Work toward pinpointing the exact sources of nutrient phosphorus in runoff from farm fields is ongoing, but the goal of finding the smoking gun continues to prove elusive.
“We really don’t know where the phosphorus is coming from on agricultural fields,” said Jane Elliot, a research scientist with the National Hydrology Research Centre in Saskatoon.
“We have some ideas, but we’re a long way from confirming them.”
Like all detective work, the task of assigning blame for the eutrophication of Lake Winnipeg is largely a case of identifying suspects and retracing their steps back from the crime scene.
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A decade-long hydrological study on the South Tobacco Creek watershed near Miami, Man., has provided possible clues for cracking the case.
Earlier work in other areas pinned much of the blame on particulate phosphorus, which occurs when moving water washes the nutrient from eroded and exposed stream banks.
However, on the Prairies, where the landscape is generally flat and rainfall and snowfall are intermittent and intense, researchers decided it could not be the only guilty party.
To close that loophole, the restoration of plant cover on riparian areas has been encouraged. Unfortunately, these efforts didn’t solve the problem, and more tests showed that phosphorus was still floating downstream from other sources.
The focus then shifted to dissolved phosphorus, a tea-like substance that intermingles with runoff water and has been proven to be most effective at creating the large algae blooms that turn Lake Winnipeg in summer into something resembling a vat of fetid pea soup.
Where does this insidious substance originate, and how can it be prevented from flowing into the lake from rivers and streams?
“We’re fairly clear that it’s dissolved phosphorus, but to get to the source of it, we’re not quite there yet,” Elliot said.
Lab experiments have found that plant tissues subjected to freezing and thawing under harsh conditions comparable to a prairie winter release soluble phosphorus.
However, what’s possible in a laboratory doesn’t always translate into the real world.
“Does it end up in the runoff water, or does it end up being reabsorbed into the soil somewhere?” she said.
“We’re pretty sure it is a source, but as to its significance, that’s something we’re not sure about.”
Making tea usually involves throwing dried leaves into a pot of boiling water. However, when Mother Nature brews up a batch of phosphorus tea, she may also throw in green vegetation.
“Lab studies have shown that the greener and fresher the vegetation, the more likely it is to leak phosphorus,” Elliot said.
“Winter wheat, in some lab studies in Saskatchewan, showed quite a lot of phosphorus coming through. In some cases, in fact, it was our highest.”
However, she said such results are only at the preliminary stage.
Spreading manure is risky beyond the end of October, even though the generally cool, damp conditions that minimize the volatilization of ammonia at that time of year are best for recycling manure’s nutrient and agronomic value.
Immediate incorporation or injection well before winter is still the best way to keep phosphorus in the field where it belongs. The later in the season it is spread, the greater the likelihood that some might be washed away in spring runoff.
“Even with hog manure, where it was injected, if it’s done late enough, we can still see it sitting there in the injection channels in the spring,” Elliot said.
“In one study, the manure was frozen stiff within the injection furrows. The runoff we got from that field really wasn’t acceptable.”
Researchers are convinced that areas with excessive levels of phosphorus, such as places where manure has been repeatedly applied, tend to “leak” their contents in runoff water, she said.
Until the phosphorus emanating from municipal sewage, natural sources and agriculture production can be identified and quantified, she said the blame game will continue over who or what is responsible.
The more stones scientists overturn in their ongoing efforts, the more questions they uncover.
“No one has really quantified the amount of phosphorus that is actually stored within the river systems themselves,” she said.
“That can be released within a flood cycle, when the bottom is scoured and phosphorus deposited there can potentially come out …. We know a lot about the system, but there’s still an awful lot we don’t know.”
