Land-Use Effects of Different Forage Systems on Deep Subsoil Nutrients in Karst.

Over 20% of global land mass and a large portion of the eastern continental U.S. is karst or pseudokarst, including nearly all of Florida. Production agriculture in karst has the potential to degrade groundwater quality due to elevated concentrations of easily leachable chemical species, such as nitrate. Forage-based agriculture is represented by many different systems, including grazing, hay, and even crop rotations, such as sod-based crop rotations. Little is known about the long-term impact these different systems have on nutrient migration to deep subsoils or recycling of nutrients from subsoils to surface soils. A survey of seven different forage-based production land-uses, along with a rotational crop system and an undisturbed, natural area was conducted in 2014 in North Florida karst having kandiudult soil types. Land-use for each system was maintained for ten years or more. Deep subsoil cores (0 – 6 m) were collected from each location and sectioned with depth. Nitrogen (total, inorganic, and natural abundance), Mehlich-3 extractable soil nutrients, and soil sequential extractions were performed. At a 6 m depth, soil nitrate averaged less than 1 mg kg^-1 from undisturbed land. In comparison, the bahiagrass pasture averaged 5 mg kg^-1, bahiagrass and bermudagrass hay fields averaged below 2.5 mg kg^-1, the perennial peanut hay field averaged below 1 mg kg^-1, sod-based crop rotation under first year grazed bahiagrass averaged  below 10 mg kg^-1 with irrigation and above 10 mg kg^-1 non-irrigated. These data suggest some additional N leaching losses below 6 m, under irrigation. Irrigated conventional rotation cotton values averaged above 12 mg kg^-1 at a 6 m depth, but above 30 mg kg^-1 at a 2 m depth. Results from this survey will be discussed in terms of potential impacts production systems may have on karst groundwater quality.