A survey on nitrate reductase activity in hardwood species in a northeastern temperate forest

Background/Question/Methods Forests in the northeastern United States are typically poor in N, especially nitrate. However, warming and continued N deposition at ~10 times preindustrial rates will likely increase total N and nitrate availability in the coming decades. The implications of these changes for plant competition could be profound, yet we do not know which canopy dominants in northeastern forests have the ability to use nitrate as a N source. To explore this issue, we investigated nitrate reductase activity (NRA) in canopy emergent and understory hardwoods, including Acer rubrum, Betula sp., Fagus grandifolia and Quercus sp., at three separate locations in the Harvard Forest LTER in Petersham, Massachusetts, USA. We assayed NRA by collecting foliage between 10AM and 2PM on a sunny day and then immediately immersing the leaves in a 0.04M KNO₃ solution. After a 2-hour incubation, we measured nitrite concentrations in the solution as a measure of NRA. To quantify soil differences between sites and evaluate whether differences in NRA led to differences in sources of N and N availability by species, we measured net N mineralization rate, soil moisture and organic matter content, and foliar %N and δ¹⁵N.

Results/Conclusions We hypothesized that N availability would differ among sites, but there were no significant differences in net N mineralization or initial NH₄⁺ and NO₃^-. Net nitrogen mineralization ranged from 2.6 - 3.9 μg N g soil^-1 day^-1 in July and 1.7 - 4.0 μg N g soil^-1 day^-1 in August. There was no detectable nitrification in July, while in August it ranged from 0.012 - 0.052 μg NO3 g soil^-1 day^-1. Across all sites, NRA in the canopy was significantly higher in Betula papyrifera and Betula lenta than any other species (p<0.0001), with rates of 110 ± 50 and 87 ± 23 μM NO2 g^-1 hr^-1 respectively. NRA was not significantly different for Fagus grandifolia, Querus sp. and Acer rubrum (32 ± 14, 16 ± 6.9 and 10 ± 4.4 μM NO2 g^-1 hr^-1 respectively). In the understory, B. lenta had significantly higher NRA (p<0.0001) than the other species. There were no significant correlations between NRA and foliar %N or δ¹⁵N at any of the three sites. This is not surprising, given the paucity of soil nitrate. Nonetheless, these analyses suggest that phylogenetic constraints in plants may play a role in shaping temperate forest responses as soil N accumulates in the coming decades.