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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?)

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Very good question! –  Mancuniensis Jul 18 '11 at 13:38
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3 Answers 3

up vote 11 down vote accepted

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.


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

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+100. Great answer, thanks for all the references. –  bstpierre Jul 19 '11 at 14:06
    
I don't suppose you could link those references to an online source? –  psusi Jul 19 '11 at 14:30
    
@psusi they were in the text, but I have linked the references too. Abstracts but in some cases not the full text, are publicly available for all of these. –  David Jul 19 '11 at 20:34
    
So this would imply that if clover (or similar) pops up in a lawn it might be a sign of low nitrogen soil but it is also a manifestation of more nitrogen being added to the soil (nature is righting itself if you like)? –  Lisa Jul 20 '11 at 2:52
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@Lisa that is correct. Nitrogen fixing plants are able to grow in low nitrogen soils because they can use the otherwise inert nitrogen in the air, i.e., they are adapted to a low-nitrogen ecological niche. Although there is no evidence that 'nature' operates with intention, the concept of 'nature righting itself' is the right idea, in line with James Lovelock's Gaia theory, which is based on the observation that many natural systems are self-regulating (homeostatic). –  David Jul 20 '11 at 15:25
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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)

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Thanks for the references. I'll have to see if the local library can get that text for me. I've been wondering about this in the context of N-fixing trees -- permaculture books make much of this but I've been skeptical because of the "locked up" issue. –  bstpierre Jul 18 '11 at 13:25
    
RE: cyanobacteria, I'm interested in azolla for growing my own fertilizer, but setting it up is a project that I can't squeeze in to this year and I'm not sure about overwintering it -- may not be able to get "permission" to overwinter it indoors if it needs it. –  bstpierre Jul 18 '11 at 13:26
    
Presumably the nitrogen is also released if the root dies. I don't know how many roots die and are renewed with a healthy tree (hopefully not too many?) but yes it sounds like you should be skeptical unless it is a long term plan with short-lived trees. –  winwaed Jul 18 '11 at 13:29
    
Just had to look Azolla up: 1 tonne/acre/year N fixing; also hypothesised as the plant that brought the Earth out of the Palaeocene/Eocene Thermal Maximum (PETM) as it took over the Arctic! - so probably a good CO2 fixer as well. (The PETM is emerging as the closest past analogue to what we're doing to the Earth's atmosphere.) –  winwaed Jul 18 '11 at 13:35
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@winwaed -1 I have to vote this down because the answer is wrong. Although some nitrogen is 'locked up' in plants until they die and decay, it is incorrect to infer that all fixed nitrogen is 'locked up' until the following season. The system is (very) leaky, and nitrogen fixing plants provide substantial N nitrogen to the soil and to non-n-fixing plants. –  David Jul 19 '11 at 16:24
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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:

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.


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.

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+1 Because I LOVED that article. Acknowledge though that it has limited application to the question at hand. –  Lisa Jul 19 '11 at 5:00
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Nice article, but it has nothing to do with this question. –  bstpierre Jul 19 '11 at 14:02
    
Sorry, I disagree slightly as it talks (briefly) about "black clover" & "clover" being a sign of "low nitrogen soil", not as a beneficial plant at getting nitrogen back in your soil... Also I clearly stated "the article doesn't directly answer your question" –  Mike Perry Jul 19 '11 at 15:24
    
@Mike: I asked what I think is a fairly precise question: "do legumes provide nitrogen to companions?" Nowhere in the article does it talk about whether clover does or does not provide nitrogen to companions. This is a good answer that should wait for a completely different question. –  bstpierre Jul 20 '11 at 2:08
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bstpierre, no offence, but you also stated in your question "Much is made of this fact -- advice that we should plant clover in our lawn to give the grass a boost", which isn't strictly true (if you believe the information given in the article I linked to, which I do), hence me posting my answer... Also I clearly stated "the article doesn't directly answer your question" –  Mike Perry Jul 20 '11 at 3:49
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