Tile drainage is a common practice used in agricultural fields to remove excess water, but it also transports harmful nutrients into water bodies, contributing to algal blooms that deprive aquatic life of oxygen. Woodchip bioreactors are an efficient way to reduce nitrogen pollution by treating the water as it exits the field. However, these denitrifying bioreactors may leach phosphorus from the woodchips into the environment. A new study from the University of Illinois Urbana-Champaign evaluates data from 10 bioreactors in Illinois to determine whether they are a source of phosphorus pollution.
- Marianne Stein
"Illinois has a goal of reducing the amount of nitrogen we send into the Mississippi River Basin by 45%. Woodchip bioreactors are one of the conservation practices that are recommended to clean nitrate pollution out of agricultural drainage water; however, we don't want to create a problem with phosphorus instead," said lead author Laura Christianson, who conducted the study as an associate professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois.
Denitrifying bioreactors typically consist of a large basin of woodchips placed at the edge of an agricultural field. When nitrogen-enriched water flows through the trench, naturally occurring bacteria consume carbon in the woodchips, fueling a process that converts nitrate to a harmless gas.
"There is phosphorus in the woodchips, because that's one of the nutrients trees take up when they grow. We wanted to assess whether or not woodchip bioreactors are leaching phosphorus, because it's also important to reduce the amount of phosphorus we're sending downstream," Christianson said.
The researchers assessed data from 10 full-size bioreactors in Illinois for a total of 23 site-years (that is, each research site multiplied by the number of years it was included in the study). Bioreactor inflow and outflow were measured to calculate daily water flow rates. The researchers analyzed water quality samples to determine the content of dissolved reactive phosphorus (DRP) - a form of phosphorus that is available for plant and algae uptake.
The results showed that rather than an increase, there was actually a small reduction in DRP concentrations for 15 of the 23 site years, indicating that bioreactors provide a net benefit for phosphorus removal. However, results fluctuated over the course of a year.
"When there is heavy rainfall, there is a spike in phosphorus removal, especially after manure application in the field," said study co-author Richard Cooke, professor in the Department of Agricultural and Biological Engineering (ABE), part of the College of ACES and The Grainger College of Engineering at Illinois.
In the first few months of a bioreactor's life, there is a significant flush of phosphorus leaching from the woodchips. After that, phosphorus mainly comes from the field's tile drainage water. The data also indicated that as the bioreactors got older, they were less effective at removing phosphorus.
The life of a bioreactor is about 10 years, so if there is a brief phosphorus surge in the first few months, but after that it removes more than it releases for most of its lifespan, then it is worth it, Cooke said.
Bioreactors are a valid practice for nitrogen removal, and finding they also provide a small amount of phosphorous removal is a double benefit, Christianson added.
"First, when phosphorus gets into the waterways, it's difficult and expensive to clean out. Second, bioreactors aren't supposed to provide this benefit at all, so it's like a freebie we're getting," she said.
Denitrifying bioreactors are a relatively recent conservation practice, and Illinois' 9 million tile drained acres provide many opportunities to implement it. Agricultural producers who are interested in installing bioreactors in their fields can contact their local Natural Resources Conservation Service for assistance. Cost-share and federal incentive programs are available to help offset the investment. Once installed, the bioreactor requires minimal maintenance; the farmer just needs to check it a few times a year to make sure nothing is blocking the flow.
There are other practices to reduce nitrogen, such as reducing fertilizer rates, better timing of fertilizer applications, or planting cover crops.
"Producers should consider what would be the best option for their farm and maybe use a combination of practices if possible. In my experience, farmers are good stewards of the land; they will think about their legacy, and what they are leaving for their children, so they will be interested in practices that work well," Cooke said.
The paper, "Denitrifying bioreactors and dissolved phosphorus: Net source or sink?" is published in the Journal of Environmental Quality [DOI:10.1002/jeq2.20568]. Authors include Ana Paula Sanchez Bustamante Bailon, Andrew Margenot, Richard Cooke, and Laura Christianson.
Funding for this study was provided by the Illinois Nutrient Research and Education Council (NREC 2017-4-360498-302; 2021-3-360498-144); a USDA NRCS Conservation Innovation Grant (NR213A750013G038); USDA NCR SARE (FNC21-1279); the Illinois Farm Bureau Bioreactor Partnership; and the University of Illinois ACES Dudley Smith Initiative. Christianson was partially supported by MN Pollution Control Agency Project #229369 through EPA Award 4F00E03272.
