This module expands on the introductory discussion of nitrogen in other modules. The various chemical forms of nitrogen found in land treatment systems are defined. Inputs from waste application as well as natural sources are quantified for typical situations. A discussion of nitrogen transformations in the soil includes mineralization and immobilization processes (organic-inorganic transformations). The importance of carbon to nitrogen ratio of applied waste is emphasized in determining what reactions will occur in the soil. Possible losses and removals of nitrogen from a land treatment system are discussed. All of the above factors are combined into a mass balance in determining wastewater and sludge loadings, on a nitrogen basis, for typical situations. Data on nitrogen loadings and removals from various existing systems are summarized in the final section of the module. An illustration of short-term nitrogen loading calculation, using the decay series concept, is presented in the Appendix. (Author/BB)