While both climate warming and increased atmospheric nitrogen deposition are expected to have large effects on primary productivity and plant species composition over the next century, the extent to which these influential global change factors may interact is uncertain.  In particular, for ecosystems that experience soil freezing over winter, warming may increase rates of nitrogen mineralization at a time when plant root activity is low, leading to increased nitrogen leaching and trace gas losses, and an increased frequency of soil freeze thaw cycles may further promote ecosystem nitrogen losses over this season.  We examined the interactive effects of warming and nitrogen deposition on primary productivity and plant species composition in a northern temperate old field.  Plots were warmed by overhead infrared heaters either year-round or solely over winter, with the latter included to isolate the winter warming effect from the year-round warming effect.  Increased nitrogen deposition was simulated through the combined addition of aqueous ammonium nitrate and slow release nitrogen fertilizer pellets.  We sampled the plots monthly to quantify plant productivity and species composition, and took weekly spectral readings to measure treatment effects on plot greenness (NDVI).

Results/Conclusions

In 2007, which featured an exceptionally dry summer, there were no significant responses of productivity or plant species composition to either warming or nitrogen addition.  However, in 2008, aboveground productivity approximately doubled in response to the combined effects of warming and nitrogen addition, and the effects of these treatments were additive.  These responses were driven by an increase in grass productivity, whereas forb productivity was insensitive to nitrogen addition and declined in response to summer warming.  Root biomass (all species) also increased in response to warming, although total root biomass was an order of magnitude lower than aboveground biomass.  NDVI increased significantly in response to nitrogen addition in both years, although these increases were not proportional to the increases in aboveground biomass, and NDVI did not increase significantly in response to warming.  Overall, contrary to our prediction, winter warming did not decrease plant productivity, despite an increased frequency of soil freeze-thaw cycles in response to this treatment.  Therefore, changes to soil freezing dynamics in response to warming do not appear to have an important influence on plant productivity in our system.