Denitrification in forest soils of Oregon

Abstract

Denitrification represents a potential loss of N from forest soils as well as a source of N oxides to the atmosphere; however, this process has not been closely examined in forest ecosystems of the Pacific Northwest. The objectives of this study were to survey insitu denitrification rates in a range of forest ecosystems and to assess the importance of selected soil properties as controlling factors of denitrification in forest soils. Soils from eight mature conifer stands, three recently clear-cut sites, and four Alnusrubra Bong, stands were sampled in spring, summer, and autumn. Denitrification potentials (anaerobic soil slurries), insitu denitrification rates, soil respiration rates, soil water contents, and soil NO3− concentrations were measured. Denitrification potentials ranged from <1 to 1900 ng N•g−1•h−1, and insitu denitrification rates varied from 0.1 to 40 g N•ha−1•day−1. Denitrification potentials were highly correlated with soil NO3− concentrations and soil water contents; these two soil variables explained more than 90% of the variation in denitrification potentials. Field denitrification rates were best correlated with soil water contents: using multiple regression, up to 79% of the variation in field denitrification rates was explained by soil water contents. Experiments on the short-term dynamics of denitrification following water addition confirmed the importance of soil water content as a regulator of denitrification and suggested that active denitrification requires formation of anaerobic microsites. Extrapolation of seasonal denitrification measurements suggests that relatively little N (<10 kg N•ha−1•year−1) is lost from Oregon forest soils as N gases.