POTENTIAL OF TARGETED WETLAND RESTORATION TO REDUCE NITROGEN LOADS TO SURFACE WATERS IN IOWA

Wetland restoration is a promising strategy for reducing surface water contamination in agricultural watersheds and in particular for reducing agricultural nitrate loads to the Mississippi River and its tributaries. More than 80 wetlands have been restored through the Iowa Conservation Reserve Enhancement Program with the explicit goal of intercepting and reducing nonpoint source nitrate loads. Over the past 10 years, we have measured nitrogen mass balances of a selected subset of these wetlands to evaluate their effectiveness at reducing agricultural, nonpoint source nitrogen loads and to develop models for predicting wetland performance at scale and in combination with other practices. The monitored wetlands were selected to ensure a broad spectrum of major external forcing functions affecting wetland performance including hydraulic loading rate, residence time, nitrate concentration, and nitrate loading rate. Nitrogen loads to the wetlands were primarily in the form of nitrate and all of the wetlands were effective in reducing both nitrate and total N loads. Nitrate removal efficiency (expressed as annual percent mass removal) ranged from 8-91% and was primarily a function of hydraulic loading rate and temperature. Mass nitrate removal ranged from 120-2800 Kg N / ha of wetland / year and was primarily a function of hydraulic loading rate, temperature, and nitrate concentration. Our results demonstrate that wetlands can be effective sinks for nonpoint source nitrate loads across a wide range of conditions and that performance can be reasonably predicted based on hydraulic loading rate, temperature, and nitrate concentration. We extended these results to project statewide nitrate load reductions for Iowa using a combination of nutrient management and targeted wetland restorations. Our analyses suggest that targeted wetland restorations will be critical to achieving a 45% reduction in annual nitrate load for Iowa.