Regulation and measurement of nitrification in terrestrial systems.

Understanding nitrification rates and their regulation continues as a key area of research for assessing human's increasing impact on the terrestrial N cycle. We review the organisms and processes responsible for nitrification in terrestrial systems. The control of nitrification by substrate availability is discussed with particular attention to the factors affecting ammonia/ammonium availability. The effects on nitrification rates of environmental controls including oxygen, water potential, temperature and pH are described. With this general understanding of the factors affecting nitrification rates as a basis, we present an in depth analysis of methods used to measure nitrification in terrestrial systems. Net, gross and potential nitrification rate measurements are explained including the use of isotopes and inhibitors to measure rates in soils. Methods for the estimation of nitrification kinetics and modeling are briefly described. Future challenges will require understanding the factors controlling nitrification across spatial scales from ecosystems to soil microsites if we are to sustainably manage reactive nitrogen in terrestrial environments.