Do legumes provide nitrogen to their companions?

Question

It's common knowledge that legumes (clover, beans, etc) can (with the help of Rhizobium bacteria) fix nitrogen from the air and hold it in the soil.

Much is made of this fact -- advice that we should plant clover in our lawn to give the grass a boost, "three sisters" plantings of beans with corn so the beans can provide the corn with nitrogen, etc.

But I've seen a couple of mentions that the nitrogen that legumes fix from the air is "locked up" on the roots of the plant until it dies. Thus they don't help their companions while they're alive -- so while clovers in a lawn have other benefits, they do not provide any nitrogen to the grasses. According to this contrary advice, the only nitrogen benefit these plants provide is to a following crop -- e.g. a soybean crop or clover cover crop will provide nitrogen to whatever follows.

Which advice is correct? Does it depend on the legume type? (I.e. are there some legume types that fix extra nitrogen that would become available for companions?)

Answer 1

Yes, legumes do provide nitrogen to associated plants, through multiple mechanisms.

Nitrogen is transfered from nitrogen fixing plants to the soil solution, where it is then available to other organisms (e.g. plants and microbes). This nitrogen transfer is in the form of root exudates, the sloughing off of root cells, and through the turnover (growth and death) of roots within the growing season. Notably, root turnover increases with root nitrogen content, and nitrogen fixers have particularly high root turnover rates. Previous studies have quantified this transfer, for example Laidlaw et al. (1996) found that the N-fixing legume clover transfers 8 mg N / m² / d to the soil whereas non-N-fixing grass transfers < 1 mg N / m² / d to the soil. Furthermore, about 50 kg N ha-1 (similar to typical agricultural fertilization rates after accounting for the inefficiency of fertilization) was transferred from clover to grass in a mixed field experiment (Boller and Nosberger, 1987).

In addition to the transfer of nitrogen from nitrogen fixers to the soil, the direct transfer from a nitrogen fixer to a non-nitrogen fixer by mycorrhizal fungi (known as 'common mycorrhizal networks' (He et al 2003)) has been observed. For example, Bethelfalvay et al (1991) demonstrated:

Maize, when associated with the N-fertilized soybean, increased 19% in biomass, 67% in N content and 77% in leaf N concentration relative to the maize plants of the N-deficient association.

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References

Bethelfalvay et al (1991) Nutrient transfer between the root zones of soybean and maize plants connected by a common mycorrhizal mycelium. Physiologia Plantarum 82:423-432

Boller, BC and J Nosberger, 1987: Symbiotically fixed nitrogen from field grown white and red clover mixed with ryegrass at low levels of N fertilization. Plant Soil 104, 219—226.

He, XH, Critchley, C and Bledsoe, C (2003) Nitrogen transfer within and between plants through common mycorrhizal networks (CMNs). Critical Reviews In Plant Sciences, 22 6: 531-567.

Laidlaw et al 1996. Effect of white clover cultivar on apparent transfer of nitrogen from clover to grass and estimation of relative turnover rates of nitrogen in roots. Plant and Soil 179(2): 243-253, DOI: 10.1007/BF00009334

Answer 2

The Wikipedia page on Nitrogen Fixation says the nitrogen is released after the plant dies. Under legumes, quote:

Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clovers, soybeans, alfalfa, lupines, peanuts, pistachios and rooibos. They contain symbiotic bacteria called Rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil(1)(5) The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (family Polygonaceae), which are often referred to as "green manure."

Wikipedia may not be the most reliable of sources with these kinds of questions, but I think the references for the statement in question look reliable and could be investigated. They are:

• Postgate, J (1998). Nitrogen Fixation, 3rd Edition. Cambridge University Press, Cambridge UK.
• Smil, V (2000). Cycles of Life. Scientific American Library.

The CUP looks like a university text, and the Scientific American text is going to be a synthesis but they do a good job of such things.

Although most of the nitrogen may well be locked up, it is also going to be a leaky system - eg. roots might died, nodules become dislodged and break down, etc.

(also interesting from that page: the list of other non-legume nitrogen fixers, such as cyanobacteria)

Answer 3

Much is made of this fact -- advice that we should plant clover in our lawn to give the grass a boost...

Have a read of the following "lawn care" article as it dispels that misconception:

• Organic Lawn Care For the Cheap and Lazy

If you believe the information given in the article, which I do -- Clover taking over a lawn is a sure sign that the soil is nitrogen deficient and the clover isn't going to help the grass. Period.

Please bear in mind, the article doesn't directly answer your question, but I believe it does give some relevant information pertaining to your question to make it a worthwhile read.

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Have a listen of Gardeners' Question Time podcast, Colne and starting listening at 33mins:38secs in.

Chris Beardshaw debunks a longstanding myth about nitrogen-fixing pea shoots.

Below is my poor effort at transcribing what is said:

Peter Gibbs: How many of you leave your pea & beans roots in the ground to get the benefit of the nitrogen that they've fixed through the Winter?

Peter Gibbs: A few hands going up there, so do I, but Chris is going to tell us we're doing it all wrong

Chris Beardshaw: Well, not necessarily doing it wrong, the principal is true and it has in fact been going on for ages hasn't it?

My grandfather taught me that all the legumes, the pea and bean family are nitrogen fixing plants.

That's to say they absorb nitrogen or in fact there is a bacteria that absorbs nitrogen on their behalf and in return the bacteria plugs into the root and gets simple sugars out of the plant, so it's a kind of symbiotic relationship.

The plant gets the nitrogen and that's why we've left our peas and beans roots in the garden thinking we're going to improve the nitrogen status of our soils.

That is until some research was recently carried out which has proved that the nodules that are formed, are present in very high quantities and that about 40% of the plants total nitrogen reserve is at the root in those nodules prior to flowering.

However as soon as the plant flowers the majority of that nitrogen then moves not into the stems and leaves, it moves away from the root into the flower, and into the pods, and of course the seeds, the peas, the beans themselves.

So much so that after harvest the nodules and the roots contain less than 3% of the original nitrogen, so if you're leaving your pea and bean roots in after harvest, actually there's very little point other than just improving the general soil structure.

Peter Gibbs: So we're not wrong, just misguided?

Chris Beardshaw: Yeah, one of the things if you're using green manures, very often we use peas and beans or things like winters beans as a green manure. But if you think about how we use them, we dig them in before they flower, that's where you get the benefit of all that fixed nitrogen. Plus all of the nitrogen which is in the plant, the upper part of the plant.

Don't bother leaving them in ground expecting to get a high nitrogen flush after they've fruited.

I'm currently in conversation with Chris Beardshaw, Peter Gibbs and Gardeners' Question Time, trying to get further information about the aforementioned study (hopefully a link to the actual document). If I'm successful, I will update this answer...

So far I've got back:

• Peter Gibbs: Chris quoted a study which found legumes only fix nitrogen up to the flowering stage, so leaving roots over winter is pointless.

• Chris Beardshaw: The important element was the huge reduction of available N in the nodules after onset of flowering, against popular beliefs.