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United States Department of Agriculture

Agricultural Research Service

Title: Soil Emissions of Nitric Oxide in Two Forest Watersheds Subjected to Elevated N Inputs

Authors
item Venterea, Rodney
item Groffman, P - INST. ECOSYSTEM STUDIES
item Castro, M - UNIV. OF MARYLAND
item Verchot, L - INTL CTR RES AGRO-FOREST
item Fernandez, I - UNIV. OF MAINE
item Adams, M - USDA FOREST SERVICE

Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 21, 2004
Publication Date: July 26, 2004
Citation: Venterea, R.T., Groffman, P.M., Castro, M.S., Verchot, L.V., Fernandez, I.J., Adams, M.B. 2004. Soil emissions of nitric oxide in two forest watersheds subjected to elevated N inputs. Forest Ecology and Management. 196:129-142.

Interpretive Summary: Long-term experiments at the Fernow Experimental Forest in West Virginia and the Bear Brook Watershed in Maine have been investigating the responses of forest watersheds to experimental inputs of excess nitrogen (N). The experimental additions are designed to simulate inputs of N which many forests located downwind of intensive industrial or agricultural areas receive via deposition from the atmosphere. During 2000 - 2001, we made first-time measurements of soil nitric oxide (NO) gas emissions in the watershed studies. At both sites, NO emissions in the N-amended watersheds were higher than emissions in the respective reference watersheds, and were positively correlated with mineral soil nitrate concentrations across all plots. Laboratory measurements indicated that nitrification was the dominant source of NO production. In soils from FEF, the role of increased acidity in promoting NO production was indicated. These results from two different ecosystems provide further evidence that elevated soil NO emissions are a characteristic response in forests subjected to prolonged and elevated N inputs. A consequence of higher NO emissions in N saturated forest ecosystems may be the promotion of ozone-related phytotoxicity. This effect may, in turn, mitigate the capacity of vegetation and soils to store carbon in response to elevated N inputs and/or increasing atmospheric carbon dioxide. These findings therefore have implications regarding the management of forests with respect to several issues, including water quality, air quality, and climate change.

Technical Abstract: The production of nitric oxide (NO) in forest soils can be an indication that the ecosystem is progressing toward a state of nitrogen (N) saturation. Emissions of NO may also have impacts on local formation of tropospheric ozone and downwind deposition of N and acidity. During 2000 - 2001, we measured soil NO emissions in two paired watershed studies. In each study, one of the forested watersheds had been amended with 2.5 - 3.5 g m-2 y-1 above background deposition rates since 1989. In high and low elevation plots at the Fernow Experimental Forest in West Virginia (FEF), and in hardwood- and softwood-dominated plots at the Bear Brook Watershed in Maine (BBWM), NO emissions in N-amended watersheds were higher than emissions in the respective reference watersheds. Mean emission rates ranged from ~ 0.61 ug NO-N m-2 h-1 in the softwood plots at BBWM to ~ 6.8 ug NO-N m-2 h-1 in the low elevation plots at FEF, compared to < 0.21 - 1.2 ug NO-N m-2 h-1 in the respective reference watersheds. Mean NO fluxes were positively correlated with mineral soil nitrate concentrations (r2 = 0.65) across all plots. Laboratory measurements indicated that nitrification was the dominant source of NO production. Net nitrification rates tended to be higher in soils from the N-amended watersheds, although not consistently. In soils from FEF, the role of increased acidity in promoting NO production was indicated. These results from two different ecosystems provide further evidence that elevated soil NO emissions are a characteristic response in forests subjected to prolonged and elevated N inputs. A consequence of higher NO emissions in N saturated forest ecosystems may be the promotion of ozone-related phytotoxicity. This effect may, in turn, mitigate the capacity of vegetation and soils to store carbon in response to elevated N inputs and/or increasing atmospheric carbon dioxide.

Last Modified: 12/18/2014