Researchers are testing a way to safely and cheaply put leftover pesticides to bed.
Tom Wolf, a research scientist with Agriculture Canada, says the “biobed” was developed in Scandinavia and legislated into use in Britain as a best management practice.
It is like a compost pile for breaking down and safely disposing of pesticides. It creates an optimal environment for microbes to thrive so they can eat pesticides much faster than they could in the soil.
“Biobeds improve on soil by binding the material more tightly and degrading it more quickly. It also holds more water, reducing leachability, and creates an environment where microbes thrive,” Wolf said.
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“By the time the water runs out the bottom, the data shows that it is essentially clean.”
Mario Tenuta, a soil microbiologist at the University of Manitoba, said microbes use the carbon in pesticides for energy, much like humans consume the starch in potatoes.
Herbicides, in particular, are designed to have specific modes of action to disrupt a plant’s physiology but are not necessarily toxic to microbes.
Pesticides may also contain nutrients that microbes crave, such as nitrogen, phosphorus and sulfur. They also consume adjuvants, stickers and surfactants used in tank mixes.
Tenuta said products containing chlorine may be more problematic but among the trillions of organisms in a cupful of soil, various kinds can be found that secrete enzymes capable of digesting complex molecules in an assembly line fashion.
“Bacteria and fungi have no stomach, so they digest their food outside their bodies. The soil solution is their stomach,” he said.
“One organism can take off the chlorine groups, then another can break it in half. And then another can actually use it as energy.”
Over time, a biobed naturally selects for those microbes best able to exploit pesticides as a food source. Tenuta said the strongest thrive and multiply, thus increasing the efficiency of decomposition.
As an example, researchers noted that residues from atrazine, a broadleaf herbicide used in the U.S. Midwest corn belt, were accumulating in the soil after decades of heavy use.
“Then in the mid-1990s, people started seeing that degradation was occurring. That’s because after many years of use, the microbial populations that could feed on atrazine had built up to a level that they could make a dent in its concentration.”
Wolf said one of the most attractive features of biobeds is their simplicity.
Previous attempts to safely dispose of agricultural pesticides usually used a drained concrete pad surrounded by earth berms, complete with holding tanks and pumps. Those plans were so expensive and elaborate that the average producer was reluctant to invest the time and expense needed to build one.
Researchers at the University of Saskatchewan are testing the system by adding chemicals on a weekly basis and measuring the leachate for chemical residues from a 20 sq. metre pit that is one metre deep and filled with a mixture of wheat, barley or oat straw, compost and soil at a 2:1:1 ratio.
“You mix all that up and then let it sit for a few weeks before adding it to the pit,” Wolf said.
“From that point onward, your pesticide waste can go into that pit.”
Because the materials are soft and spongy, a simple bridge made from a steel grate supported by I-beams can be built to allow the sprayer unit to be driven over top for rinsing.
Another option is a cement pad wide enough to accommodate equipment and that drains into a biobed.
Wolf said data from Sweden shows that the life expectancy of a biobed is about eight years. At that point, the straw is broken down and the humus has degraded into humic acid. European regulations advise producers to excavate the biobed’s contents, store it for a year and spread it on a field.
“That’s a fairly cautious approach,” Wolf said.
“We believe that it is likely that there will be pesticides still in it. We don’t know for sure if the pesticide has been in all cases broken down or simply held. Therefore, you have to be cautious …. While I don’t think it would be compost for sale, it certainly wouldn’t hurt to put it on your field.”
Some European jurisdictions require that biobeds be built on top of impermeable liners to prevent liquids from leaching into ground water.
However, Wolf said the liner increases the complexity of the system as well as the chances that the matrix would become saturated. Too much water would inhibit microbial activity, he added.
As well, the typically dry prairie climate probably makes a liner unnecessary, he said.
In soil, 2,4-D remains present at detectable levels for about five weeks. In a biobed, it is 99 percent gone in one to two weeks.
The U of S researchers are also looking at biobed performance for disposing of glyphosate because literature on the popular herbicides is lacking and results are difficult to analyze in a biobed.
However, researchers know that because glyphosate binds tightly with soil, it is not recognized as a major contaminant of water.
“There are some chemical issues, but we’re hopeful that we will have some good results. It’s too soon to say,” Wolf said.
“Obviously, glyphosate is of major importance and we want to make sure it works.”
Could it be used for dumping that last 100 gallons of spray sloshing around in the tank?
Wolf said preliminary results show that a biobed can handle concentrated amounts of pesticides, but the current recommendation is to use it only for diluted solutions such as rinse water.
He said more research is needed into the behaviour and degradation rate of various agricultural chemicals as well as the limits of biobed effectiveness.
“We really have no choice but to evaluate them all on a case-by-case basis.”
