5.6 Nitrogen Fixers and Native Growing Practices

Nitrogen Fixing

Plants require nitrogen for the formation of  nucleic acids and proteins. Nitrogen, therefore, is one of the most important macronutrients for plantlife. Atmospheric nitrogen,the diatomic molecule N₂ ( dinitrogen), is the largest pool of nitrogen in terrestrial ecosystems. However, plants cannot take advantage of this nitrogen because they do not have the necessary enzymes for conversion into biologically useful forms. nitrogen can however be “fixed,” which means that it can be converted to ammonia (NH₃) through biological, physical, or chemical processes. As you have learned, biological nitrogen fixation (BNF) is the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃), exclusively carried out by prokaryotes such as soil bacteria or cyanobacteria. Biological processes contribute 65 percent of the nitrogen used in agriculture. The following equation represents the process:

N₂+16 ATP + 8 e− + 8 H⁺  2NH₃ + 16 ADP + 16 P_i + H₂

The most important source of BNF is the symbiotic interaction between soil bacteria and legume plants, including many crops important to humans (Figure 1). The NH₃ resulting from fixation can be transported into plant tissue and incorporated into amino acids, which are then made into plant proteins. Some legume seeds, such as soybeans and peanuts, contain high levels of protein, and serve among the most important agricultural sources of protein in the world.

Figure 1. Some common edible legumes—like (a) peanuts, (b) beans, and (c) chickpeas—are able to interact symbiotically with soil bacteria that fix nitrogen. (credit a: modification of work by Jules Clancy; credit b: modification of work by USDA)

Soil bacteria, collectively called rhizobia, symbiotically interact with legume roots to form specialized structures called nodules, in which nitrogen fixation takes place. This process entails the reduction of atmospheric nitrogen to ammonia, by means of the enzyme nitrogenase. Therefore, using rhizobia is a natural and environmentally friendly way to fertilize plants, as opposed to chemical fertilization that uses a nonrenewable resource, such as natural gas. Through symbiotic nitrogen fixation, the plant benefits from using an endless source of nitrogen from the atmosphere. The process simultaneously contributes to soil fertility because the plant root system leaves behind some of the biologically available nitrogen (Figure 2).

Figure 2. Soybean roots contain (a) nitrogen-fixing nodules. Cells within the nodules are infected with Bradyrhyzobium japonicum, a rhizobia or “root-loving” bacterium. The bacteria are encased in (b) vesicles inside the cell, as can be seen in this transmission electron micrograph. (credit a: modification of work by USDA; credit b: modification of work by Louisa Howard, Dartmouth Electron Microscope Facility; scale-bar data from Matt Russell)

The Three Sisters, Native Growing Practices

Native populations in the Americas for hundreds of years have cultivated the Three Sisters plants: corn, beans, and squash. These plants in conjunction with one another save as an ecologically beneficial and traditional planting method that sustains agricultural systems in a multitude of ways. The plants compliment each other by providing mutually beneficial outcomes. Corn provides structure for beans to latch and grow off of, beans supply soil with nitrogen, and squash due to its heavy ground cover reduces weeds while maintaining healthy water levels in the soil. This practice can also be referred to as companion planting. Such a methodology supports biodiversity, increases fertility of soil, and reduces requirement of external nutrients that larger farms might use (i.e. chemical rich fertilizers or pesticides).

Decolonized approaches to farming center around honoring land by working with its natural ecosystem, rather than forcing a new one. Three Sisters have maintained food security for innumerable communities overtime

Figure 3. Three sisters garden