ABSTRACT
Nitrogen (N) is generally the most common growth-limiting nutrient in agricultural produc-
tion systems. The N taken up by crops is derived from a number of sources, particularly from
fertilizer, biological N fixation and mineralization of N from soil organic matter, crop
residues, and manures (Keeney 1982). The contribution of mineralization to crop N supply
may range from <20 to >200 kg N ha1 (Goh 1983; Cabrera et al. 1994) depending on the quantity of mineralizable organic N in the soil and environmental conditions (soil tempera-
ture and moisture) that control the rate of mineralization. Large amounts of mineralizable
N can accumulate under grassland with the result that crops grown immediately after
cultivation of long-term grass may derive much of their N from mineralization. In contrast,
soils that have been intensively cropped often mineralize little N, leaving crops heavily
dependent on fertilizer N.
