The influence of atmospheric CO2 enrichment on plant-soil nitrogen interactions in a wetland plant community on the Chesapeake Bay

We investigated plant and soil nitrogen pools and soil processes in monospecific stands of the C3 sedge Scirpus olneyi and the C4 grass Spartina patens grown in the field in open top chambers in a brackish marsh on the Chesapeake Bay. Stands of S. olneyi responded to eight years of elevated CO2, by increased rates of net ecosystem gas exchange and a large stimulation of net ecosystem production. We conducted our study in the summer of 1994 and 1995 when soil cores were collected and aboveground biomass was estimated. Nitrogen concentration in elevated CO2 treatments was reduced 15% in stems of S. olneyi and 8% in the upper 10 cm of the soil profile. While total plant nitrogen per unit of land area remained the same between treatments, total soil nitrogen showed a non-significant tendency to decrease in the upper 10 cm of the soil profile in elevated CO2 both years of study. A significant decrease in soil bulk density largely contributed to the observed decrease in soil nitrogen. Exchangeable nitrogen and potential denitrification rates were also reduced in elevated CO2, but net nitrogen mineralization was unchanged by elevated CO2 treatment in S. olneyi both years. Plants and soils in a pure stand of the C4 grass, S. patens, showed none of these effects of elevated CO2 treatment. Our data provides evidence of changes in nitrogen dynamics of an ecosystem exposed to elevated CO2 for eight years; however due to the variability in these data, we cannot say if or how these changes are likely to impact the effect of rising CO2 on primary production or carbon accumulation in this ecosystem in the future.