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Search For Nitrogen Fixers

Dr. Chantal Hamel with Agriculture Canada’s Semiarid Prairie Agricultural Research Facility in Swift Current, Sask. has spent the past few years trying to establish a zero input native pasture and she has had to throw out a few assumptions along the way.

Hamel and her team wanted to see how much nitrogen could be fixed by legume plants when mixed with native grass species. The researchers used purple prairie clover and alfalfa, in the absence of any other viable native legumes. Both are well adapted to the prairie environment and had shown good results in earlier, related research by some of Hamel’s colleagues.

The results, however, were surprising in that although the legumes did fix nitrogen very actively in the plants themselves, the level of fixed nitrogen in the soil was no higher than it was in plots that were comprised only of native grasses and excluded the legumes.

“When we measured the nitrogen fixed we looked at the plant tissue and we saw that the legumes took almost all their nitrogen from the air, so they were fixing nitrogen,” says Hamel. “But when we looked at the whole system; the grass and the soil, we didn’t see a difference. So this tells us that there is another source of nitrogen which I don’t think comes from the legumes, but other sources.”

Which raises the question; how does nitrogen get into the soil in a natural, native prairie ecosystem? The next step in Hamel’s research is to look at the roll of naturally occurring soil fungi in the process of making nitrogen available to grass stands, when no nitrogen-fixing plants are present.

Results to date showed native soil fungi, called arbuscular mycorrhizal (AM) fungi, send out long filaments to interact with plant roots and help the plants obtain nutrients like nitrogen and phosphorus. Researchers noted a seasonal variation in the abundance of AM fungi, which peaked in August.

Another common occurrence in grass roots was the presence of dark septate (DS) fungi. They occupied about half the length of plant roots no matter the season. DS fungi, AM fungi and free-living biological nitrogen fixers may contribute significantly to the nutrition of native grasses in mixed pastures.

“We suspect it is maybe because of the natural mechanisms that are acting in the prairie,” says Hamel. “Maybe we have input from other nitrogen fixing bacteria in symbiosis with the grasses.”

Along with continued monitoring of nitrogen levels in the soils, the next step is to introduce cattle onto the native grass paddocks and see how grazing will affect the system.

A number of paddocks have been established with different treatments — one with only brome grass as a control pasture to compare data against. The others have been seeded with a mixture of seven native grass species; little bluestem, blue grama grass, western wheat grass, northern wheat grass, Canada wildrye, awn wheat grass, and green needle grass. Some of the paddocks also have the addition of legumes, (alfalfa and purple prairie clover), whilst others do not.

All the sites will be grazed and results compared to see how the systems react to grazing, what it does to the levels of nitrogen in the soil, which mix responds the quickest to replace depleted nitrogen, and which species end up dominating in each system.

“We also want to look at the persistence of the plants,” says Hamel. “We have seven grasses, but we don’t know what is going to happen with them. Probably some will disappear, some will take over and maybe we will end up with some species that do well together and we would also like to look at the persistence of the alfalfa and the purple prairie clover.”

It’s a project that will take at least another five years, as the scientists are trying to replicate what would occur in a natural environment. They believe that some of the natural processes that are occurring in a native prairie system may be more important than adding legumes to the mix.

“We started off with the idea of how we could input nitrogen into our native prairie system, so we didn’t have to add nitrogen fertilizer and we wanted to do that by introducing legumes, but now we see that introducing legumes may not be that important,” says Hamel.

This could provide both environmental and economic benefits, especially for livestock producers.

“Nitrogen production is a very energy intensive process,” says Hamel. “And the living system has evolved so that if it doesn’t need to produce nitrogen it doesn’t do it. If you have enough nitrogen in the soil you will not have biological fixation. So if you put legumes in a soil where free-living or associative bacteria fix nitrogen actively, the nitrogen fixed by legumes will soon become available to other components of the system. This includes these bacteria, that will easily make use of this other source and thereby reduce their own level of nitrogen fixation. This may well be what we are seeing.”

Once they have a better understanding of how nitrogen is produced in a native prairie the next step for Hamel and her team is to try and identify the organism responsible for fixing these inputs, through the analysis of the DNA coding for their nitrogen fixing enzyme, and tweak the process to improve it.

Projects to select durum and chickpea genotypes for their association with good soil microorganisms, using conventional methods, have also recently been initiated.

“We have lots of cattle production here and we have lots of native prairie and we expect that in the future, with climate change, we will have more arid zones where we cannot grow many crops and where we could establish native prairie for cattle production,” says Hamel.

With the cost of synthetic fertilisers also certain to rise as the fossil fuels that produce them become scarcer and more expensive, the search for cheap forage will become a mission.

“We are looking at ways to supply forage at essentially no cost because of the adaptation of these plants and maybe of the whole system,” says Hamel. “Our goal is to find something that doesn’t cost anything and uses the soil bio-resources better.”

Angela Lovell is a freelance writer based in Manitou, Manitoba.

About the author


Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at or follow her on Twitter @angelalovell10.



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