Edge-of-Field (EOF)
What It Is
In addition to incorporating conservation practices in the field, many Iowa farmers are adopting edge-of-field practices that help to significantly improve water quality in the state by managing the loss of nitrogen from cropland.
Approximately 50 percent of Iowa cropland has subsurface drainage, based on United States Department of Agriculture Census of Agriculture data from 2012 and 2017. While this subsurface drainage makes it possible to farm previously wet soils, the drainage system also carries dissolved nitrogen from farm fields to the streams, rivers, lakes, wetlands, and other surface waters of the state.
Excess nitrogen in surface water leads to local, regional, and national-level alterations to aquatic ecosystems resulting in decreased water clarity, increased algal growth, and oxygen deficiencies that cause fish kills and reduce biotic diversity. Excess nitrogen also harms potable water supplies. Nitrate-nitrogen concentrations above the 10 mg/L NO³-N drinking-water standard established by the United States Environmental Protection Agency are not uncommon in Iowa in surface waters and groundwater.
Conservation Practices
Edge-of-field practices that address nitrogen loss can significantly reduce the amount of nitrate-nitrogen that leaves subsurface drainage networks. On average, nitrate-nitrogen is reduced by:
Several types of wetlands can be used in agricultural settings, depending on your goals. If the primary goal of a wetland is to improve water quality, water quality enhancement wetlands help to remove nitrogen through conversion of nitrate-nitrogen to nitrogen gas by microbial activity and through plant uptake. Ideally, water quality enhancement wetlands should have a pool footprint greater than or equal to 1 percent of the watershed area to be treated. The topography of the site should allow for a drop in elevation from the tile outflow to the surface of the standing water in the wetland to prevent backflow of water into the tile drain system.
Additional land is needed to allow a diverse buffer of wetland vegetation to develop around the shallow water pool. If the wetland footprint is in an area that could experience high sediment flow, a sedimentation basin or other structure will need to be considered. It is also important that water quality enhancement wetlands remain fish-free to reduce sediment disturbance and prevent unwanted loss of sediment, phosphorus, and nitrogen from the system.
If the primary goal is to provide additional wetland habitat, identifying low-profitability wet zones within the field can reveal locations that could be planted in perennial wetland vegetation.
Some must-have pieces of information for determining if a wetland could be a suitable edge-of-field practice for the site include a soils map, profitability maps, and knowledge of relationships to district infrastructure if the site is in a drainage district.
Multipurpose oxbows are restored wetlands located adjacent to a stream or river that receive tile drainage water and function to both reduce nitrate and provide aquatic habitat. Oxbow wetlands are old stream channel bends that have been cut off from the main stream channel over time either through channelization or natural processes.
Ideally, multipurpose oxbow restorations should be situated in the floodplain in an area along the stream that does not exhibit any signs of channel instability, such as steep banks, bank sloughing, or bank undercuts. The multipurpose oxbow will also need to be in a site that has the topography to allow for a drop in elevation from the tile outflow to the surface of the standing water to prevent backflow of water into the tile drainage system. The basin footprint should be 1 to 2 percent the size of the drainage area to allow for optimum nitrate reduction. Multipurpose oxbows improve water quality through the conversion of nitratenitrogen to nitrogen gas by microbial activity. In addition to nitrate reduction and habitat, multipurpose oxbows can also provide flood water storage.
Some must-have pieces of information for determining if a multipurpose oxbow could be suitable for your site are historic aerial photos, a topographic map, and a drainage map.
Saturated buffers are established near streams or ditches by diverting the existing tile drainage outflow so that water passes through the subsurface of a vegetated buffer prior to entering a waterway. Saturated buffers help to remove nitrogen through conversion of nitrate-nitrogen to nitrogen gas by microbial activity, as well as through plant uptake. In addition to improving water quality, saturated buffers can also enhance stream- and ditch-side habitat.
For this practice, it is beneficial to have a clay base layer to prevent undesired movement of water that could result in subsurface water bypassing the
saturated buffer. Sites with open surface intakes in the drainage system are not ideal, as the soil and residue that may get into the drainage system via the surface intake could interfere with the movement of water into the saturated buffer. If surface intakes are present, additional precautions will be needed to reduce sediment flow into the saturated buffer.
Trees, both existing and newly planted, are an option within saturated buffers. If there are existing trees within the footprint of the saturated buffer, take extra care in setting distribution lines. Work to identify and remove aggressively rooting species, such as boxelder, willow, silver maple, and cottonwood, within 20 feet of the distribution lines. If tree seedlings are to be established within a saturated riparian forest buffer, space seedlings at least 20 feet from distribution lines and control structures and avoid establishing aggressively rooting species.
Use the USDA’s Saturated Buffer Viewer (https://acpfdata.gis.iastate.edu/ACPF/satbuff/) to determine if a saturated buffer will work on your site.
Bioreactors treat water from subsurface drainage systems by diverting tile flow into an excavated trench filled with woodchips. The woodchips provide carbon and attachment surfaces for microbial communities that convert nitrate-nitrogen to nitrogen gas. Ideally, bioreactors need relatively consistent tile flow to maintain saturated conditions in the bioreactor. Bioreactors cannot be placed in areas where surface flows may cause ponding of water on top of the bioreactor. It is important to keep the bioreactor footprint out of highly trafficked areas to prevent the compaction of woodchips within the trench. The presence of surface intakes requires additional consideration.
Source: ISU, 2022 https://store.extension.iastate.edu/Product/15823
