Grassland community responses across a gradient of atmospheric CO2 and N fertilization

Background/Question/Methods

Fossil fuel emissions and deforestation are increasing atmospheric CO₂ concentrations, and fertilizer use and pollution are increasing nitrogen inputs to the biosphere. There is a pressing need to estimate to what extent ecosystems are responding to these inputs, yet few studies have quantified responses at more than two levels of CO₂ and nitrogen simultaneously, making the magnitude and shape of ecosystem response uncertain. We assess how these global changes affect community-level responses of experimental grassland mesocosms across six levels of CO₂ and nitrogen addition. Specifically, we measure how two species (Holcus lanatus and Anthoxanthum odoratum) respond to sub- and super-ambient atmospheric CO₂ and how soil nitrogen availability—applied both acutely and chronically to simulate atmospheric deposition and agricultural fertilization—modifies the community response. In a controlled environment facility, communities of 10 individuals were placed under six levels of headspace CO₂, crossed with 11 different nitrogen treatments, each replicated five times. Every two weeks across 6 months carts were exchanged between growth chambers to ensure there were no chamber effects. Plant communities were cut every 28 days to 6 cm above the soil surface. The foliar biomass was sorted by species and the biomass obtained was used to estimate community and per species net primary productivity.

Results/Conclusions

For the total community biomass produced across 6 months, there were no three-way interactions between CO₂ concentration, nitrogen addition rate and nitrogen addition form (chronic or acute). However, there were two-way interactions between the factors. First, the relative and positive response of community biomass to increasing CO₂ concentration was dependent on the nitrogen addition rate, with a greater relative increase when nitrogen was applied at higher (between 70 and 240 kg N ha^-1 y^-1) rather than lower rates (10 to 35 kg N ha^-1 y^-1). Second, at the lower nitrogen addition rates, there was no difference in total community biomass response whether nitrogen was applied chronically or acutely, but acute additions yielded greater biomass at higher addition rates. In contrast to the community biomass response, the proportional biomass of species was dependent only on nitrogen addition rate. Holcus lanatus was dominant at lower levels of nitrogen (55 percent of the community) and A. odoratum was dominant at higher levels (60 percent). Our data suggest that total biomass responses – in contrast to community composition responses – will likely be harder to predict given that they were a product of interactions between the global change treatments.