Rhizoliths formation: mechanistic models and implications for paleoenvironmental reconstructions

Rhizoliths, cylindrical concretions formed mostly by CaCO₃  accumulation around plant roots, serve as valuable indicators of environmental conditions and ecosystem dynamics such as carbon sequestration and water balance. Despite increasing attention to rhizolith formation, there remains a lack of numerical, laboratory, and field experiments. For the first time, we developed a dynamic model of rhizolith formation in CaCO₃-containing loess soils, considering water fluxes toward roots, Ca²⁺ and CO₃^2- concentration in soil solution, and potential evapotranspiration rates (ET_o). Using numerical simulations with the HYDRUS-1D model, we explored the interplay between these factors and their impacts on rhizolith development. Hydraulic fluxes facilitate Ca²⁺ (simulated at 0.13, 0.15, 0.3, and 1 mmol L^-1) transport towards the rhizosphere as a function of root water uptake at low (ET_o = 0.03 cm d^-1) and high (ET_o = 1 cm d^-1) water flow rates under initial optimal (h_o = -100 cm) and intermediate (h_o = -1000 cm) moisture conditions. An extensive simulation run was critical for achieving zero-suction gradient (dh/dz =0) in the model, which was attained at 374-year run (T_ԑ), with equilibrium water content Ɵ_ԑ of 0.089 cm³ cm^-3 and yields 0.23 cm³ cm^-3 threshold porosity for calcite saturation ɸ _Casat, equivalent to 72% of the loess porosity ɸ of 0.32 cm³ cm^-3. The equilibrium properties at T_ԑ enabled differentiation between hydraulic constraints and jamming of the porous medium by calcite saturation as the causes of the standstill in the calcification function. On top of that, our work unfolds root encasement and reliquary varieties with their concomitant physical and biogeochemical mechanisms underlying rhizolith transformations. At intermediate soil-water conditions with 1 mmol L^-1 Ca²⁺, tempo-sequential evolution of rhizoliths of radii 0.2, 1, 2, and 3 cm occurs in respectively 1.5, 9.5, 85, and 150 years. Each rhizolith layer harbors CaCO₃ constituents (namely, δ¹⁸O, δ¹³C, ⁴⁴Ca, ⁴⁶Ca, and ⁴⁸Ca), organic biomarker compounds from root (e.g., lignin), and clumped isotopes (Δ⁴⁷) among others which are preserved across time into the future. Therefore, this work conceptualizes rhizolith as a ‘time-capsule’ with each CaCO₃ layer encapsulating a snapshot of vital environmental proxies, providing a window into otherwise inaccessible historic ecosystem dynamics.