How does elevated CO2 affect nitrogen cycling in a brackish marsh?
Marshes can act as "sponges" for nitrogen pollution from land runoff, which mostly comes from agriculture and municipal sources in the Chesapeake Bay region. Nitrogen pollution can be stored by plants or in soil. However, in waterlogged environments nitrogen can also be converted into gaseous forms by microbes and lost from the ecosystem. This process is called denitrification and can produce nitrous oxide (N2O), a very potent greenhouse gas. I sought to discover how the nutrient filtering capacity of marshes may be altered as atmospheric CO2 rises. I determined how elevated CO2 and nitrogen pollution affect nitrogen retention by plants and soil, emissions as N2O, and lateral movement (an indicator of tidal nitrogen export) in a brackish marsh along the Chesapeake Bay. I used open-top chambers, as seen in the picture to the right, to elevate CO2 while allowing natural rainfall, sunlight, etc. to reach the plants.
My results suggest that nitrogen polluted marshes may retain a smaller fraction of ecosystem nitrogen inputs because of greater N2O emissions and nitrogen export, thus contributing to climate change and eutrophication of coastal waters. However, as CO2 concentrations rise, high levels of nitrogen inputs to marshes may actually enhance ecosystem nitrogen accumulation despite greater nitrogen losses and thereby sustain plant responses to elevated CO2.