## 2 Materials and Methods  
### Study Site  
A set of 6 artificial plumes at the University of Mississippi Field Station in Abbeville, MS was employed for this experiment (Figure 1A). Flumes were designed to simulate shallow, low-gradient ditches or streams which drain farmed lands of the lower MAP. Each flume had a 0.34 m\({}^{3}\) head pool followed by a sequence of run pool sections, where each section had a volume of 0.91 m\({}^{3}\), resulting in a total volume of 3.96 m\({}^{3}\) per flume. Flumes were filled with 1.54-1.74 m\({}^{3}\) of a mixture of Dundee silty loam and Sharkey clay soil typical of the nearby MAP region to provide a relatively consistent sediment depth of \(\sim\)5 cm across all sections. Water was supplied by a pond located \(\sim\)50 m from our experimental setup. This water had stable nutrient concentrations (0.091-0.224 NO\({}_{3}\)-N mg L\({}^{-1}\), 0.004-0.012 PO\({}_{4}\)-\({}^{3}\)-P mg L\({}^{-1}\)) and entered each flume through a PVC pipe at the main headpool and flowed through the run, pool, run, pool sequence before exiting through an outlet standpipe in the final pool. Outlet water flowed into a series of small wooded and vegetated wetlands. Half of the ditches (# 2, 3, and 6) were vegetated with _Leersia oryzae_ while the other half (# 1, 4, and 5) had bare soil (Figure 1B). Three months prior to the start of the experiment, _L. oryzae_ was planted at a density of approximately 132 stems m\({}^{-2}\) (\(\mu\) \(\pm\) SE, 132 \(\pm\) 27) from a nearby floodplain wetland complex area located on the field station property. We chose _L. oryzae_ as our experimental plant because our previous small-scale lab and mesocosm studies have demonstrated its potential to enhance nutrient removal [36]. The ditch complex was maintained with flowing water gravity fed by the nearby pond and monocultures in vegetated ditches were maintained by hand weeding.  
We conducted three experimental loading events across all ditches on 3 different dates in a 15 d period. Each ditch was randomly assigned to one of 3 HRT so that over the 3 sampling events, each ditch experienced a 2, 4, and 6 h residence time [43]. HRTs were set manually with PVC spigot structures and verified with conductivity tracer tests [43]. During each nutrient loading event we dripped a solution of 17.92 and 8.3 g L\({}^{-1}\) of NaNO\({}_{3}\) and KPO\({}_{4}\) into each ditch at a constant rate with a peristaltic pump. Pumps were calibrated to deliver nutrient stock solution at 5 mL min\({}^{-1}\) into the headpo  
Figure 1: Schematic diagram of (**A**) experimental ditch setup depicting dosing and sampling method. (**B**) Picture of simulated drainage ditches at University of Mississippi Field station.  
the solution could mix thoroughly with the inflow water. We purposely loaded all ditches (15.5 \(\pm\) 0.6 and 9.9 \(\pm\) 0.4 g of NO\({}_{3}\)-N and PO\({}_{4}\)-P, respectively; Table 1) at the same rate regardless of HRT treatment to better separate the effects of HRT on reduction in a specific load. We acknowledge that this creates different nutrient concentrations experienced by experimental ditches under each HRT treatment. The estimated expected nutrient concentrations for each HRT treatment were 1, 2 and 4 mg L\({}^{-1}\) NO\({}_{3}\)-N and 0.5, 1 and 2 mg L\({}^{-1}\) PO\({}_{4}\)\({}^{-3}\)-P for HRT 2, 4 and 6, respectively.