Nitrogen mineralization in temperate agricultural soils: processes and measurement

Soils form a major repository of nitrogen (N) within both natural and agricultural terrestrial ecosystems, containing, on a global basis, an estimated 2.4 x 1011 tons of N. The soil receives N inputs through fertilizer additions and from the atmosphere in precipitation and dry deposition or via biological fixation; inputs are also made in plant and animal residues. N is removed in the harvested crop and is lost by leaching and surface run-off of soluble forms, by gaseous transfer as N gas and N oxides (during nitrification and denitrification processes), and by ammonia volatilization. In some circumstances, erosion may also be important. In addition to these interactions with the total ecosystem, internal cycles also operate within the soil, so that even if gains and losses are in balance, then N still continues to cycle in the soil. This chapter describes the current understanding of the conceptual basis of the processes involved in mineralization, relationships among the processes and other factors, and how their effects can be determined practically. The aim is to present this in a way that is relevant to current and future agricultural development and to environmental issues.