Paul was right: Rhizodeposition

I have forwarded a few articles to Paul to put to rest the "dialogue"(s) about the significance of root exudates of nitrogen fixing plants providing nitrogen to adjacent plants without the decomposition and decay of either leaf- or root matter. The process is coined rhizodeposition and the best review article I could find was published in Agronomy of Sustainable Development (Volume 30, Number 1, January-March 2010) by a group of researchers (Joëlle Fustec, Fabien Lesuffleur, Stéphanie Mahieu and Jean-Bernard Cliquet).
Rhizodeposition is assessed to vary anywhere between 4% and 71% of total Nitrogen incorporation from root exudates ONLY dependent on donor and host species and methodology. The Abstract ( http://alturl.com/w94e4), the comparative table of studies referenced and the Conclusions are the best parts.

The other review article is from Soil Biology & Biochemistry (Volume 40, 2008 pp. 30–48) Nitrogen rhizodeposition in agricultural crops: Methods, estimates and future prospects by Florian Wichern, Elmar Eberhardt, Jochen Mayer, Rainer Georg Joergensen, Torsten Müller, of a few of the same research and some more

1)  I like the subject line

2)  I mentioned your emails in a podcast recorded today

3)  Before sending on to helen, I want to read it first, but since the kickstarter thing, I have been deluged by email.  I'm trying to catch up.

Thank you and take all the time you need: this is important stuff. There are also studies trying to assess the nitrogen fixing contributions of the non-rhizobial micro-organisms (rhizobial bacteria form an endosymbiotic nitrogen fixing association with roots of legumes forming macroscopic colonies in the form of root nodules.)

More to come!

You are welcome to attach files here (where appropriate) or to link to the studies. 

Maybe this thread will grow into the thing I send to helen!

Would someone please edit that post with the long URL? - it's making the page too wide to read. thanks

Search for rhizodeposition returned 1666 articles so far. I will try to do a reasonably accessible review of the recent ones; it seems that recent research strongly support the notion that nitrogen fixing bacteria enrich the soil to benefit adjacent plants significantly.

What i wonder is if the plants that are being used for nitrogen fixing are yield crops or just support species.

It is of course well known that rhizodeposition takes place, but common practice in regular agriculture is to use lupins and stuff like that which gets underplowedbefore setting seed or making pods.

I am really curious if research has been done with plants that were left to crop, and crop was yielded, how uch of the nitrogen has become available.

I will dive into your links, and search a bit for myself.

This discussion keeps popping up everywhere.

It seems that nitrogen fixing intensifies with demand and drops off with N saturation. To re-state: even when crop plants are allowed to achieve high yields (like beans and peas, AND are harvested) their contribution to adjacent plants is not diminishing. So both "supporting plants" and crops are companions in polyculture.

Now, reading research about rhizodeposition fills me with humility: modeling the complexities of ecosystems is virtually impossible. Just imagine, adding associative nitrogen-fixing bacteria to the endophytic (root-nodule forming) mix combined with  Paul Stamet's nutrient and mineral carrying mycelial conduits creates  an immense web of interconnectedness, cooperation and competition. Truly humbling and yet glorious.

The conclusion after having read well over a dozen of the peer-reviewed scientific publications, both studies and review articles (aka "white papers") is that Helen Attowe's position that companion planting of nitrogen-fixing plants benefits the non-nitrogen fixer ONLY by means of plant tissue decomposition is based on the research consensus of the mid-'90s agronomy—probably constrained by the limits of isotopic-N study methodologies. In other words she is either not a careful reader or has not read the literature since. Research of the last 15 years is OVERWHELMINGLY argues the rhizodeposition through exudates is a significant source of N in companion planting AND that the pruning of nitrogen-fixing plants increases their contribution to the soil's available N content over ten-fold.
What is most interesting is that rhizodeposition is bidirectional: N-exudates from non-nitrogen fixers contributes about 10% of total plant tissue N in nitrogen-fixer companion plants.
Helen is in the illustrious group of fervent deniers of things unpalatable to tastes for one reason or another. (I have a theory for its source—subversive and explicit psychoanalytic formulations tend to alienate more than enlighten—but I will remain temptingly and cautiously silent. Hint: it's libidinal)
[font=Verdana][size=14pt]Case closed.[/size][/font]

Thanks Cini.  Can you recommend one or two papers in particular from those you have posted that give the best evidence of this phenomenon?

Hugh H. wrote:
Thanks Cini.  Can you recommend one or two papers in particular from those you have posted that give the best evidence of this phenomenon?

yes,
I have uploaded them in order of importance so you could start with
Nitrogen Rhizodeposition in Legumes

From what I've studied the average N sequestration of legumes in a monocult is 300-500kg/hectar. (thats about 2.4 acres)

Is that 300-500kg what makes it into the soil, or just the total the plants produce?  It seems like a lot.

Anyone have info on how much might make it into the soil with a specific management practice (chop & drop, tilling in, etc?)

[ftp][/ftp]Oh sorry thats the total but seeing as I use only 100kg per hectar to grow my corn it's alot

Can anyone help me understand the benefits to nitrogen fixing trees over nitrogen fixing legumes like clovers.  Last year I experimented with putting alfalfa under tomatoes to see if they could co-exist well.  Unfortunately it didn't work well so I'm back to cover/manure crops and am fine with that but wonder if I should add some siberian pea plants to the fruit tree guilds (even though I know I should I don't want to since I mix fruit trees in with my garden beds, it would shade too much land).  For now I only have bulbs, buckwheat and clover under the fruit trees.  I want to maximize my rhizodeposition and since it requires a living plant..... Thoughts?

A bit OT, but I don't think tomatoes are N-loving (correct me if I'm wrong).  Many gardeners traditionally put something like potassium sulfate, so I imagine its the K which you typically need to boost.  Remember the general rule that N gives more leaves and K gives more fruit.

How did your experiment not "work well"?  How did the tomatoes grow (or not)?

I'm not aware of many differences between trees and herbaceous plants in terms of nitrogen fixation (apart from deciduous trees which drop a lot of nitrogen in their leaves in autumn).  Martin Crawford uses the same accumulation rate, 10g/m2/yr for both in his book.  This equates to only 100kg/ha/yr so quite a bit lower than what BDAFJeff suggests in his previous posts to this thread.  Perhaps the figure isn't too accurate; it's not referenced either, so no way to see how he came to that number.

Crawford says that nitrogen fixation is affected by temperature (not too hot/cold), soil pH (not too acid), soil nitrogen (not too high), moisture (not too dry), and of course light availability.

"not work well" meaning that the same tomato varieties growing next door to the undersown alfalfa tomatoes were stunted in their growth and production.  They did fine but the tomatoes that were simply mulched with 3" of straw did much better.  Fun point though was that the undersown alfalfa test subjects had zero disease problems, some grass hopper damage and end rot from oddly timed rain but no slugs or disease to speak of.  Not worth the yield difference but still intriguing so next year I'm going to try an Aslike clover (perennial) and see how that does though I'm thinking now buckwheat for the mulch and the K, I've had good luck with clover and buckwheat in sync so maybe I'll go that route. 

I'm looking for no till soil conditioners that I can either plant right in to or undersow for mulching/soil building.  I'm tired of buying straw and my wheat and rye crops are not developed well enough yet to produce the level of straw that I'm after.  The new maris widgeon variety this spring should prove a good candidate though.  Once I can get one naturalized to my alkaline soil I'll know more.

Thanks for the info on trees vs herbaceous, I still need to better research how often to lightly mow the alfalfa and clovers to produce good mulches without killing them.

Hugh H. wrote:
A bit OT, but I don't think tomatoes are N-loving (correct me if I'm wrong).  Many gardeners traditionally put something like potassium sulfate, so I imagine its the K which you typically need to boost.  Remember the general rule that N gives more leaves and K gives more fruit.

How did your experiment not "work well"?  How did the tomatoes grow (or not)?

I'm not aware of many differences between trees and herbaceous plants in terms of nitrogen fixation (apart from deciduous trees which drop a lot of nitrogen in their leaves in autumn).  Martin Crawford uses the same accumulation rate, 10g/m2/yr for both in his book.  This equates to only 100kg/ha/yr so quite a bit lower than what BDAFJeff suggests in his previous posts to this thread.  Perhaps the figure isn't too accurate; it's not referenced either, so no way to see how he came to that number.

Crawford says that nitrogen fixation is affected by temperature (not too hot/cold), soil pH (not too acid), soil nitrogen (not too high), moisture (not too dry), and of course light availability.

You might try comfrey under tomatoes.  It will help build potassium.  A quick google search confirms that tomatoes shouldn't be fed too much nitrogen, as I suggested in my last post.  I suspect they are not a good 'test crop' for your underplanting experiments because of this.

How about trying it with some leaf crops that will really like the extra N?  I'd be interested to hear what techniques you find success with.

Hugh H. wrote:
You might try comfrey under tomatoes.  It will help build potassium.  A quick google search confirms that tomatoes shouldn't be fed too much nitrogen, as I suggested in my last post.  I suspect they are not a good 'test crop' for your underplanting experiments because of this.

How about trying it with some leaf crops that will really like the extra N?  I'd be interested to hear what techniques you find success with.

Good idea with the comfrey, what level do tomatoes feed on.  If they're surface feeders the comfrey combination would be ideal.  I'll need to come up with other plants that would work with the comfrey though so that I could continue a rotation while leaving the perennial comfrey in place.