Monday, August 2, 2010: 1:50 PM
406, David L Lawrence Convention Center
Background/Question/Methods
Coastal wetlands purify water and are thought to sequester carbon based on their high rates of productivity. However, microbial processes that generate potent greenhouse gases, including N2O, CH4 and CO2, may be active in wetlands, and can influence the climatic roles of these ecosystems. In particular, denitrification, which produces N2O, increases in response to nutrient loading, and wetlands receive substantial inputs of nitrogen from human sources (including agricultural fertilizer, sewage and atmospheric sources). In field experiments at a Spartina patens marsh in Plum Island (MA), we tested the hypothesis that nitrate loading stimulates N2O production, using flux chambers, while also measuring changes in fluxes of CH4 and CO2. To test whether changes in fluxes would be sustained, we measured fluxes immediately after a nitrate addition and again 2 days later. We also evaluated the influence of different chamber methods on gas flux measurements in the field, by comparing results obtained from light and dark chambers placed (in random sequence) on the same experimental plots.
Results/Conclusions
N2O fluxes increased significantly in the field, from undetectable levels to an average of 6.6 μmol N2O m-2 h-1, within an hour of a single experimental nitrate addition (equivalent to 1.4 g N m-2). Neither CH4 or CO2 fluxes from the same plots were affected. In both light and dark chambers, N2O fluxes were higher in the presence of nitrate additions than in control plots, although N2O fluxes were greater overall in dark chambers than in light ones, possibly reflecting an influence of oxygen from photosynthesis. Two days later, N2O fluxes did not differ between nitrate-addition and control treatments in light chambers. Although short-lived, the average global warming potential of nitrate-enhanced N2O fluxes (expressed in CO2 equivalents) was comparable in magnitude to average reported C sequestration rates for wetlands. Thus nitrate loading may have substantial effects on climatic roles of wetlands. Further manipulations nitrate and oxygen availability in wetland sediment slurries in the laboratory have shown that, although N2O production is stimulated by nitrate additions (comparable to those applied in the field), the magnitude of N2O fluxes depends the presence of oxygen, which may inhibit denitrification. These studies increase our knowledge regarding greenhouse gas production in key coastal ecosystems and further support the need to reduce nitrogen loading to the environment. They may ultimately improve our ability to not only restore coastal wetlands but also to ameliorate human impacts on climate change.
Coastal wetlands purify water and are thought to sequester carbon based on their high rates of productivity. However, microbial processes that generate potent greenhouse gases, including N2O, CH4 and CO2, may be active in wetlands, and can influence the climatic roles of these ecosystems. In particular, denitrification, which produces N2O, increases in response to nutrient loading, and wetlands receive substantial inputs of nitrogen from human sources (including agricultural fertilizer, sewage and atmospheric sources). In field experiments at a Spartina patens marsh in Plum Island (MA), we tested the hypothesis that nitrate loading stimulates N2O production, using flux chambers, while also measuring changes in fluxes of CH4 and CO2. To test whether changes in fluxes would be sustained, we measured fluxes immediately after a nitrate addition and again 2 days later. We also evaluated the influence of different chamber methods on gas flux measurements in the field, by comparing results obtained from light and dark chambers placed (in random sequence) on the same experimental plots.
Results/Conclusions
N2O fluxes increased significantly in the field, from undetectable levels to an average of 6.6 μmol N2O m-2 h-1, within an hour of a single experimental nitrate addition (equivalent to 1.4 g N m-2). Neither CH4 or CO2 fluxes from the same plots were affected. In both light and dark chambers, N2O fluxes were higher in the presence of nitrate additions than in control plots, although N2O fluxes were greater overall in dark chambers than in light ones, possibly reflecting an influence of oxygen from photosynthesis. Two days later, N2O fluxes did not differ between nitrate-addition and control treatments in light chambers. Although short-lived, the average global warming potential of nitrate-enhanced N2O fluxes (expressed in CO2 equivalents) was comparable in magnitude to average reported C sequestration rates for wetlands. Thus nitrate loading may have substantial effects on climatic roles of wetlands. Further manipulations nitrate and oxygen availability in wetland sediment slurries in the laboratory have shown that, although N2O production is stimulated by nitrate additions (comparable to those applied in the field), the magnitude of N2O fluxes depends the presence of oxygen, which may inhibit denitrification. These studies increase our knowledge regarding greenhouse gas production in key coastal ecosystems and further support the need to reduce nitrogen loading to the environment. They may ultimately improve our ability to not only restore coastal wetlands but also to ameliorate human impacts on climate change.