Thursday, February 26, 2009

Rhizobium Symbiosis with Woody Plants: Leguminous Nitrogen-Fixing Trees

Rhizobium Symbiosis with Woody Plants: Leguminous Nitrogen-Fixing Trees
By Douglas Barnes


As mentioned in the previous article in this series, beneficial partnerships are the way of nature. In particular, some microbes (Frankia and Rhizobium) form associations with certain plants allowing them to fix atmospheric nitrogen into a form that plants can use. These symbiotic partners can help us to rehabilitate damaged landscapes, preparing the soil for a succession of more long-term plants.


This piece focuses on woody plants that associate with the bacteria of the genus Rhizobium. We can see from the diagram below that there are 3 subfamilies of the family Fabaceae (AKA Leguminosae). These families are Faboideae (AKA Papilionoideae), Mimosoideae, and Caesalpinoideae. Note that not all the trees in these subfamilies are nitrogen-fixers. Among the Caesalpinioideae, 23% are nitrogen fixers. For Mimosoideae, the figure is 90%, and for Faboideae, 97% are nitrogen-fixers.


As the diagram shows, Mimosoideae contains the nitrogen-fixers Acacia, Albizia, Calliandra, Enterolobium, Leucaena, Mimosa, Paraserianthes, and Pithecellobium. Caesalpinoideae's nitrogen-fixers are Chamaecrista, Cordeauxia; and Faboideae has Cajanus, Dalbergia, Erythrina, Flemingia, Gliricidia, Pterocarpus, Robinia, Sesbania, and Tephrosia.

To rapidly revegetate a damaged landscape, be sure to include plenty of these species to help quickly build up the soils. In areas of very problematic soil, such as arid, tropical and subtropical regions, make 90% of your initial planting of trees nitrogen fixing, pioneer species (associating with either Frankia or Rhizobium), and 10% of species your long-term canopy overstory species. When the system reaches maturity, the proportions will be reversed with 10% nitrogen-fixing, support species and 90% canopy species. The same formula could be followed for humid temperate regions, but the soils in these area are not so fragile and can stand a lower percentage of nitrogen fixers. A 70/30 or even lower may suffice in these areas, as the seasonal cycles of death and regrowth feed these soils well.

As the diagram below shows, the nitrogen-fixing support trees can be pruned (coppiced, pollarded, shredded or sacrificed) to provide mulch, fodder, fuel or fibre. As this is done, the roots of the tree self-prune, releasing nitrogen into the soil.


The highest concentrations of nitrogen are to be found in descending order in the seeds, the seed pods, the flowers, the leaves and then the woody parts of the tree. Inter-planting with fruit or nut trees naturally provides more soil nitrogen. But interplanting also makes the job of chop-and-drop mulching that much easier.







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3 comments:

Scott A. Meister said...

Associating this article with the actual design part of permaculture begs the question...WHERE should we do we do this?

I highly recommend doing this to cover tops of hills and higher elevations of a property to take advantage of passive gravitational nutrient flow.

Planting dense at the very top, thinning out as you go down the hill. The trees will protect the hill from erosion, while simultaneously combing the wind for nutrients (dust, wings, etc) while also providing habitat for pest management species like birds, rodents and things which will also fertilize the soil below them...further building the soil around them, and compounding the nutrient flow down hill as you go. Perhaps strip-intercropping as you go down...this could also be done in cooperation with a rainwater harvesting and storage swale system which has the added benefit of recharging underground water supplies and springs.

It's also worth noting that nitrogen isn't the only thing needed in a soil. Phosphorus and Potassium are just two of the other many major things needed in a soil...and there are a number of plants that have the ability to accumulate and store these nutrients in their biomass...and can be chopped and dropped too.

I believe all hilltops should be covered with NPK interplantings. They protect from erosion, build soil, provide soil stabalization and help conserve water.

We should also not give the impression that nitrogen fixers are all that is needed to have a healthy soil...but they certainly are a start.

DJEB said...

That is a great point, Scott. The top is the catchment is the best place to start. In fact, Gangi Setty of the Green Tree Foundation has been sending me shots of the catchment around his town of Talupula, India to plan out regreening that catchment.

Scott A. Meister said...

I just realized while re-reading my comment that when I was talking about combing the wind for nutrients...I mentioned "(dust, wings, etc.)"
Here...my reference to wings was not meant to imply whole bird wings or anything of that sort...but was rather a random reference to insect parts that are often floating around in the wind as debris...this debris is of great value for building soil...and often a source for nitrogen.