Nutrient limitation, hydrology and watershed nitrogen loss

Forty years ago scientists framed the small-watershed concept to examine how ecological and hydrological processes interact to regulate biogeochemical cycling in terrestrial ecosystems (Likens and Bormann, 1995). This approach inspired new ways of thinking, particularly at the watershed scale, about what controls nutrient losses from terrestrial ecosystems. How can we continue to link ecology and hydrology to provide new insights into long-standing, fundamental questions in biogeochemistry? At least one approach would be to frame questions that are consistent with an understanding of nutrient limitation—its causes and consequences—in terrestrial ecosystems. Nutrient limitation as a scientific concept originated in 19th century agricultural chemistry, and it continues to serve as an organizing principle in modern biogeochemistry. A limiting nutrient is defined as that element in shortest supply relative to demands for plant growth. The addition of a limiting nutrient will stimulate plant growth (i.e. net primary productivity) more than will additions of any other elements, and co-limitation by two or more nutrients is also possible. Nitrogen (N) is the most common limiting nutrient in the temperate zone, as indicated by its widespread use in agricultural fertilizers, and by experimental additions of nutrients to a range of natural terrestrial ecosystems (Vitousek and Howarth, 1991). Limitation by N is common in so many regions because it is not supplied by rock weathering (with few exceptions), and must accumulate from atmospheric deposition and biological fixation as ecosystems develop. With sustained inputs of N from atmospheric deposition (especially air pollution) and biological fixation, it is possible to overcome limitation by N, though experimental tests of limitation by nutrients other than N are rare for temperate terrestrial ecosystems (see Tanner et al. (1998) for a brief review of tropical forests). Nitrogen limitation has important consequences for the cycling and loss of N from watershed ecosystems. Plants and their mycorrhizal associates rapidly assimilate plant-available forms of N (i.e. NH4, NO3, simple amino acids) in N-limited ecosystems, and retain this N by conversion to more complex organic forms. Microbial communities in soils release available N during organic matter decomposition; however, they can assimilate it almost as rapidly in order to satisfy metabolic requirements for the decomposition of low-N litter. In this way, nutrient limitation drives tight retention of