TRACKING SOIL CARBONATE DISSOLUTION AND PRECIPITATION THROUGH HIGH-PRECISION MONITORING OF SOIL PORE SPACE O2 AND CO2

Authigenically formed soil carbonates occur in many dryland soils. They can accumulate in soils over thousands of years, gradually driving changes in soil hydrology, chemistry, and nutrient availability. Developing a complete understanding of the seasonal processes that control soil carbonate formation has proven difficult, due in large part to uncertainties in relating short term instrumental records to the geochemistry of carbonates that have been accumulating for millennia.

In an effort to better understand the processes controlling soil carbonate pedogenesis, we designed a series of controlled laboratory and field experiments in which powdered calcite was added to previously carbonate-free soils. In order to non-destructively track soil carbonate dissolution and precipitation, soil moisture, temperature and the concentrations of pore space O₂ and CO₂ were monitored. Soil CO₂ and O₂ were measured using a continuous flow analyzer coupled with an automated soil gas sampling system. The concentration of soil CO₂ is affected by the dissolution/precipitation of calcium carbonate and changes in soil respiration, both of which are mediated by soil moisture and temperature, while soil O₂ levels are primarily controlled by changes in soil respiration. These simultaneous measurements allow us to separate the carbonate dissolution/precipitation CO₂ signal from changes driven by respiration.

Our results indicate that after soil wetting, large, rapid calcium carbonate dissolution events are followed by significantly longer periods of gradual calcium carbonate formation. Similar trends in oxygen were observed between treatment and control soils, while treatment exhibited a smaller and lagged peak in CO₂. Additionally, large differences in the δ¹³C values of soil-respired CO₂ were observed between treatment and control soils during carbonate dissolution events, with δ¹³C values 10 ‰ higher in the treatment soils. These results indicate that the conceptual open system with fixed, respiration-controlled CO₂ does not adequately describe the soil system during rapid wetting events. Instead, a numerical model in which gas-water equilibration occurs in a closed system with constant sum CO₂ constraint at each time step more closely explains the experimental observations.