AGRICULTURAL IMPACTS ON GEOCHEMICAL AND HYDROLOGIC FLUXES IN THE CRITICAL ZONE (Invited Presentation)

Widespread intensive row crop agriculture in the Midwestern United States is vital to food and fuel production but has replaced native prairie vegetation and potentially altered geochemical and hydrologic feedbacks within this critical zone. To investigate the impact of agriculture on critical zone structure and function, two sites have been developed in Iowa and Nebraska as part of the Intensively Managed Landscapes Critical Zone Observatory (IML CZO). Like much of the Midwest, the sites are underlain by thick (8 – 20+ m) last-glacial loess and have experienced agricultural land use for ~150 years. Both study sites also contain small parcels of land restored to prairie 20 – 50 years ago, providing insight into critical zone processes under historical land use in the region. In both Iowa and Nebraska, basic atmospheric and soil moisture, temperature and electrical conductivity data are recorded by meteorological stations, and lysimeters sample soil porewater at four depths down to 1 m deep under both agriculture and restored prairie land uses. At the Nebraska site, streams draining agriculture or restored prairie are also sampled to measure cations, anions, dissolved organic carbon, alkalinity and oxygen isotopes. Soil moisture is significantly lower in the surface restored prairie soils compared to agricultural soils, implying rapid infiltration under restored prairie likely due to enhanced pore connectivity from deep root networks and soil aggregation. Soil electrical conductivity and Ca and Mg concentrations are greater in agricultural soils at all sampled depths compared to restored prairie soils, indicating enhanced water-mineral interactions in agricultural soils. Likewise, Ca and Mg fluxes in streams draining agricultural land use exhibit higher Ca and Mg fluxes compared to streams draining restored prairie. These observations lead to a conceptual model where agricultural land use, through the disruption of pore connectivity in surface soils, has altered soil hydrology, and therefore soil porewater and stream chemistry, compared to the critical zone under prairie vegetation. Furthermore, geochemical and hydrologic processes appear to respond rapidly to land use change in this critical zone.