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The role of vegetation, drying, and rewetting on water and sediment phosphorus concentrations in an experimental stream channel experiment.
Proceedings of the 2023 Mississippi Water Resources Conference

Year: 2023 Authors: Pawlowski E., Witthaus L., Taylor J., Moore M., McNamara S.

Agricultural drainage ditch networks are an important pathway for nutrients. Changing environmental conditions within drainage ditch networks caused by intermittent drying and rewetting can control nutrient fate and transport. We delivered a phosphorus pulse to six experimental stream channels that subsequently underwent four wet and dry cycles to describe phosphorus retention and mobilization in agricultural ditch environments. Experimental stream channels were filled with sediment from the Mississippi Alluvial Plain and three of the channels were planted with vegetation (Leersia oryzoides) that was allowed to establish prior to the nutrient addition. We monitored discharge, conductivity, pH, salinity, and dissolved oxygen during each of the rewetting periods and during the initial addition of phosphorus salt. Following each wet period, the streams were drained and allowed to dry before the next wet cycle. We collected water samples during each of the rewetting periods and during natural precipitation events that generated flow within the constructed channels. We sampled sediments before and after each rewetting event and measured moisture content, particle size, and water extractable phosphorus on both field moist and air-dried samples. Sediment samples were stratified across channel features (two runs and two pools). Average dissolved phosphorus concentrations were approximately four times higher in unvegetated (0.35 mg L-1) verses vegetated streams (0.09 mg L-1) during the initial wetting event, but subsequent rewetting events had similarly low concentrations (0.01 to 0.03 mg L-1) among both treatments. Sediment increased in phosphorus concentrations following the initial phosphorus addition in all channels. By the fourth wet cycle those concentrations had generally decreased but showed variability throughout each cycle based on channel position and which wet-dry cycle the sediment was collected in. Vegetated sediments did not have as substantial of an increase in phosphorus as sediment from unvegetated channels (2.36 mg kg-1 vs. 4.07 mg kg-1 respectively). These results suggest that both vegetated and unvegetated ditches can store phosphorus through repeated drying and rewetting cycles, but vegetation and likely biological uptake reduces phosphorus in both the water column and sediment. These results show that vegetation may be a promising mitigation tool in ditch networks with intermittent flow but future work is needed to assess phosphorus storage over longer temporal scales.

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