Introduction. Mass Spectrometry. Optical Emmission Spectrometry. Natural Abundance of 15N: Fractional Contribution of Two Sources to a Common Sink an Use of Isotope Discrimination. Nitrogen Fixation in Soil and Plant Systems. Nitrogen Fixation in Flooded Rice Soils, Aquatic and Sediment Systems. Nitrification and Dentrification. Mineralization and Assimilation in Soil and Plant Systems. Mineralization and Assimilation in Aquatic, Sediment, And Wet Land Systems. 13N Techniques. Chapter References. Index.