Nitrogen Mineralization, Immobilization, and Nitrification

This chapter outlines the methodologies and the concepts used to assess soil internal nitrogen(N)-cycle processes. It discusses laboratory and field methods for determining gross and net N-transformation rates. Three 15N techniques have been used to study gross N- transformation processes: 15N natural abundance techniques, where slight differences in isotopic enrichment of soil pools, resulting from biological discrimination, are observed over time; 15N tracer techniques, where a substrate pool is labeled and movement of the isotope through the system is monitored over time; and 15N isotope-dilution techniques, where a product pool is labeled and the rate at which production alters the isotopic enrichment of the pool is monitored. Tracer techniques have been widely used to determine directions of mass flow and connections among ecosystem compartments, but they have also been used to measure gross rates of flows.