Often considered a byproduct of forestry and landscaping, wood chips could also become central to decreasing nitrogen run-off from tile drainage systems.
Dipping his hand into a plastic container full of wood chips at the Agriculture and Agri-Food Research Centre in Morden, Steve Sager said that increased use of tile drainage in Manitoba, as well as other regions, have made reducing nutrient run-off a priority in traditional horticultural systems.
“With the expansion of tile drainage in Manitoba, we’re looking at different practices to reduce some of the nutrient loss we see,” he told producers during a recent field day hosted by the Manitoba Pulse and Soybean Growers association. “And wood chip bioreactors are used a lot in the central states, and in the Midwest states. So we’re kind of monitoring or investigating their application or potential under Manitoba conditions.”
Simple to construct, wood chip bioreactors are essential wood chip-filled trenches, lined with waterproof membranes. The wood chips provide a source of carbon that allows nitrogen to be converted into nitrogen gas as it is absorbed, then released into the atmosphere, by the supplied organic matter.
“So as the tile effluent moves through the wood chips, we reduce the amount of nitrates leaving the system at the edge of the field,” Sager explained. “And this really relates to our surface water bodies, Lake Winnipeg in particular.”
Poor surface drainage and untreated tile effluent can also decrease yields and affect farmland, he added.
The study is now in its second year and will run officially for one more season, but the soil scientist said the low-maintenance system will be monitored for years into the future. Agriculture Canada is also testing the bioreactor at sites in Ontario and Newfoundland, as well as at a location on Prince Edward Island.
Studies done in Minnesota and Illinois have shown the wood chip-based system decreases nitrogen in tile drainage effluent, as well as some biological material. While denitrifying bioreactors are still relatively new, Sager believes they hold great possibility — even in areas with harsh conditions like the Canadian Prairies.
However, it will still take some time for the official results of the project to come in.
“We have more lab results and like a lot of our samples from this year we’re waiting to get analyzed at our Brandon station,” Sager said. “Last year we put through about 80 per cent of the tile effluent through the two bioreactors and we — in most cases, because it is temperature dependent — reduced the amount of nitrogen by half.”
In some cases, where the flow of water was very low, there was essentially no nitrates left after treatment, he said.
The federal research centre has also embarked on a study looking at the viability of “vegetative filter strips” or “grass buffers” in the fight against nutrient run-off, according to Jason Vanrobaeys.
“What I’m focused on really is looking at the surface flow aspect of it, and practices that can really, really reduce nutrient loss,” he told producers, explaining that the buffers include the construction of small berms or dams to retain water in areas with some vegetation.
“Not a whole lot of results in terms of water quality from that yet, but we should have those results by this fall,” said Vanrobaeys. He added that the berms were inexpensive to construct and could service a relatively large area of cropland.
Sager added that bioreactors can also be constructed at minimal cost, noting the wood chips that fill them can last years, if not decades.