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Legume nitrogen fixation learning module

The effects of legume species and varietal differences on N fixation

The characteristics of different legume species such as life cycle (annual vs. perennial), growth potential, root vs. shoot size, the efficiency of the symbiotic relationship and the plant’s response to its environment all interact to determine how much N will be fixed by a given legume crop. At the time of incorporation the amount of new N that will be contributed by the green manure is a function of:

  1. The concentration of N in the legume tissue.
  2. The size of the legume biomass.
  3. The proportion of total plant N that was derived from the atmosphere.

Consider for example, if one legume (Legume A) is more sensitive to high soil N, the proportion of total plant N from fixation may be lower than another legume (Legume B) in a high fertility field. However, if Legume A produces more total biomass, it may fix more total N than Legume B even with a lower rate of N fixation.

Within a legume species, there can be high variability in N fixation rates due to varietal differences. For example, our research has shown that different varieties of hairy vetch can have very different N fixation rates. This is probably due to different breeding histories and selection criteria that were used to select these different varieties.

Nitrogen fixation has not been a trait selected for in most legume forage and cover crop breeding programs, however, it could be inadvertently altered during the selection process. If breeding is done on relatively nutrient rich soils, legumes with lower N fixation rates may have inadvertently been developed. These changes in N fixation capacity could be due to changes in plant physiology or modifications in the way the plant interacts with its N-fixing microbes.

Recent research has shown that newer varieties of soybean are not able to regulate the amount of energy that is going to rhizobia that are inefficient N-fixers. Older soybean varieties are able to limit their support of rhizobia that are not fixing N in exchange for sugars and are not as susceptible to “lazy” rhizobia.

How do you think growing a grass in an intercrop with a legume will affect the legume’s N fixation?
Go to the next unit:
Managing legume N fixation by planting species mixtures

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