Where is Nitrogen most abundant on Earth?

Why is it that the most common gas in the earth's atmosphere is nitrogen but not so many living organisms developed to consume it?

• Some bacteria and some plants have the ability to fix nitrogen present in air, but as far as I know it is not widely used by most of the living organisms. How come forms of life that could use a very common element did not develop/evolve through time?

• Answer:

  Nothing actually "consumes" nitrogen in the sense that they consume oxygen, for energy. Oxygen is an energy source, or rather, the "oxidation" half of the key metabolic reactions that turn some kind of fuel (such as sugar molecules) into very stable components like water and carbon dioxide. The more stable a component is, the less energy can be extracted from it. Nitrogen is essentially useless for providing energy. Nitrogen is different. Its primary function is that it is a key part of proteins. It's not an energy source; it's just the atom that provides the right "shape" for amino acids to do their thing. In that sense every organism uses nitrogen, but very few are capable of getting it right from the air, because it's so stable. It has to be transformed into a less stable, more energetic intermediate form, such as ammonia, called "nitrogen fixation". Most complex organisms don't do their own nitrogen fixation. Why? Because they don't have to. It's the same reason animals don't photosynthesize: it's metabolically cheaper to let somebody else do it, and then eat them. There's no benefit to being self-sufficient, not when the world is full of other organisms who will make nitrogen for you. They don't particularly "want" to do that, but it's their fault for being primitive organisms. Since nitrogen fixation is so important, it developed early on by bacteria. The multi-cellular organisms (plants and animals) take advantage of the bacteria in pretty much the same way carnivores eat herbivores and herbivores eat plants: the plants steal from the bacteria. In fact, some of the most primitive organisms are those that photosynthesize and fix nitrogen, the cyanobacteria. They're so busy being self-sufficient that they don't have any energy to do anything else. What people like to think of as "advanced" organisms are precisely those that aren't self-sufficient, but instead aggregate the work of other organisms. That gives them the energy to do what we think of as important. Unfair? Whatever. Life ain't fair. The winners are the ones who step on the losers.