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�. .r ` NITROGEN GEOCHEMISTRY This Appendix presents a general discussion of the chemical forms of nitrogen with special attention to nitrate and the transformation processes for nitrogen in soils and groundwater. This discussion is presented in order to assist the reader in understanding the chemical and physical mechanisms leading to the formation and transport of nitrate. CHEMICAL FORMS AND TRANSFORMATIONS OF NITROGEN Nitrogen is present throughout the atmosphere, surface soils, soil pore liquid, soil atmosphere, surface water, and groundwater in a variety of chemical forms. The earth's atmosphere is approximately 70% Nz, the most abundant form of nitrogen. Specialized soil microorganisms are capable of chemically reducing N2 to form amino acids and protein. This process, called nitrogen fixation, is the ultimate source of nitrogen in the earth's soil, since few minerals contain nitrogen in any form. The nitrogen present in proteins and amino acids, which occur in all living, dead and excreted cells is organic nitrogen. Organic nitrogen is strongly adsorbed to surface soils and is the predominant form of nitrogen in non-agricultural soils. Once organic nitrogen enters soils or surface waters, a variety of microorganisms convert organic nitrogen to inorganic nitrogen in the form of ammonia (NH3). Ammonia may be lost to the atmosphere or become dissolved in water and form ammonium (NH4*). Ammonium is strongly adsorbed to soils and is rarely detected below a depth of 5 feet below ground surface (bgs). Ammonium, is oxidized and converted into nitrate (NO;) by a two-step process termed nitrification' by a different group of microorganisms occurring primarily in surface soils. Nitrate moves rapidly through soils and is accumulated by plant roots which reduce nitrate into amino acids and proteins completing the "nitrogen cycle". An additional microbial reaction in surface soils may convert nitrate into nitrogen gas (NZ) or nitrous oxide gases (NO, N20). This process, termed `denitrification,' is important from an environmental perspective since it may protect groundwater areas where excess nitrate is present. FATE AND TRANSPORT OF NITROGEN The geochemistry of nitrogen in surface soils is complex. Figure 1 summarizes the important transformations of nitrate in shallow unsaturated soils (the vadose zone). Soil microbiological activity near the surface determines the chemical form and therefore the - 1 -