Elevated concentrations of CO2 and nitrogen alter DOC release and soil phenolic content in wetland microcosms

Physiological responses of plants to elevated carbon dioxide (CO2) and nitrogen (N) availability are ecologically important because of increased atmospheric CO2 concentrations and N enrichment in many ecosystems. Here, the effects of N availability on the responses of six wetland plant species to elevated CO2 levels are examined in terms of growth and root exudation. Six species of emergent plant species typically found in marshes were incubated under two levels of CO2 (370 and 740 ppm) and two levels of N (0 and 8.8 mg N L-1). Elevated CO2 did not affect shoot biomass, root biomass, and height significantly, regardless of N levels. The C/N ratio of plant species increased in response to elevated CO2 levels, but this effect varied by species. All species released higher amounts of dissolved organic carbon under elevated CO2 compared with ambient air conditions. This response was limited under low soil N concentrations. By contrast, phenolic content increased significantly with elevated CO2 under low-N treatment. The findings suggest that elevated CO2 is not responsible for biomass accumulation of emergent wetland plant species, but does elicit changes in the quantity and quality of root exudates, which are, in turn, dependent on N availability to plant species.