How to Reduce Nitrate Loss
Reducing nitrate loss using innovative methods of drainage water management
Another way to reduce the amount of nitrate leaving fields is to more intensively manage the movement of shallow ground water. Conventional subsurface drainage systems work by lowering the water table level to the level of the subsurface drains. Subsurface drainage waters and associated nitrate flow uncontrolled from the field to surface waters. Water table management is the practice of controlling the drainage flow and water table level using the subsurface drainage system. With controlled drainage and/or subirrigation, a control structure is used to manage the rate at which drainage water leaves the field, or to pump water back into the drainage system. A crop production benefit is that water tables can be raised during dry weather to irrigate the crops from beneath the soil surface (subirrigation). Drainage during wet weather is maintained to prevent the flooding of crops.
This controlled release of drainage water keeps soil below the root zone wet for a longer period of time. Wet soil is a favorable condition for conversion of left-over nitrate to the gaseous form of nitrogen by soil microorganisms (denitrification). Nitrate at deeper soil depths has little value to plant growth and is susceptible to leaching. During the non-growing season when plants do not require nitrogen, water table levels can be raised higher to create seasonal wetland-like conditions.
In addition to promoting denitrification, some water table management systems link subsurface drainage systems with wetlands and/or riparian areas that capture, treat, and recycle drainage water.
Field-scale demonstration of agricultural drainage systems linked to wetland and/or riparian systems are being studied under real-world farming conditions to learn the full range of costs and benefits for landowners and the environment.
The projected benefits of new drainage water management techniques include improved crop yields, improved water quality, and increased riparian and wetland areas.
When nitrogen application is more closely matched to the needs of the crop, nitrate leaching can be reduced substantially. Applying nitrogen at the optimal rate and time of year for plant uptake reduces the amount of unused nitrogen in the soil and the potential for leaching. However, a change in application regime is not always practical for farmers.
Further research will investigate the practicality of different cropping, tillage, and fertilization systems, and provide specific nitrogen recommendations for the Midwest’s different climate zones.