Floodwater monitoring and soil incubations reveal complementary nutrient retention patterns in a reconnected floodplain wetland

Stream channelization and levee construction have disconnected many streams from their floodplains, reducing nutrient storage potential within watersheds. Levee breaks along historically channelized rivers can restore floodplain–river connections, but quantifying nutrient retention changes with reconnection is difficult because of complex flow paths and floodplain structural heterogeneity. We assessed potential nutrient retention patterns across a reconnected floodplain wetland with 2 complementary methods: 1) by measuring spatial changes in floodwater nutrient concentrations with automated water samplers and 2) by measuring areal nutrient flux, including N gases, at the soil–water interface with flow-through soil-core incubations. Changes in floodwater nutrient concentrations indicated the floodplain could be a net sink for total N and total P during 3 floods. Aqueous N∶P ratios consistently increased with distance from levee in all floods, indicating P was more readily removed from floodwater, likely via particulate deposition. Soil cores indicated lower PO4-P retention potential compared with trends in floodwater nutrient concentrations, likely because short-term particulate nutrient deposition was not captured by cores. Soil-core results for N flux generally supported floodwater patterns of N concentration change. High production of N2-N gas at the soil–water interface and low production of N2O-N provide evidence for N-removal potential of this floodplain with limited greenhouse gas production. Floodwater nutrient concentration patterns and areal nutrient flux rates from soil-core incubations cannot be directly compared because they measure different processes occurring across a floodplain. However, using both methods together supports the idea of floodplains as net nutrient sinks while highlighting nuances of floodplain nutrient retention capacity and tradeoffs among ecosystem services gained through restoration projects.