Q for Robert Kourik - Nitrogen fixer debate

Hi Robert. Welcome to Permies, though for all I know, you've been hanging out here as long as anyone.

If you have been a Permies fan for a while, you may already know of an ongoing debate (less active recently) between our host, Paul Wheaton, and Helen Atthowe, his instructor in his Master Gardener course way back when. Helen says that there is little improvement in soil fertility from nitrogen fixers until they die and release their accumulated nitrogen (this applies to annuals primarily, I assume). Paul, on the other hand says that root die-back happens as a matter of course as conditions like moisture change throughout the nitrogen fixer's life, and that this releases significant nitrogen into the soil.

As someone who has studied the latest research on roots (and probably plenty of other areas), have you seen any papers that discuss this issue? Would you care to weigh in on this question?

Hi Micky,

Excellent question, but may I be so bold as to add to it? Does the nitrogen fixer have to *die*? What happens when the local deer come along and "prune" the tops of my nitrogen-fixing Sea-buckthorn (Hippophae rhamnoides). Do the roots really prune themselves in response to the reduction in above ground mass and do they release nitrogen during this process? I have a number of young Hippophae rhamnoides that I've planted beside fruit trees specifically to support the larger tree by adding nitrogen to the soil. The plum tree that's beside the one that gets pruned a couple times/year thanks to hungry deer is certainly doing better than before I planted Hippophae rhamnoides beside it, but that could just have been chance.

Thanks

Hi Micky, I'm more aligned with Helen. I see the release of nitrogen to adjacent roots when the nodules "shed" due to death, grazing (mowing), and stress (limited release) I would say, however, that severe drought might release the N2. But I wouldn't support drought for this process. Don't forget mycorrhiza also help transfer nitrogen t roots. From my book 3 years in the making: • Mycorrhizae may have an important role in nitrogen cycling in acidic and highly organic soils. (mycorrhiza.ag.utk.edu/) Nitrogen is transferred from the soil to trees via mychorrhizal association. EM helps in the mineralization of nitrogen where plant litter is rich in lignin (the cells that make trees woody and sturdy) and tannins (astringent plant products). In An Introduction to Agroforestry by P.K. Ramachandran Nair, 1993, we read: “Mycorrhizal fungi are also often associated with the roots of nitrogen-fixing trees. Endomycorrhizae, which penetrate the host roots, …the vesicular arbuscular mycorrhizae (VAM) are the most common and are the most important for plant nutrition. Nodulation and nitrogen fixation require a high P level in the host plant, which can be facilitated by the mycorrhizal symbiont.”

I would think that when the grazing animal poops, pees or dies, we will get some of the nitrogen from forage plants that are still living. Chop and drop is a partial killing of the plant.

Thanks for your reply Robert. I found the info about mycorrhyzal transport particularly helpful. I knew mycorrhyzae are important for plants, but I didn't have any specific knowledge of what nutrients they exchange between them.

It seems Paul is swimming against the current on this one, but I think that's a role he's pretty comfortable in. I hope he shares some of his reasons for holding his unpopular opinion one day, but to this point I don't think he has, or I've missed it. In any case, as you and Dale point out, there are ways to hasten nitrogen release: grazing, mowing, chop & drop etc. These will do for me.

Maybe I missed something, but it seems the arguments are closer than I first read. I believe I just read from Robert K. that he sees "the release of nitrogen to adjacent roots when the nodules 'shed' due to death, grazing and stress." Is that so far from what Paul W. is saying?

I would think it is a good assumption to say that there is more N2 released from complete plant death than just root slough from grazing, mowing or stress. But, are Paul and Robert as far apart as some are thinking? I also assume that if one is grazing rather than plowing under or crimping, one would get less N2 at that one-time release, but by keeping the plant alive and growing and grazing again, you get multiple N2 releases of a perennial life-cycle of the plant. Am I understanding this properly?

I basically agree with you Dan. I sure messed up not mentioning the obvious - the nodules shed like gang busters (all of them I presume) when the plant is turned under as a green manure. But I guess that is included with death.

This seems to have been inferred already, but I don't see why mycorrhizae couldn't transfer nitrogen directly from one live plant to another. Paul Stamets would take the view that mycorrhizal fungi are huge players in moving water, sugars, and minerals between plants as needed. It would make sense for them to handle nitrogen the same way, though I don't have any evidence to back that up.

As far as experimental testing goes, I don't think it would be too difficult to use radioactive nitrogen to track nitrogen movement in and between plants. This is a rather common means of tracking, and it should be fairly simple to set up for this. A researcher could simply add radioactively-marked N2 to the air around a plant and leave the soil with all of its original, non-radioactive forms of nitrogen. Soon, the N-fixing plants would start showing marked nitrogen as their bacteria started fixing N2 into other forms. From there it would be fairly easy to see whether the other, non-N-fixing plants were getting their nitrogen from the N-fixers or from preexisting soil nitrogen. This seems like a simple enough experiment that I would be kind of surprised if no one has done this and published the results. I haven't looked for such results, though, so I don't know whether they're out there.

Jonathan

Jonathan Krohn wrote:As far as experimental testing goes, I don't think it would be too difficult to use radioactive nitrogen to track nitrogen movement in and between plants.

Hi Jonathan,

I've seen a documentary in which researchers wrapped a clear plastic bag around a tree branch, sealing it around the branch and then injecting C14-marked CO2 into the bag. The plant took up the CO2 through the leaves, manufactured sugar out of it and proceeded to share it with nearly every other plant in the area through its associated mycorrhizae. No big deal to contain the C14 and ensure that any detected in other plants has gone through the hypothesized process.

It would take more ingenuity to do a similar experiment with nitrogen. For starters, the fixation site is in the roots and so are the mycorrhizae. I don't know how you'd go about containing the radioactive nitrogen isotope where the N-fixing nodules are without also isolating the plant from its mycorrhizal partners. Maybe a bag around part of the root system, I suppose. Not as straight forward as the CO2 experiment, it seems to me, but a clever scientist could probably work it out.

Jonathan Krohn wrote:This seems to have been inferred already, but I don't see why mycorrhizae couldn't transfer nitrogen directly from one live plant to another. Paul Stamets would take the view that mycorrhizal fungi are huge players in moving water, sugars, and minerals between plants as needed. It would make sense for them to handle nitrogen the same way, though I don't have any evidence to back that up.

I agree with this. I think that one reason there may no be much evidence is most nitrogen studies I have read are regarding annuals for farming purposes. I do however think it would be a lot less nitrogen than what you get from die back.

From my book in progress:

• Mycorrhizae may have an important role in nitrogen cycling in acidic and highly organic soils. (# __ mycorrhiza.ag.utk.edu/) Nitrogen is transferred from the soil to trees via mychorrhizal association. EM helps in the mineralization of nitrogen where plant litter is rich in lignin (the cells that make trees woody and sturdy) and tannins (astringent plant products). In An Introduction to Agroforestry by P.K. Ramachandran Nair, 1993, we read: “Mycorrhizal fungi are also often associated with the roots of nitrogen-fixing trees. Endomycorrhizae, which penetrate the host roots, …the vesicular arbuscular mycorrhizae (VAM) are the most common and are the most important for plant nutrition. Nodulation and nitrogen fixation require a high P level in the host plant, which can be facilitated by the mycorrhizal symbiont.”

But the nitrogen transfer is much less than phosphorous.

Micky Ewing wrote:It would take more ingenuity to do a similar experiment with nitrogen. For starters, the fixation site is in the roots and so are the mycorrhizae. I don't know how you'd go about containing the radioactive nitrogen isotope where the N-fixing nodules are without also isolating the plant from its mycorrhizal partners. Maybe a bag around part of the root system, I suppose. Not as straight forward as the CO2 experiment, it seems to me, but a clever scientist could probably work it out.

One approach would be to simply let the marked N2 into all of the soil. I see three potential sources of error with this: 1) fixation of nitrogen by free soil bacteria, 2) fixation of nitrogen by non-biological sources, 3) fixation of nitrogen by plants not previously known to fix nitrogen, and 4) direct absorption of nitrogen from the atmosphere by other plants. The last two, of course, might be ignored since currently they are generally believed to be impossible. All of these potential sources of error, however, could be accounted for by adding a control without any known N-fixing plants.

One could also, as you suggested, limit the marked N2 to only part of the root system (or split the root system, or graft two N-fixers together, etc.). This would test nitrogen transfer by the pathway: atmospheric N2 > fixation by bacteria in root nodules > transport throughout N-fixing plant, including to distant roots > transfer to mycorrhizae > transport to other plants. The critical link here would be that the soluble nitrogen would have to travel from the original root nodules into a major root or even the stem, and then travel back into a different system of smaller roots. One could avoid the need for this link by having the mycorrhizae, rather than an N-fixing plant, be the bridge between the marked-N2 and non-marked-N2 systems. Basically, the N-fixer would be in a chamber with marked N2, and its attached mycorrhizae would run through a small tube or hole to the other body of soil. This would test the pathway: atmospheric N2 > fixation by bacteria in root nodules > transfer to mycorrhizae > transport to other plants. It might not be as robust, however, because mycorrhizae don't handle exposure to air as well as plant roots or shoots do, and it might be susceptible to error sources 1 and 2 above, if the mycorrhizae absorbed any nitrogen directly from the soil.

Given that, the logical conclusion would be to do all three experiments, and then see whether they showed the same results. Isn't experimental design fun? (I actually think it is, but I already enjoy playing with complex systems.)

Jonathan