Fossil fuel = decomposition of living organisms --> liquid or gas, they"re actually fossils made through millions of years. Complete recycle of water back to the eco system. Proteins and dna, what makes who we are, requires nitrogen which is an important element. 78% of the atmosphere is made out of. Nitrogen cannot be used unless converted into useable forms , after it goes through ammonification then it can be used in a biological way. I t has to go through a complicated five steps, details will be in the text book go check it. In the past, b/c nitrogen in the atmosphere isn"t in a biologically useable form we relied on bacteria to convert the nitrogen into the biological useable form which is called nitrogen fixation. Lots of nitrogen in the air, but not in useable form unless it"s converted.