By Robin Rivet.
Nitrogen is an essential component of the nucleic acids found in the DNA of all living organisms. Despite the fact that 78% of the earth’s atmosphere is composed of nitrogen, our urban forest typically suffers from nitrogen deficiency. Most vegetation is unable to break down this vital nutrient from its gaseous state, largely due to the stable properties of the N2 molecule. Instead, the majority of plants must acquire essential nitrogen either through the natural processes of biomass breakdown or from supplemental fertilizers like ammonium nitrate or NPK (Nitrogen, Phosphorous, and Potassium) blends compounded in a lab.
Rhizobia, Frankia, and Faux Fixes
Vegetable legumes (Fabaceae) like peas and beans nurture a symbiotic partnership with nitrogen-fixing bacteria called rhizobia (which include species of various genera). Together, they create root nodules that can convert plentiful nitrogen gas into useable plant nutrition. If specific bacteria are absent, supplemental inoculants must be added to stimulate this process.
Similarly, although most trees derive 90% of their energy from the sun through photosynthesis, a few species supplement their food needs through nitrogen-fixing processes. However, leguminous trees have not been consistently studied or extensively utilized in agriculture. Like their legume cousins, such trees can develop a partnership with rhizobia, as well as with Frankia (a genus of bacteria that partner with actinorhizal plants). In both cases, these soil bacteria devour sugars exuded via tree roots, and in turn deposit nodules of available plant nitrogen for the host plant. Surprisingly, documentation suggests that only about 20% of species in the Fabaceae family perform this tactic seamlessly.
Trees of Life: Boosting Ecosystem Fertility
Seeking perennial species to improve natural fertility is what the concept of permaculture ecosystems are built upon. And, as climate change is pressing modern agriculture to improve productivity in any way possible, academic research is leveraging more intensive vetting of useful tree species. Despite current advice coming from both agroforestry and the native plant movements, assertions about which trees are truly best suited for this purpose still seem largely anecdotal. The local tree list below is not comprehensive, nor totally vetted, but it was derived from fairly reliable data pulled from around the world where the various species originate.
Even though planting more nitrogen-fixing trees is one solution to fading urban fertility, the ecology and biology involved are complicated. Regardless of taxonomy, stressed plants do not give up their sugars willingly, and temperature extremes, drought, salinity, pH, and pesticides also affect the breakdown of stable nitrogen. Further, a few nitrogen fixers are known to succeed so vigorously that they may suppress, rather than bolster, surrounding plants. Some species, like redbud, Moringa, and carob trees, frequently claim nitrogen-fixing properties, but most documentation suggests otherwise. However, all three remain excellent landscape choices.
Lightning as a Last Resort
My take is that few legumes or Frankia-linked actinorhizal trees are harmful, and most would benefit our tree-challenged suburban landscapes. Sometimes it might seem that the only other sane alternative, short of increasing chemical fertilization, is waiting for lightning to enhance available nitrogen. Amazingly, lightning bolts do spill nitrate fertilizer from the sky, but last I checked, we seldom get enough of that connection to keep our plants irrigated, never mind fed.