Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/1/2007
Publication Date: 9/1/2007
Citation: Hunt, P.G., Matheny, T.A., Ro, K.S. 2007. Nitrous oxide accumulation in soils from riparian buffers of a Coastal Plain watershed - Carbon/nitrogen ratio control. Journal of Environmental Quality. 36(5):1368-1376. Interpretive Summary: Nitrogen is an essential nutrient for life, and it cycles through the earth’s ecosystem in both liquid and gaseous forms. The most prevalent form is di-nitrogen gas, an inert form that composes over 78% of the earth’s atmosphere. Nitrogen is added to terrestrial ecosystems by both natural and human activities, and excessive addition can cause contamination of water bodies. However, water bodies are often protected from excess nitrogen by riparian buffers – wet ecosystem contiguous to stream and lakes. In the riparian buffer, liquid phase nitrogen is converted to gaseous di-nitrogen by a microbial process called denitrification. Thus, the maintenance and installation of riparian buffers has become one of the major water quality protection practices. However, in some instances, denitrification is not complete; and a greenhouse gas (nitrous oxide) is produced. To date, there are relatively few studies that access the level of nitrous oxide production or the conditions that controlled its production in riparian buffers. In this investigation of an eastern Coastal Plain watershed, we found hot spots of nitrous oxide production in riparian buffers. We also found the controlling factor was the carbon to nitrogen ratio of the soil. If the soil carbon was 25 times greater than the nitrogen content, the denitrification formed inert di-nitrogen gas. Thus, we have both a measure of where nitrous oxide may be a problem and a potential method (addition of carbonaceous materials) for mitigation of nitrous oxide production.
Technical Abstract: Riparian buffers are used throughout the world for the protection of water bodies from nonpoint source pollution, particularly nitrogen. Yet, relatively few studies of riparian or treatment wetland denitrification consider the production of nitrous oxide. The overall objectives of this research were to ascertain the level of potential nitrous oxide production in riparian buffers and identify controlling factors for nitrous oxide emissions within the soils of an agricultural watershed in the southeastern Coastal Plain of the USA. Soil samples were obtained from seven sites with distinctly different agronomic managements and landscape positions. Soil samples were collected from the soil surface, midway between the soil surface and the water table, and above the water table. Denitrification enzyme activity (DEA) was measured by the acetylene inhibition method. Nitrous oxide accumulations were measured after incubation with and without acetylene (a representation of baseline nitrous oxide production). The mean DEA (with acetylene) was 80 ug N2O-N/kg soil/h (Std. Dev. ± 136) for all 283 soil samples from the entire watershed. If no acetylene was added to block conversion of nitrous oxide to di-nitrogen gas, only 15 ug N2O-N/kg soil/h (Std. Dev. ± 44) was accumulated. Half of the samples accumulated no nitrous oxide. The highest level of denitrification was found in the soil surface layers and in buffers that were impacted by either livestock waste or nitrogen from legume production. Nitrous oxide accumulations (with acetylene inhibition) were well correlated to the soil nitrogen. Without acetylene inhibition, correlations with soil and site characteristics were poor, but there was a controlling factor. Nitrous oxide accumulations without acetylene were found to be essentially zero, if the soil C/N ratios exceeded 25. Soil C/N ratios may be an easily measured and widely applicable parameter for identification of potential hot spots of nitrous oxide emissions from riparian buffers.