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

摘要

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 CO _2 in soil air and shallow groundwater DIC (dissolved inorganic C). Soil air CO _2 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 CO _2 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 pCO _2 reached its maximum of 5% in October; the lag-time of 2-3months may correspond to the travel time of soil-generated CO _2 to the water table. The time-variable CO _2 caused an annual carbonate-mineral saturation cycle, intensifying limestone dissolution and DIC production when CO _2 was high.The C flux depended on respiration and rainfall regimes, and had two main pathways. Transport of soil CO _2 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 CO _2 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 CO _2 downward. Calculated C flux from the unsaturated zone to the unconfined aquifer in the monitoring period was 0.26±0.03M/m ~2/a of C, which is less than 1% of the CO _2 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.