Downy brome alters soil, then makes itself at home

An American researcher believes downy brome does more than out-compete other plants in the pasture.

Bob Blank said the invasive weed also changes the soil.

“We actually believe that it sees the soil differently than some of our natives. It can actually access nutrients that some natives can’t (through) a myriad of processes,” said Blank, a soil scientist with the U.S. Department of Agriculture’s Agricultural Research Service.

“There’s lots of things going on.”

The plant spreads quickly, out-competing plants and reducing diversity.

“You’ll be shocked at how quickly it does it,” he said.

“You’ll be shocked at the size of it, the densities of it and you’ll be shocked where you have a fire where you may not have had one before.”

Downy brome is a problem weed in many parts of the continent. In Blank’s state, Nevada, ranchers are forced to accept it as the dominant forage.

Livestock can eat the plant, al-though other plants are preferred. However, its prevalence can lead to more wildfires.

“Any grazer, rancher out here deals with cheatgrass (another name for downy brome). That’s just a reality. They deal with both the positive and the negatives aspects,” said Blank.

“It burns like crazy and we get wildfires in this country in certain years that are absolutely enormous in size, devastating, just costly to control, and we don’t really know how to rehabilitate them either.”

In Saskatchewan, the annual plant is designated as a noxious weed. Its presence is noted in the province’s southwest, as well as in Alberta.

Chemical controls are available for the weed, but seeds can lay dormant for several years before germinating. Alberta Agriculture instructs producers to plan for at least three years of control.

Blank has studied downy brome for a number of years in both green-house and field studies.

In a study published this fall, Blank wondered if the weed would grow better in soil already infested with downy brome. He compared the production of plants grown in uninfested soil with that of plants in infested areas.

After two growing seasons, the invaded soil had double the biomass.

Blank’s team believes the plant, and some other weeds, are able to engineer the soil, altering the nutrient cycle to access greater amounts of nitrogen, phosphorus and manganese.

“It modifies the soil to favour its invasiveness. This is not really a new idea,” said Blank.

“It’s been speculated for many invasive plants. Cheatgrass is just another one.”

Blank said it’s a separate phenomenon from nitrogen fixation.

A 2005 ARS study suggested that rising carbon dioxide levels may benefit some invasive plants, including fire-tolerant species such as downy brome.

It is known to invade areas that have been disturbed by grazing, cultivation or fire.

“Disturbance creates open sites that it can occupy, colonize and then go from there,” said Blank.

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