical water fluxes) into the deeper carbonate zone and export
reaction products at dissolution equilibrium. The numerical experiments
explored the potential effects of partitioning 40 % of infiltrated water
to depth in woodlands compared to 5 % in grasslands. Soil CO2 data
suggest relatively similar soil CO2
distribution over depth, which in woodlands and grasslands leads only to 1 % to
∼ 12 % difference in
weathering rates if flow partitioning was kept the same between the two land
covers. In contrast, deepening roots can enhance weathering by ∼ 17 % to
200 % 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 at
all, underscoring the essential role of roots in general. Numerical
experiments also indicate that weathering fronts in woodlands propagated
> 2 times deeper compared to grasslands after 300 years at an
infiltration rate of 0.37 m/a. These differences in weathering fronts are
ultimately 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 promoting
recharge and CO2–carbonate contact in the deep
subsurface and (2) the hydrological impacts of rooting characteristics can
be more influential than those of soil CO2 distribution in modulating
weathering rates. We call for colocated characterizations of roots,
subsurface structure, and soil CO2 levels, as well as their linkage to water
and water chemistry. These measurements will be essential to illuminate
feedback mechanisms of land cover changes, chemical weathering, global
carbon cycle, and climate.