Soil and Sediment Nutrient Dynamics Under Variable Climate Conditions In A Mississippi Agricultural Watershed

The Mississippi Alluvial Plain is a region that experiences highly variable climate and hydroperiod fluctuations. Over-bank flooding from regional rivers and back-flooding from the Mississippi River often inundate soils and sediments for weeks, if not months, of the year. Conversely, dry summer conditions can desiccate water-logged sediment in ditches and ponds. Wetting and drying of soils and sediments can alter redox properties, microbial content, and induce nutrient transformations that have been shown to lead to releases of phosphate and ammonium and release or uptake of nitrate. Soils located in the agriculturally rich Mississippi Alluvial Plain of the USA have medium to high phosphorus content and may experience long periods of saturation followed by drying, as well as periods of inundation due to flooded conditions. This study was designed to estimate the potential phosphorus and nitrogen fluxes that may occur when dry soils and sediments are re-wetted and remain submerged in water for various residence times. Soils/sediments were collected from two locations within five different habitats (cropland, Conservation Reserve Program land (CRP; i.e. reclaimed agricultural land), forest/riparian wetland, sediment retention pond, and drainage ditches) within Beasley Lake Watershed, Mississippi. Soil sample locations were selected to include one location per habitat that typically experiences ephemeral standing water, such as topographic depressions within the cropland, CRP, and forested areas. All sediment sample locations included ephemeral or perennial standing water. All soil/sediment samples were dried, ground, and sieved for consistency to 2 mm. To simulate flooding, samples were inundated with water, and replicates were incubated at 5, 15, 25 and 30°C to represent different seasonal temperatures throughout the year. Water was sampled at 4, 8, 12, 24 and 48 hours to evaluate the kinetic properties of nutrient fluxes. Preliminary data indicated that phosphate release increased with increasing temperature, ammonia release increased up to 25C and then decreased, and nitrate release generally decreased with increasing temperature. Overall, phosphate and nitrate releases were highest in the dry riparian habitat, the cropland sites, and the ditches. Ammonia releases were highest in the consistently wetted areas with high organic matter, e.g., sediment retention pond, flooded riparian wetland, and ditches. As many areas of this region experience recurring flooding due to overbank flow and backwater, these results may help to understand the timing and potential extent of nutrient fluxes caused by these extreme events.