Seasonal hysteresis in soil respiration reveals hydroclimatic control on pco2 dynamics and calcite precipitation in vertisols

Abstract

To better understand the controls on CO2 dynamics in modern soils and the timing of pedogenic carbonate precipitation, soil pore space gases (N2, Ar, O2, and CO2) were measured in a north Texas Vertisol over a period of 15 months. Soil CO2 concentration is controlled primarily through soil productivity (defined here as root respiration and microbial oxidation of organic matter) consuming O2 and diffusion from the atmosphere. Thus, the relationship between gaseous O2 and CO2 is modified by molecular diffusion but stoichiometrically predictable. We found that throughout the year there are seasonal deviations from the stoichiometric trend, where there is much less CO2 than would be predicted from soil O2 concentrations. The most pronounced periods of CO2 deficit coincide with the wet season, when the water saturated soil provides conditions favorable for the dissolution of pedogenic calcite and the pore space gas to dissolve. Both processes remove CO2 from the air-filled pore space and induce a seasonal hysteresis that appears repeatable. To determine if these processes could explain the CO2 discrepancy, we modeled the system in PHREEQC, with and without calcite precipitation. Our results show that gas solubility alone was not enough to account for the CO2 deficit, but the combination of 1) calcite dissolution/precipitation and 2) pore space gas dissolution/exsolution closely matches the CO2 behavior observed in the Vertisol. This seasonal hysteresis appears to be driven by fluctuations in soil water content and temperature. The respiration produced CO2 is removed due to calcite mineral dissolution and gas solubility as soil water increases during cool, wet seasons. When soil water decreases, calcite precipitation and degassing returns CO2 to the system, thus the CO2 to O2 ratios reflect what is expected from a respiration dominated environment.