Integrated Drainage-Wetland Systems for Reducing Nitrate Loads from Des Moines Lobe Watersheds
The main subject area for this project is the Des Moines Lobe in north-central Iowa drained mostly by the Des Moines, Raccoon, Iowa, and Skunk Rivers. The Des Moines Lobe represents the southernmost extent of the Prairie Pothole Region and is a relatively flat landscape with very poor natural drainage. However, as a result of extensive subsurface ―tile drainage,‖ the Des Moines Lobe has become some of the most valuable and productive land in the world. In 2011, the average land value in north-central Iowa was $7,356 an acre, and 80.5% of that area was in row-crops (42.9% in corn and 37.6% soybeans). Unfortunately, the Des Moines Lobe has also become a source of significant NO3- loading to downstream waters.
The elevated nitrate loads from the Des Moines Lobe are largely a result of the changes in land-use and hydrology brought about by subsurface ―tile drainage‖. Tile drainage has historically been treated and continues today as a conservation practice because it decreases surface runoff, thus reducing surface transported contaminants such as phosphorus, sediment, ammonium-nitrogen, pesticides and pathogens. However, tile drainage also increases subsurface flow and leaching losses of NO3-. This is due mostly to an increase in the subsurface fraction of the total annual discharge and the "short-circuiting" of subsurface flow, but also in part to the increased aeration of organic-rich soils with potentially increased mineralization and nitrification and decreased denitrification in the soil profile.
Iowa and the other Corn Belt states contribute a large share of the N and P loads transported by the Mississippi River and these have been identified as primary drivers of Gulf hypoxia. The Hypoxia Action Plan calls for 45% reductions in both N and P loads. Although some alternative cropping systems, such as small grains, alfalfa, or other sod-based crops/rotations, can substantially reduce both N and P losses, these alternatives have major economic implications as well as environmental concerns of increasing row crop production pressure on more highly erosive and environmentally sensitive lands. Improved N management has the potential to reduce NO3- leaching, but that potential is much less than needed to address the problem of Gulf hypoxia. It is clear that a combination of approaches is needed, but it would be a mistake to approach practices on a piece meal basis, especially since some practices that reduce N loads can substantially increase P loads.
In this project, we evaluated a structural approach integrating nitrate-removal wetlands with the emerging technologies of drainage modification. In combination with in field management, the integration of drainage and wetland systems will provide a dual nutrient strategy with potential to reduce N loads, reduce P loads and increase crop production. We evaluated the effects of drainage systems designed to reduce subsurface flow (controlled drainage and shallow drainage), drainage systems designed to reduce surface runoff, and targeted wetland restoration. The integration of drainage and wetland systems has the potential to simultaneously increase the number of wetland sites, push those sites closer to the NO3- source, and enhance wetland performance by increasing the average residence time in the wetlands.
The integration of these approaches also provides opportunities for developing market-based solutions. Private and public interests coincide if we are able to couple increased water-use efficiency and crop yield due to drainage modification with improved water quality due to integrating drainage and wetland systems. This opens an array of possible strategies for leveraging funds, capabilities and activities of private and public sources.