Location: Soil Drainage Research
Project Number: 5080-13210-002-28-T
Project Type: Trust Fund Cooperative Agreement
Start Date: Jul 1, 2018
End Date: Mar 31, 2019
We will determine the role of two-stage ditch floodplains in retaining or removing nitrogen (N) and phosphorus (P) from tile drain discharge during baseflow conditions. Our objectives are to 1) quantify N and P uptake in tile drain discharge on two-stage floodplains and 2) determine the effect of flowpath length on N and P uptake.
We will first identify tile drains within the edge-of-field (EOF) network that flow directly onto two-stage floodplains. We will conduct steady-state additions along flowpaths of varying lengths to determine the influence of two-stage floodplains on N and P removal. Briefly, we will pump a solution of nitrate (NO3-), phosphate (PO43-), and conservative tracer (i.e., NaCl) into three flowpaths that have been manipulated to span a range of flowpath lengths and residence time. We will measure conductivity at the bottom of each flowpath to determine when plateau has been reached; at plateau, we will collect and filter triplicate samples from five sampling locations that are longitudinally-distributed along each flowpath. NO3- and PO43- will be determined using standard methods on a flow injection autoanalyzer. We will then plot the natural logarithm of the ratio of N and P concentration to conservative tracer against sampling location distance and use this regression relationship to calculate assimilatory uptake metrics, including longitudinal uptake rate, uptake length, and uptake velocity. At plateau, we will also collect dissolved gas samples that will stored on ice and later analyzed for N2 and O2 using membrane inlet mass spectrometry in order to similarly determine rates of denitrification and community respiration along each flowpath. We will conduct nutrient release experiments in two tile drains located in three ditches at three time points during the growing season (May-July 2018). Following each experiment, we will also collect soil cores (0-5 cm) from five locations along the flowpaths in order to determine soil organic matter, carbon, N, and P content of the floodplain soils. Soil organic matter and N content are important controls on N removal in floodplains, and P content may provide an indication of internal loading (i.e., P release) from the two-stage floodplains. Using this strong experimental design will allow us to accurately quantify the influence of two-stage floodplains on N and P uptake from tile drain discharge and determine the role of increasing flowpath length as a management tool to maximize nutrient removal during baseflow conditions.