Final answer:
Nitrogen is added to the largest reserve (the atmosphere) mostly through nitrogen fixation, a process where atmospheric nitrogen is converted into ammonia by soil bacteria, especially in a symbiotic relationship with legumes.
Step-by-step explanation:
Nitrogen is a fundamental component of the biosphere, essential for plant and animal life. The largest reserve of nitrogen is added to the atmosphere via the process of nitrogen fixation. This process involves the conversion of atmospheric nitrogen gas (N₂) into a more usable form, such as ammonia (NH₃), which can then be utilized by plants. Biological Nitrogen Fixation (BNF) is a significant source of nitrogen fixation, often occurring through the symbiotic relationship between soil bacteria and legumes. These bacteria, which can form nodules on the roots of legumes, are able to convert atmospheric nitrogen into organic molecules that are essential for the growth of plants and the production of proteins and nucleotides.
During this biological process, soil bacteria chemically combine atmospheric nitrogen with hydrogen to create ammonia, which plants can directly incorporate into their tissues. This process plays a critical role in the nitrogen cycle and greatly improves agricultural productivity, as nitrogen is an important nutrient that supports the formation of vital biological compounds within plants.