By Amy Petherick
Researchers at the University of Waterloo say that phosphorus, which causes algal blooms in Lake Erie, is partly caused by field runoff. Farmers can play a role in helping to solve the problem.
Farmers have long been regarded as stewards of the land. Although that opinion may be less popular in recent times, researchers at the University of Waterloo (U of W) are proving that restoring Lake Erie absolutely requires agriculture’s help.
Phosphorus (P) is feeding Lake Erie’s problematic algal blooms, and one of the many ways it’s getting to the lake is from field runoff. Merrin Macrae is an associate professor at the U of W who is particularly interested in surface-water chemistry research. Together with Chris Van Esbroeck, a student who recently earned a Master of Science degree under her supervision, Macrae recently concluded a monitoring study that attempts to identify where all the water in the lake is really coming from and, as a farm boy, Van Esbroeck is particularly interested in seasonal patterns of P exports from reduced-tillage farming.
“Hydrology is a big part of what’s driving phosphorus losses,” Macrae says. “When water’s on the move, phosphorus is on the move.”
As many farmers would likely have expected, Macrae says the majority of the field runoff they’re interested in happens between October and April. Those who remember the heavy spring rains in 2011 might not be surprised to recall the largest algal bloom in Lake Erie’s history occurred during the very warm summer that followed. That particular disaster inspired the establishment of the Lake Erie Ecosystem Priority (LEEP) in 2012 and their research soon identified that non-point sources are a big part of the problem. In fact they discovered more than 50 per cent of incoming loads are from non-point sources and they also pinpointed which watershed systems are delivering the most P.
“Certainly there is a lot coming into the lake from the U.S.,” Macrae says, “but it’s important to realize Ontario also has a contribution and it’s an appreciable contribution.”
One cause for concern behind non-point loads may be higher concentrations of P in soils. Macrae says the best estimate of just how much is coming in from Ontario cropland amounts to roughly 10 per cent of all non-point source loads or 0.33 kg per hectare of total P annually, when the estimated tributary loads are averaged across all of the cropland contributing to runoff. But most researchers agree monitoring has been poorly conducted in Ontario, offering Macrae and Van Esbroeck the opportunity to undertake their current monitoring study.
Although dissolved reactive phosphorus (DRP) is the soluble form of P they monitor, Macrae says they also monitor total P, which includes many types and forms of P, namely the particulate or sediment-related form. Although researchers have established the total amount of P reaching the lake is not really increasing, concentrations close to shorelines are. The culprits behind this development are zebra and quagga mussels, which consume particulate P and excrete the DRP that algae populations love.
“We know these mussels are a complicating factor,” Macrae admits, “but they are not the ones that put the phosphorus in the lake, they are just making that lake more sensitive.”
Since they began monitoring the edge of fields at two sites in the Maitland and Thames watersheds, between May 2012 and April 2013, Van Esbroeck says the Maitland site demonstrated that although tile sources contributed 78 per cent of total runoff, surface runoff contained 81 per cent of the 0.096 kg of DRP lost per hectare and an equal amount of total P. The Thames site was a very similar story – just less pronounced – allowing them to conclude surface runoff is a very important pathway for annual P loss.
“The majority of all reactive phosphorus is coming from overland flow, even though overland flow really only accounted for 15 per cent of runoff,” Van Esbroeck says. “Because such a small amount of surface runoff contributes so much to reactive and total phosphorus, any steps you can make to improve soil structure and infiltration will produce a better situation.”
“With clay, we have more of that tendency to swell and shrink, forming cracks in the soil and that provides a direct conduit for surface water to get into our tile,” he theorizes. “If you’re doing straight no-till and you’re not incorporating fertilizer, you end up with two centimetres of soil that are actually quite stratified.”
At their sites, levels of P in the top six inches were not excessive, but the phosphorus that was there was concentrated near the surface due to the tillage systems in place. With only one year of data from Essex, he says there are too many moving pieces to identify the cause of the differences in their observations. But they can say with more certainty when runoff is occurring. Van Esbroeck says they observed large losses of P in fall runoff events that occurred after fertilizer was applied in October at their sites. This is why he says P applications made in the fall are a concern.
“If you’re applying fertilizer in the fall, you lay it down on the surface, and you’re a lot more likely to have it leave than if you apply it preplant in the spring and you incorporate it,” he says.
Solutions with potential
As a result of their observations, both Van Esbroeck and Macrae are able to offer farmers lake-friendly fertilizer management recommendations. First of all, Macrae says, soil test before you apply P if you’re farming land that drains into Lake Erie, and be sure you know what you have. If your soil is already testing high for P, then aim for more moderate levels. Cover crops can help with this, and she also recommends them for erosion protection. Because the non-growing season is so critical, simply increasing crop diversity with cover crops, winter wheat and forages helps to hold more total P back from waterways. Macrae says it’s all about making it more difficult for water to get off the field.
“A small loss can have a big impact downstream,” she explains. “Anything you can do is going to improve the situation.”
Where water flow is particularly concentrated, she urges farmers to add strips, buffers, or even Water and Sediment Control Basins (WASCoBs). Macrae says she’d also like to see farmers apply fertilizer that contains P in the spring, not the fall, and follow that with some form of incorporation. She believes banding it within two inches of seed placement could prove to be the ultimate solution, but admits she’s no expert on the agronomic or economic implications farmers would face in making such a change.