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Harnessing the power of the rhizosphere

Biochar — heat-treated organic material — can give the root zone a boost

Have you ever noticed how lush the crabgrass can get around an old firepit? It’s a small yet compelling example of the benefit that burnt wood residue can have on plant growth.

But there’s a lot more going on than meets the eye in that interchange between plant, charcoal, soil and air, and Prem Pokharel is the man to ask about it.

Pokharel, a PhD student at the University of Alberta, is the recipient of the 2018 WGRF Endowment Fund Scholarship. He’s examining what happens when biochar is added to soil — to plant growth, to nutrient mineralization and to greenhouse gas (GHG) emissions. He’s discovered that these changes are different in the rhizosphere (the root zone) than in bulk soil in the rest of the field.

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That distinction is key. People have been using biochar to increase soil’s carbon-storage capacity and reduce GHGs for years. But Pokharel points out that the focus has always been on bulk soil, not the rhizosphere. He aims to close that knowledge gap.

“The rhizosphere is a very dynamic system,” Pokharel says. “It’s a very active zone.” That area around plant roots is where microbial life is influenced by chemicals released from the roots and where, as a result, biological and chemical processes are completely different from what happens in bulk soil.

Pokharel’s research is comprised of four separate studies that take on various aspects of how certain biochars interact with the rhizosphere. He’s completed the first two and has already garnered some interesting results.

Experiments with biochar

Biochar is not quite the same thing as charcoal. Both are produced in a similar way by heating biomass under conditions of little to no oxygen, a process known as pyrolysis. This thermal decomposition results in a black, carbon-rich substance. In biochar, it’s highly absorptive and has a high ion-exchange capacity which charcoal lacks.

Not all biochars are the same — it depends on the feedstock. In his work, Pokharel is using biochars created from manure pellets and from willow chips, which are both readily available in Western Canada.

For the first of his four studies, Pokharel used specially designed rhizoboxes — boxes with rhizosphere and bulk soil compartments — in which he grew wheat in a greenhouse. He had five treatments: unprocessed wood chips and manure pellets, biochars from each and an untreated check. Pokharel measured plant production, biomass and CO2 emissions from both the rhizosphere and bulk soils.

The results were mixed for greenhouse gas. “Using a biochar soil conditioner does not always give a positive result in reducing GHGs,” Pokharel says. In one experiment, the manure biochar did not have any effect on CO2 in bulk soil but did reduce it in the rhizosphere. However, results showed that the addition of biochar from either source did reduce CO2 emissions overall.

“I was also looking at the crop productivity, specifically wheat biomass, and the result was different,” he says. “The manure biochar increased total plant biomass and grain yield, while the wood chip biochar reduced both those things.”

The second study used the same setup, but this time Pokharel looked at the effect of biochar on nitrogen mineralization in both the rhizosphere and bulk soil.

“On cropland, we’re mostly concerned with nitrous oxide,” he says. (According to Agriculture and Agri-Food Canada, about 10 per cent of Canada’s GHG emissions come from crop and livestock production, with the two biggest emitters being methane from livestock and nitrous oxide from cropland.)

“Biochar’s role in decreasing nitrous oxide emissions from soil depends mainly on the rate of nitrogen mineralization and its subsequent effect on the denitrification process,” Pokharel explains. How fast N mineralizes depends on the microbial environment. Specifically, it depends on what microbes are present and how plentiful they are. He is looking at the bulk and rhizosphere soils from the first study to see what if any changes the biochars make to the microbial communities. He’s still analyzing the data.

Taking it to the field

The next two experiments will move from the greenhouse to the field. One will look at how biochar amendments affect nutrient leaching and nitrogen recovery by crops, and the other will examine the ideal application rates for biochar and chemical fertilizer used together to get the best crop results while reducing GHGs.

Pokharel has a long way to go with his research, but he’s already uncovered differences between how biochar affects biogeochemical processes — in other words, how biochar influences how things like CO2 and N2O cycle through earth, air and plant, and how that cycle is different in the rhizosphere compared to those same processes in bulk soils.

“What I am trying to see is the mechanism of how biochar affects the rhizosphere,” he says. “It’s important information in understanding the process around the root zone.”

There is real power in the rhizosphere that he hopes can be harnessed through proper use of biochar.

– By Clare Stanfield for WGRF

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