GEOCHEMICAL REACTION FRONTS UNDER AGRICULTURE, RESTORED PRAIRIE, AND NATIVE PRAIRIE LAND USE IN A MIDWESTERN LOESS CRITICAL ZONE

In the Midwestern United States, intensive row crop agriculture has replaced native prairie vegetation, altering surface soil structure and pore networks that can influence how rapidly and deeply water infiltrates into the critical zone to drive chemical weathering processes. Here, we aim to investigate how land use influences soil geochemical profiles formed in last-glacial loess in eastern Nebraska with a mean annual temperature of 10 °C and mean annual precipitation of 78 cm. Deep soil cores (7 – 10 m) were collected from soils subject to ~150 years of row crop agriculture, soils restored from row crop agriculture to prairie ~50 years ago, and soils that have remained under historic native prairie vegetation. Soil cores were analyzed for bulk geochemistry and mineralogy, pH, particle size distribution, total and organic carbon, and cation exchange capacity to determine changes in geochemical profiles under each land use. Under agricultural land use, a carbonate reaction front appears at 1.5 m depth, where both Ca and Mg enrichment, as well as an increase in pH and the abundance of carbonate minerals, is observed. In contrast, soils that were restored to prairie vegetation from agriculture ~50 years ago are depleted of Ca, Mg and carbonate minerals to a depth of 3.5 m, where carbonate appears to accumulate. A native prairie soil profile that has no record of land use conversion to agriculture has depleted Ca and Mg to a depth of 2.5 m, at which point carbonate accumulates. Interestingly, the carbonate accumulation under both soil profiles influenced by agriculture either currently or in the recent past show an accumulation of organic carbon at the carbonate reaction front, whereas no increase in organic carbon is evident where carbonate accumulates at depth in the native soil core. Thus, vegetation may influence hydrologic fluxes that in turn control the depth of carbonate dissolution in loess parent material, which has implications not only for modern critical zone evolution and solute transport to streams, but also potentially for interpreting the climate and evolution of past critical zones.