The role of vegetation, drying, and rewetting on water and sediment phosphorus concentrations in an experimental ditch channel experiment

Intermittent drying and rewetting of agricultural drainage ditch networks along with vegetation presence play a significant role in phosphorus (P) fate and transport to the receiving water bodies. To examine how vegetation influences the response of P transport to drying and rewetting, we lined six low-gradient (0.01% slope) stainless steel experimental ditches with alluvial sediment and planted vegetation (rice cutgrass, Leersia oryzoides) in three ditches while maintaining bare sediment in the other three ditches. We then added a 26-g P pulse to each ditch and recirculated the ditch water for maximum retention prior to draining. Three subsequent wetting periods, separated by 10-day drying periods, used water with no added P that flowed through the ditches (i.e., water did not recirculate). We monitored discharge, water quality, and P concentrations during the initial P addition and subsequent rewetting periods. Sediment properties, including water-extractable P, were sampled before and after each rewetting period. The vegetated ditches rapidly removed P from the water column, but similar total removal was observed in all ditches following the first wet period. Subsequent rewetting periods released concentrations between 14 and 45 µg P L⁻¹ in both treatments. Integrated release rates showed 99% P retention in the vegetated ditches and 98% retention in unvegetated ditches after all wet and dry cycles. Water-extractable P was not found to be significantly different based on channel feature, vegetation presence, or time. Our results suggest P is likely retained in both vegetated and unvegetated low-gradient ditches with intermittent flow regimes and similar sediment properties.