The effect of precipitation events on inorganic carbon in soil and shallow groundwater, Konza Prairie LTER Site, NE Kansas, USA

Monthly sampling for 1year at the Konza Prairie LTER (Long-Term Ecological Research) Site in northeastern Kansas shows a connection between the annual cycles of CO₂ in soil air and shallow groundwater DIC (dissolved inorganic C). Soil air CO₂ reached 6–7% in July to mid-August, when moisture was not limiting to soil respiration. Following the annual maximum there was a sequential decrease in CO₂ in three soil horizons to less than 0.5% because of moisture deficiency in the late summer and temperature decline in the fall and winter. Groundwater pCO₂ reached its maximum of 5% in October; the lag-time of 2–3months may correspond to the travel time of soil-generated CO₂ to the water table. The time-variable CO₂ caused an annual carbonate-mineral saturation cycle, intensifying limestone dissolution and DIC production when CO₂ was high. The C flux depended on respiration and rainfall regimes, and had two main pathways. Transport of soil CO₂ in the dissolved form with diffuse flow of recharge water was the most effective during the growing season so long as soil moisture was present. Downward movement of gaseous CO₂ and equilibration with groundwater at the water table was favorable in July to August. Storm rainfall events recharged the aquifer within a few hours through preferential flow and stream–groundwater interaction, resulting in dilution of groundwater rather than forcing entrapped CO₂ downward. Calculated C flux from the unsaturated zone to the unconfined aquifer in the monitoring period was 0.26±0.03M/m²/a of C, which is less than 1% of the CO₂ that is released by soil to the atmosphere via efflux. However, meteoric precipitation was only 72% of average annual precipitation during the study period, so this study represents dry-condition flux. In addition, increased respiration rates due to warming of the atmosphere have the potential to cause a higher C flux to the saturated zone, intensifying weathering and groundwater acidification, so that further study is suggested.