ABSTRACT
Most of the nitrogen (N) within the Earth system is contained in the atmosphere. More than 99.9% of atmospheric nitrogen is present as nitrogen gas, which comprises 78% of the atmosphere by volume. Nitrogen is mainly present in the oceans as unreactive and non-bioavailable dissolved N2, comprising 95% of the total nitrogen. Anthropogenic nitrogen fixation also implies a net atmospheric output, which is borne out by the negative atmospheric budget that ensues when either the minimum, mid-range or maximum values of inputs and outputs are used. Nitrogen is an essential element, being present in the fundamental building blocks of living tissues. It mainly exists in biomass as amino nitrogen in protein structures and amino acids and in nucleotides in DNA and RNA. It is excreted in urea, (NH2)2CO. Ammonia is directly toxic to aquatic organisms. Nitrogen oxides are the main precursors of photochemical smog.
