MISSISSIPPI RIVER ISLANDS AND THEIR POTENTIAL TO AFFECT NITROGEN CYCLING NEAR MEMPHIS, TN, USA

This study addresses the stratigraphy and subsurface hydrology of river islands and their relationship to nitrogen cycling in the Lower Mississippi River (LMR). The Mississippi River drains over 1.2 million square miles of North America, including a number of densely populated cities and highly fertilized agricultural land, and thus acts as a collection system for nitrogen, causing downstream eutrophication. Studies show that some nitrogen is removed or retained by sediment and vegetation in riparian wetland environments. The LMR has numerous mid-channel bars and vegetated islands that could play a significant role in nitrogen cycling. For this project, two sites were selected for observation; one vegetated, and one non-vegetated island. These two sites account for as much as 2/3 of the overall channel width at their respective reaches suggesting that they are of sufficient size to have a significant impact on nitrogen cycling.

Wells were installed at several sites on each island to assess stratigraphy and hydrology. Soil samples were characterized in the field and then later classified using sieve analysis. Representative soil samples from each 1-ft borehole increment were subjected to Loss on Ignition (LOI) analysis to assess their relative organic content and potential to promote nitrogen cycling. The non-vegetated island was found to be mostly sand with minor lenses of fine material. The vegetated island displayed alternating layers of fine sands and silt/clay representing deposits from seasonal flood events. Historical observations of the islands indicate that the vegetated island is more susceptible to vertical accretion, whereas the non-vegetated island is more susceptible to broader morphological changes. Data suggest the non-vegetated island behaves similarly to an unconfined system with potential perched zones, whereas the vegetated island may act as a series of confined to semi-confined aquifer units. The non-vegetated island samples showed ranges of LOI₃₆₀ between 0% and 1%, and LOI₅₅₀ below 0.3%. For the vegetated island samples, LOI₃₆₀ ranged from 0.3% to 5.1% and LOI₅₅₀ ranged from 0% to 2% indicating a higher organic content in the sediment of the vegetated island. These findings suggest that islands could potentially play a significant role in cycling and the overall nitrogen budget of the LMR.