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 using extractants in the laboratory and thus differs from porewater pH, 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 the application of enhanced weathering (EW), a technique aimed at counteracting increasing anthropogenic carbon dioxide from burning fossil fuels. An increase in pH is thought to be one of the key advantages of EW, given that the process can mitigate soil acidification and increase crop yields. As a result, fully evaluating the biogeochemical and agronomic consequences of EW approaches requires accurate simulation of both soil pH (pHs) and porewater pH (pHpw). 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 measurements 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 modeling frameworks enabling the calculation of bulk pHs. The validity of the model is examined through comparison of model results with soil pH measurements from mesocosm experiments on maize production with crushed basalt amendments. Finally, illustrative example simulations are shown, demonstrating that a difference between pHs and pHpw can lead to significantly different estimates of soil alkalinization and carbon capture by EW for a given targeted pH in cropland systems.