by Sally Colby
Most farmers who have manure to spread think about some of the measures that can help preserve nutrients and prevent them from entering bodies of water. One long-time field crop measure — tile drains — can help reduce nitrogen loss.
“Subsurface, or tile drainage, is an essential part of how we do agriculture,” said Dr. Laura Christianson, Department of Crop Sciences at the University of Iowa. “There’s a long history of farmers doing tile drainage because it improves crop growth, crop yield, helps us get into fields earlier in spring and reduces risk.”
Christianson says installing tile drainage is one of the most consistent infrastructures that a farmer can do to provide a positive return on his investment. “In recent years, we’ve learned that tile drainage changes the natural hydrology — how the water naturally leaves the field,” she said. “It also changes the pathway of nutrients, primarily nitrogen, to move from one field where we want those nutrients to be — to downstream waters where we don’t want those nutrients to be.”
Christianson lists 10 practices that can aid in reducing nitrogen loads from drained cropland. The first three directly involve cropping.
When it comes to improving tile drain management, nitrogen management practices are centered on reducing nitrogen fertilizer application rate to university-recommended rates and coordinating nitrogen fertilizer timing so it’s present when the crop needs it — less in fall and more in spring. “Fine tuning nitrogen management is very difficult and it’s challenging to get nitrogen management exactly right every year,” said Christianson. “Crops have different needs during the season, each field is different and every year is different.”
A second nitrogen management cropping practice is incorporating cover crops. “Winter cover crops are typically planted at harvest time and protect the soil over winter,” said Christianson. “Sometimes cover crops overwinter and come back in spring. The idea is to extend the period of the year so we have a crop actively growing, actively taking up water, actively taking up nitrogen and holding onto that nitrogen. In doing so, there’s less nitrogen downstream.”
A third field cropping strategy involves using a perennial in the cropping system. An example would be two years of corn followed by three years of alfalfa. Water quality improves with a perennial like alfalfa similarly to a cover crop — extending the time a crop is growing and taking up water and nitrogen.
Other nutrient management practices are more directly related to drainage, such as drainage water management, which is the practice of implementing an adjustable control structure throughout the drainage system to adjust the level of the tile outlet. In such a system, water from tile drainage water flows over a series of gates before it leaves the drainage system and continues downstream. “By holding water back and adjusting the outlet levels during certain times of the year when we don’t need to drain fields, water and nitrogen is also held back and prevented from moving downstream,” said Christianson.
The next practice is reducing drainage intensity. This can be accomplished in two ways: installing drain tiles wider apart or installing drain tiles at a more shallow depth (closer to the soil surface) so less water goes downstream. Drainage systems are designed to meet crop productivity goals, but the end result is less nitrogen downstream.
Another practice is recycling drainage water. This practice involves storing drainage water onsite in a pond or reservoir during spring and early summer drainage. When the crop needs supplemental irrigation during the growing season, the stored water can be used. This practice affords the potential to preserve all nitrogen because it’s being stored onsite. “If you’re in a year when you need supplemental irrigation, there’s potential for a big crop yield benefit,” said Christianson, adding that this practice can be expensive, but may be a good option in the future if weather patterns become more variable.
The next set of practices fall into the ‘edge of the field’ category and include wetlands, alternative open ditch design, saturated buffers and bioreactors.
Christianson explains that wetlands are a dynamic system of plants, water, soil and bacteria that all work together to clean nitrogen out of tile drainage water. In addition, wetlands provide wildlife habitat, carbon sequestration and flood retention. The challenge with wetlands is that they might require land to be taken out of production, which can be a hard sell to some producers.
The alternative open ditch design is essentially a two-stage ditch. “We’re retrofitting traditional trapezoidal drainage ditches to mimic more natural systems,” said Christianson. “With a two-stage ditch, we have a small main channel in the middle of the ditch, which conveys most of the flow most of the time.”
Christianson says although buffers provide great benefits, the tile drain pipe goes straight to the buffer in tile drain landscapes. “With a saturated buffer, we’re re-routing the drainage water at the edge of the field so the drainage water actually flows as shallow groundwater through the buffer rather than through the pipe,” she said. “In doing so, those great microbes and bacteria that live in the buffer soil and the plants living in the buffer now have access to that nitrogen before the nitrogen seeps out to the stream.”
Although a bioreactor might sound like a complex and pricy concept, it’s actually fairly simple. “It’s a trench filled with wood chips that you route the tile drainage to,” said Christianson. “What happens inside the trench is that there are natural denitrifying bacteria that live on the wood chips and use the carbon as their fuel source.” As the nitrate in the drainage water flows through, the denitrifying bacteria convert the nitrate in the water to dinitrogen gas — the natural process of denitrification. “By adding the extra wood chips, we’re simply super-powering, or enhancing, this process,” said Christianson. “We’re giving the bacteria extra food, and it’s the bacteria that are cleaning the water.” Christianson added that because the bacteria are doing the work, it’s a biological process, hence the name.
Christianson says an important part of the bioreactor is the inflow control structure, which routes water from the field to the bioreactor but also allows some water to bypass the bioreactor when excess water is coming from the field. An outflow control structure holds water back in the wood chips for a long enough period of time to allow the bacteria to work.
Research on bioreactors shows about 25 to 45 percent nitrogen load reduction annually. Wood chips last about 10 years before they need to be replaced. The average cost of a bioreactor varies widely based on labor cost, materials and location but average between $3,000 and $25,000. “Bioreactors at the end of their life don’t run out of carbon,” said Christianson. “They have reduced hydraulic capacity. The wood chips have broken down and it’s harder to push water through the chips.”