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.