Spatial and process-based modeling of soil inorganic carbon storage in an arid piedmont

Abstract

Inorganic carbon (C) comprises approximately a third of the total C pool in soils worldwide, largely in the form of pedogenic calcite. Pedogenic calcite occurs in semi-arid and arid regions in the form of calcareous, calcic, and petrocalcic horizons. Understanding the processes governing the storage and flux of this inorganic C pool is crucial to the development of accurate regional and global C budgets. Objectives of this study were to: (1) develop a process-based pedogenic calcite model that accounts for landscape hydrology and geomorphic position, (2) calibrate model parameters with field data from a Mojave Desert landscape, and (3) apply the model to understanding soil inorganic C response to elevated atmospheric CO 2 concentrations. Two parameters are introduced in this study to account for surface water redistribution on the landscape: the precipitation threshold (Ω) and the topographic index threshold (Λ). These parameters account for runoff-generating rainfall events (Ω) and the potential to shed or collect water at a location (Λ). A one-dimensional compartmentalized thermodynamic model is used to simulate calcite precipitation and dissolution. Geomorphic delineation, sediment collected from dust traps, and detailed soil descriptions including percent sand and clay, and bulk density from 16 soil pits of the southern Fry Mountain piedmont served as inputs into the model. Precipitation and temperature inputs were simulated using stochastic models. Time steps for the sample sites used to calibrate the model were determined from optically-stimulated luminescence dates. Incorporation of precipitation and topographic index thresholds into the model allowed more information to be extracted from carbonate depth distributions than previous pedogenic calcite models. The model predicts a net loss of inorganic C from the upper 10 cm soil depth under elevated atmospheric CO 2 and a consequent gain in the 10–20 cm depth. The leaching of calcite from shallow depths to below 10 cm is evident in all landforms including those most susceptible to wind erosion. This leads to the conclusion that increased atmospheric CO 2 levels may result in a concentration of inorganic C at a depth that is protected from wind erosion creating a more geomorphically stable pool of C.