CHEMICAL WEATHERING RATE INCREASING AT THE KONZA PRAIRIE LTER SITE, NE KANSAS, USA

The Konza Prairie Long-Term Ecological Research Site is a mesic, mid-continental, unplowed grassland where shallow groundwater and ephemeral-stream chemistry at a 1.2 km² upland watershed has been changing over at least the past 16 years. Along with an increase in groundwater CO₂ and decrease in pH, major species solute concentrations have increased. Annual amounts of watershed-scale chemical weathering, calculated by using water-rock interaction processes to account for the dissolved species, are highly variable: chemical weathering, expressed as mass of minerals exported as dissolved species in a sampling year by the stream, averaged 774 kg/hc/yr from 1992-2007, with 72% relative standard deviation (% RSD), and a range of 11% to more than 290% of the average. This variability results from the extreme variability in both meteoric precipitation and stream discharge. Meteoric precipitation from 1992 – 2007 averaged 865 mm (similar to the long-term average of 835 mm) with 19%RSD and a range of 77% to 148% of mean annual precipitation. Annual stream flow, 1992-2007, averaged about 268,000 m³ per year with 66% RSD, and a range of about 10% to 270% of mean annual stream flow. Normalizing the chemical weathering amounts to stream discharge amounts allows assessment of changes in weathering rates: over the 16-year study period, the rate of chemical weathering increased steadily by more than 3%. This increase is the likely outcome of global warming increasing soil respiration and groundwater CO₂ and thus groundwater acidification, resulting in steadily increasing rates of chemical weathering.