Nitrogen Fixing Root Nodules
Microscopic view of a cross-section of a legume root nodule from the tropical tree Inga oerstediana
What you are looking at in this photo is a microscopic image of the inside of a root nodule taken from a leguminous tree. Have you ever seen the small balls, or nodules, located on the roots of certain plants such as clover, beans, or peas?
Nodules on roots of Inga oerstediana
Root nodules are small structures that form on plants of a certain taxonomic group called legumes that participate in a mutually beneficial relationship with soil bacteria known as rhizobia. A photo of nodules attached to a plant root can be seen below.
Rhizobia live inside the nodules and ‘fix’ atmospheric nitrogen gas into a form the plant can use as a fertilizer for its own growth and development. This process is called nitrogen fixation. In return, the rhizobia bacteria use some of the plant’s carbohydrates produced through photosynthesis. Since both legume and bacterial partners benefit from this relationship, it is called a mutualistic symbiosis.
Many familiar legumes are herbaceous plants such as beans and peas, but there are also many legumes that are trees. The nodules in these photos were taken from a legume tree of the species Inga oerstediana found on an organic coffee farm in Chiapas, Mexico where it shades the smaller coffee bushes. A picture of an organic coffee farm shaded by leguminous trees can be seen below.
Nitrogen fixing bacteria require oxygen-free conditions to fix nitrogen, so they only inhabit the cells in the central core of the nodule (the darker mass of cells in the center). Though not visible in this photo, the interiors of legume nodules are normally pink due to the presence of leghemoglobin (similar to the oxygen-carrying hemoglobin that causes your blood to be red). Leghemoglobin has a high affinity for oxygen, and it locks up oxygen, thus fostering the oxygen-free conditions needed for nitrogen fixation.
Coffee growing underneath the leguminous tree Inga oerstediana in Chiapas, Mexico.
Organic coffee production in the state of Chiapas, México is a critical means of income generation for many poor Mayan peasants. The utilization of the ‘free’ nitrogen provided by nitrogen fixation can help eliminate dependence on environmentally damaging chemical fertilizers that are too expensive or not accessible to small-scale coffee producers. Many farmers in Mexico and elsewhere are currently learning how to harness this valuable form of bio-fertilizer to help them produce more and better coffee beans.