Species differences in nitrate reductase activity are unaffected by nitrogen enrichment in northeastern US forests

Forests in the northeastern United States are typically nitrogen (N) poor, but anthropogenic N deposition may begin to overcome this limitation in the coming decades. Concomitant with this change, soil nitrate may increase in systems previously dominated by ammonium, and thus the ability to reduce nitrate may become an increasingly important plant trait. Here we present results from a survey of nitrate reductase activity (NRA) in foliage and roots of four canopy dominant tree genera (Betula spp., Fagus grandifolia, Quercus spp., Acer spp.) in two northeastern temperate forests (Harvard Forest, MA, and the Catskill Mountains, NY). We found that NRA varied by ∼9× across genera, and was consistently greatest in birch (Betula) and lowest in maple (Acer) species. Root NRA was measured at one site, and was ∼10× lower than in foliage. In both Harvard Forest and the Catskill mountains, the amount of NRA in a particular species, and the difference in NRA between species, was unaffected by long-term N fertilization. Despite the order of magnitude differences in NRA between species, NRA did not correlate with foliar %N, C:N ratio, N:P ratio or response of these variables to fertilization. We postulate that NRA may be related to ecological strategy, and that high NRA may be more common in early successional species that thrive in relatively N rich conditions. These species-level differences remain, even as forests mature and N availability diminishes. High NRA may be an important trait for some species, particularly early in ontogeny. But these results suggest that species-level differences in NRA may not be a large driver of species response to continuing N deposition in northeastern forests.

► Tree species in northeastern US forests differ in their ability to produce nitrate reductase.
► Species differences in nitrate reductase activity are not affected by long-term fertilization.
► High nitrate reductase activity does not correlate with response to fertilization of foliar N, C:N or N:P.