The Components of Nitrogen Availability Assessments in Forest Soils

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The availability of nitrogen (N) limits production in many forest ecosystems, and many methods are available for estimating N availability (Keeney, 1980; Binkley 1986; Mahendrappa et al., 1986; Binkley and Vitousek, 1989). The concept of soil availability may represent the rate at which N is converted from unavailable to available forms within the rooting zone (Scarsbrook, 1965). Alternatively, it may refer to the extent to which plant production is constrained by a limited supply of available N. These two aspects of N availability were termed N supply rate and N limitation by Chapin et al. (1986). In agroecosystems, N supply rate and N limitation are often closely linked. In forest ecosystems, differences in species composition, stand age, and soil moisture may uncouple low N supply from N limitation (Chapin et al., 1986). In addition, the nonuniform rooting distribution of trees and the presence of forest floors add spatial complexities to forest N cycles that make it more difficult to estimate N availability in forests than in agroecosystems.