Nitrogen Fixation
78% of the earth’s atmosphere is nitrogen. Abundant and useful, it stabilizes other elements in the atmosphere and is an essential nutrient for plants and animals to produce amino acids, which are a component of proteins and DNA. Proteins enable plants to grow strong stems and green leaves. Protein also gives the plants more nutritional value for animals. Nitrogen is the nutrient with the greatest effect on crop yield.
Nitrogen is pretty important. That’s why we need fertilizers, right?
Fertilizer supplies nitrogen in forms that plants can use immediately or with a simple chemical conversion. This rapid delivery of nutrients is useful as compared with manure and compost, which require longer-term decomposition: most of the nitrogen will not be available to the plants for years.
The use of nitrogen-rich inorganic fertilizers is common, and is becoming even heavier as the demand for food grows. The International Fertilizer Industry Association predicts that we will use 100 million tons in the year of 2010-2011. But too much nitrogen can diminish the quality of crops. Excess nitrogen from fertilizer – usually more than half of the nitrogen – pollutes lakes and rivers (eutrophication) with rainwater runoff. Once they have found their way to the oceans, these high concentrations of nutrients create “dead zones,” killing algae and suffocating fish. Nitrate in groundwater holds the risk of methemoglobinemia (“blue baby syndrome”), a condition in which the blood cannot carry oxygen properly. Nitrogen fertilizer is also a huge contributor to greenhouse gas emissions, and uses natural gas intensively.
Yikes! How else can plants get nitrogen instead?
Gaseous nitrogen – the N2 found in the air – is unavailable for use by organisms because of its stability as a molecule. In order for this nutrient to benefit crops, it must be “fixed:” combined with hydrogen or oxygen to create compounds such as ammonium (NH4), nitrate (NO3) or nitrogen dioxide (NO2).
Not surprisingly, there are natural processes which can give plants what they need and are superior to synthetic fertilizers, avoiding all of the abovementioned negative repercussions. Some types of bacteria fix nitrogen and reduce the need for artificial fertilizer. The bacteria live in root nodules of certain plants, achieving a symbiotic relationship with their host: the plants provide a site for growth and carbohydrates for nourishment; the bacteria provide nutrients like nitrogen.
Which plants fix nitrogen?
Normally, this nitrogen fixation is performed by members of the legume family (Fabaceae), like beans, peas clover, alfalfa and lupin. The decaying of plants also contributes to soil fertilization.
Another type of plant that has the capacity to fix nitrogen is angiosperms (like alder, ceanothus and Russian olive) nodulated with actinomycete fungi (the Frankia genus). As the trees discard leaves, this decomposing matter enriches the soil with nitrogen.