Phylogeny constrains nitrate reductase activity in northeastern forests even under nitrogen enrichment

Background/Question/Methods

Forests in the northeastern United States are typically nitrogen (N) poor, but anthropogenic N deposition may increase N availability in the coming decades. Concomitant with this change, soil nitrate may increase in abundance in systems once dominated by ammonium, and thus the ability to reduce nitrate may become an increasingly important plant trait. Here we assessed the activity of nitrate reductase (NRA) in leaves and roots of several canopy-dominant northeastern hardwoods, and asked how NRA is affected by N fertilization. We further asked whether the observed genus-level variation in NRA resulted in significant differences in nutrient acquisition, as measured by foliar N, C:N and N:P ratios and δ¹⁵N. In the summer of 2009, we conducted a survey of hardwood foliar NRA at three hardwood-dominated sites at Harvard Forest, MA. The following summer, we measured NRA at the Harvard Forest Chronic Nitrogen Addition Experiment on the day of fertilization, and 4 and 8 days after fertilization. We also compared NRA level in control and N-fertilized monospecific stands in the Catskill Mountains, NY. At the sites we focused on maples (Acer spp.), birches (Betula spp.), beeches (Fagus grandifolia) and oaks (Quercus spp.).

Results/Conclusions

At all sites, foliar NRA varied by ~10x across genera, and was highest in Betula and lowest in Acer species (0.4±0.1 and 0.02±0.01mmol NO₂^- g^-1 hr^-1 for B. lenta and A. rubrum at Harvard Forest control plot; 0.3±0.1 and 0.07±0.01mmol NO₂^- g^-1 hr^-1 respectively for B. alleghaniensis and A. saccharum in the Catskill Mountains control plots). Root assays showed that NRA was ~10x lower in roots than in foliage of the same canopy trees, and did not differ between species or genera. The difference in NRA between species was not affected by long-term fertilization, nor was the level of foliar NRA within a species. Contrary to our original hypothesis, NRA did not correlate with foliar %N, C:N, N:P or δ¹⁵N, nor with the magnitude of response to fertilization. Based on these data, we conclude that while there is a strong phylogenetic constraint on foliar NRA, NRA is unlikely to substantially affect the relative competitive ability and community composition of northeastern forests under continuing anthropogenic N deposition.  However, if nitrate becomes a more dominant form of N in these forests, these phylogenetic constraints on NRA may become increasingly important.