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, yet the extent to which they may interact is uncertain. In particular, for ecosystems that experience soil freezing, warming may increase nitrogen losses over winter by stimulating nitrogen mineralization at a time when plant root activity is low, and by increasing the frequency and intensity of soil freeze thaw cycles. We examined the interactive effects of warming and nitrogen deposition on plant biomass in a northern temperate old field over three years. 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 used shoot height-mass allometry to non-destructively sample plant biomass from permanent subplots, and we estimated percent cover of all species within the main plots.
Results/Conclusions
Over the first summer, which was exceptionally dry, there were no significant responses of plant biomass or cover to either warming or nitrogen addition. However, over the second, wetter summer, plant biomass 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 biomass, whereas forb cover was insensitive to nitrogen addition and it displayed species specific responses to warming. Root biomass (all species) also increased in response to warming, although total root biomass was an order of magnitude lower than aboveground biomass. Over the third summer, warming and nitrogen addition both increased plant biomass, although these effects were dampened relative to the second summer, and litter accumulation in the high biomass plots from the previous summer appeared to partially suppress plant growth. Overall, our results do not support the hypothesis that warming-induced changes to soil freezing dynamics limit plant biomass accumulation in our system. On the contrary, they demonstrate that winter warming alone can increase biomass to the same extent as year-round warming, and that this effect may interact strongly with inter-annual variation in precipitation and litter production.