NITROGEN-RETENTION IN FOREST SOILS

Factors affecting N retention in both fertilized and natural forest ecosystems are reviewed. Collectively, the literature suggests that current conceptual models describing N competition among heterotrophs, plants, and nitrifiers in the soil may need some modification. Nitrifiers seem to be better competitors for N than previously recognized, especially in situations where N inputs are continuously elevated (such as in polluted sites or with repeated fertilization). The assumption that heterotrophs are the most effective competitors for N may be valid for the short term with relatively low N inputs, but may not hold true over the long term or with high N inputs. The apparent "mining" of N from soils by trees in N-poor sites suggests that they, rather than heterotrophs, are the most effective long-term competitors for N in such cases. Nonbiological reactions of N with soil organic matter are known to be a major cause of N immobilization following fertilization and may represent significant competition for N during decomposition in unfertilized sites as well.