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Leguminous tree pruning as a means to shed roots and cycle nutrients toward a crop

Posts: 51
Location: Costa Rica
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Hi all,

Some context; I work in Costa Rica at Rancho Mastatal Sustainability Education Center. We are in the humid tropics and have approximately 15 acres of agroforest on site. One of our principle fertility strategies involves the following process:

We plant thousands of leguminous trees (Gliricida, Erythrina, Shizolobium, Flemingia, Inga, Calliandra and so forth) on contour, very densely, throughout our fruit and nut tree orchards. All of these legumes are pollarded, usually around 2 meters of height. Depending on the species and age this occurs one to three times per year. All of the pruned material is used as mulch around the base of the tree crops. In addition to this we hope that this process of pruning the above ground biomass also leads to a forced root pruning of the trees. The roots, full of nitrogen rich nodules, decompose, and are quickly cycled back into living biomass, due to the rapid nutrient cycling of the tropics.

My question is: is it possible that the nutrients from the decomposing roots ever reach our nearby tree crops' roots and enhance their growth?
or worded differently; is the above a viable fertility strategy based on the root to root interactions, or is this interaction unlikely?

Obvious variables to me include timing of pruning, distance between root systems, and general health of the soil food web.

Most of the research in this field stems from the coffee/cacao interactions with their shade trees. See for one example: http://link.springer.com/article/10.1007/BF00046846

This study concludes that the litter production is more important than the N fixation of the trees, which leads me to believe that the root decomposition and cycling is not as pronounced as I hope it to be.

Any insights would be great appreciated!

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Hi Scott, I work only in the temperate one, so my response is limited. I do know when a nitrogen-fixing plant is stressed (season changes, drought, pruning, grazing or death) the nodules "shed" and the nitrogen is available to other plant's root. But most of the nitrogen is elsewhere in the plant than in the roots. It's the nodules to focus on. If the tree has seed most of the nitrogen is in the seed and not the root's nodules. With annual legumes the roots have the most nitrogen in the nodules just as the plant begins to bloom. There is a good drawing of this in my Designing & Maintaining Your Edible Landscape -Naturally book published in 1986. I hope this helps.
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