Legumes touted as emission fighters - Plants that actively fix nitrogen result in much lower nitrous oxide emissions than other crops

By Jeffrey Carter, Special to Ontario Farmer, Tuesday, February 6, 2007

By returning to legume-based nitrogen fixation, agriculture could significantly reduce greenhouse gas emissions, the Canada Research Chair in organic agriculture says. Agriculture and Agri-Food Canada research from 2005 shows that legumes that are actively fixing gaseous nitrogen (N₂) through biological means result in far lower nitrous oxide (N₂0) emissions than previously estimated, Dr. Derek Lynch says. In fact, unless the legumes are plowed down, nitrous oxide production is negligible.

Lynch, who works at the Nova Scotia Agricultural College, wants to investigate further. "It should be looked at and that's what I'm trying to do," Lynch says. "In Canada, right now, we do not have a good sense of how much of the nitrogen can come from legumes."

Organic farms, to a large extent, depend on nitrogen derived from perennial and cultivated legumes. In comparison, the use of synthetic nitrogen sources contributes significantly to nitrous oxide loading. "We could do what they did in the 1880s to get good yields - having legumes in the crop rotation," Lynch suggests.

It was during the latter part of the 19th century that agricultural researchers began to refine the use of legumes in rotations as a nitrogen source. In the estimation of some historians, the development was as important as the industrial revolution since it resulted in a dramatic increase in food production.

During the course of World War I, another source of nitrogen as plant food was developed by German scientist Fritz Haber. His invention was a machine that combined nitrogen and hydrogen to produce ammonia synthetically. "The development was driven by the demand for explosives," Lynch says. "It was his invention that was the most important invention of the 20th century, in terms of impact."

Unfortunately, the synthetic creation of nitrogen fertilizers also led to the nitrous oxide problem. Agriculture is the biggest source, Lynch says. According to the Cooperative Research Centre for Greenhouse Accounting, there is 1,000 times more carbon dioxide in the atmosphere but nitrous oxide has 310 times the impact on climate. "Nitrous oxide is produced primarily from excess reactive nitrogen accumulating in agricultural soils, cascading through terrestrial and aquatic systems. During the past few decades, the production of new reactive nitrogen by humans, primarily as industrial fixed fertilizer nitrogen, has been greater than the reactive nitrogen derived from all natural sources," Lynch writes. "The negative consequences... for the health and welfare of people and of ecosystems, is only now being fully realized."

Lynch also touched on what may be a concern for organic farmers. In organic dairy operations, adequate, but not excessive, levels of nitrogen and potash can be generated. However, there may be a phosphorus shortfall. "I think that's something we need to be careful of because it could limit legume productivity and that could have a cascade effect on nitrogen fixation," Lynch says. One possibility may revolve around the use of rock phosphate as a soil amendment, perhaps in combination with green manure.

The Organic Agriculture Centre of Canada is located at the Nova Scotia Agricultural College at Truro. Lynch spoke at the recent Guelph Organic Conference.

 

The OACC gratefully acknowledges permission from Ontario Farmer to post this article on our website.

Posted February 2007