Carbon Sequestration in Wetland Ecosystems: A Case Study

Wetlands occupy a significant part of the land surface of the globe. In addition to acting as sinks for carbon, wetlands will respond to rising atmospheric CO₂ and climate change in various ways. Rising sea level and increasing storm frequency are expected to cause deterioration of coastal wetlands while rising atmospheric CO₂ and rising temperature may increase primary production, carbon sequestration and increased emission of methane in marshes dominated by C₃ vegetation.A brackish wetland on Chesapeake Bay, Maryland has been the subject of a long-term study of the response to elevated CO₂. In this study, carbon sequestration depended mainly on the direct effects of elevated CO₂ on ecosystem photosynthesis and respiration and most of the additional carbon accumulated into fine roots, the decay of which increased soil carbon. The addition of carbon to the sediments altered biogeochemical cycles, evidence of which was increased nitrogen fixation and methane production. Elevated CO₂ significantly increased the rate of photosynthesis while causing a reduction in the concentration of soluble protein, primarily the carboxylating enzyme, Rubisco, which caused a significant redistribution of nitrogen by decreasing the amount of N in green photosynthetic tissue aboveground and increasing it in roots and rhizomes belowground. The increased C:N had important consequences for plant/insect interactions as well as decomposition. One of the most significant findings was that elevated CO₂ reduced mitochondrial respiration, probably through a mechanism which involves a direct effect of CO₂ on the activity of certain key enzymes controlling the rate of respiration.This study suggests that rising atmospheric CO₂ will alter fundamental processes at all levels of organization in wetlands and presents us with the challenge to understand the mechanisms by which rising atmospheric CO₂ regulates carbon balance in plants and ecosystems.