Deep Subsoil Attenuation of Nitrate and Other Elements in Floridian Karst Terrain.

Production agriculture in karst terrain has the potential to degrade groundwater quality from elevated concentrations of easily leachable chemical species, such as nitrate.  Elevated nitrate is often found in Florida soils.  Highly-weathered soils with low pH and high iron oxide content have the potential to adsorb migrating nitrate and offer some protection to groundwater resources.  Little is known about the chemical composition of deep subsoils under production agriculture in karst terrain within the Florida panhandle.  Deep subsoil cores (1.5 – 11.3 m) were collected from a rotational cropping system at the UF-NFREC, Jackson County, FL.  Soil cores were sectioned and analyzed for soil pH, organic C, KCl-extractable NH₄-N, NO₃-N, metal oxides of Mn, Fe, Al, and Si, and elements associated with the metal oxide fraction.  Deep subsoil mineralogy was dominated by mixed metal oxides of Mn, Fe, Al, and Si, along with limestone layers.  Subsoil pH values were generally less than the PZNC of common soil Fe and Al oxide minerals.  Elevated nitrate was identified in layers dominated by high Fe oxide content.  Elevated Cr and P were also found in high Fe oxide layers.  Elevated Co and Ni were associated with high Mn oxide layers. Sulfate-S was uniformly distributed throughout the subsoils.  Estimated nitrate in the subsoils ranged from 300 to 2,700 Mg/ha at 1.5 to 11 m depths. It is unclear the potential impacts deep subsoil N stores may have on karst groundwater quality, particularly during extreme events, such as intense rainfall. It may be necessary to further reduce N inputs of cropping systems located in karst landscapes. Future work includes monitoring deep subsoil nitrate in a conventional crop rotation versus a rotation using variable rate irrigation and fertigation and that also includes a sod-forming (deep-rooted) perennial grass in the rotation.