In-silico calculation of soil pH by SCEPTER v1.0

Abstract. One of the soil properties most commonly measured to describe agronomic and biogeochemical conditions of soils is “soil pH”. Soil pH measures the concentration of exchangeable H⁺ that resides in bulk soil samples taken from the field, through aqueous H⁺ measurements of extractants (e.g., deionized water or electrolyte solutions) added to dried bulk soil samples in the laboratory. Therefore, “soil pH” differs from “porewater pH”, the latter of which we define here as an in-situ measure of porewater H⁺ concentration in soil/weathering profiles. The difference between the two pH measurements is often not fully known for a given system but could lead to a misunderstanding of soil conditions if the two measurements are directly compared. Agricultural soils are one of the targeted loci for application of the “Enhanced Rock Weathering” (ERW), a technique aimed at counteracting increasing anthropogenic carbon dioxide from burning fossil fuels, and an increase in pH is thought to be one of key advantages of ERW as this can mitigating soil acidification and secure crop yields. As a result, fully evaluating the biogeochemical and agronomic consequences of ERW approaches requires accurate simulation of both soil pH (pH_s) and porewater pH (pH_pw). This paper presents an updated version of the reactive transport code SCEPTER (Soil Cycles of Elements simulator for Predicting TERrestrial regulation of greenhouse gases), which enables simulation of bulk soil pH measurement in the laboratory in addition to porewater pH as measured in the field along with a more comprehensive representation of cation exchange with solid-phase constituents of bulk soil. We first describe the implementation of cation exchange in the SCEPTER model, then introduce conceptual modelling frameworks enabling the calculation of bulk pH_s. The validity of the model is examined through comparison of model results with soil pH measurements from mesocosm experiments of maize production with crushed basalt amendments. Finally, illustrative example simulations are shown demonstrating that a difference between pH_s and pH_pw can lead to significantly different estimates of carbon capture by ERW for a given targeted pH in cropland systems.