Title: Nitrous Oxide Emissions from Riparian Buffers and Treatment Wetlands Authors
Submitted to: Recycling of Agricultural Municipal and Industrial Residues
Publication Type: Proceedings
Publication Acceptance Date: July 6, 2006
Publication Date: August 15, 2006
Citation: Hunt, P.G., Matheny, T.A., Ro, K.S., Szogi, A.A. 2006. Nitrous oxide emissions from riparian buffers and treatment wetlands. In: Petersen, S.O., editor. Proceedings of 12th Recycling of Agricultural Municipal and Industrial Residues (RAMIRAN), September 9-12, 2006, Aarhus, Denmark. p. 155-157. Technical Abstract: Riparian buffers and treatment wetlands 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. Nitrous oxide emissions are frequently found at low levels in agricultural soils. They can be stimulated by wet soil conditions and/or high soil nitrogen. The objectives of this research were to 1) ascertain the level of potential nitrous oxide production in a riparian buffer that was heavily impacted by nitrogen from swine wastewater, 2) compare this heavily impacted site to other riparian buffer sites within the watershed, and 3) identify controlling factors for nitrous oxide emissions. The research was conducted in a watershed within the Coastal Plain of North Carolina, USA. Soil samples were obtained from seven sites with distinctly different agronomic managements and landscape positions. Soil samples (5-cm diameter x 15.2-cm length) were collected from three depths at each site: (i) at the upper 15 cm of the soil surface, (ii) midway between the soil surface and the water table, and (iii) 15 cm above the water table. Denitrification enzyme activity (DEA) was measured by the acetylene inhibition method. All samples received chloramphenicol to inhibit protein synthesis. Nitrous oxide accumulation was then measured after incubation with and without acetylene. The mean DEA (with acetylene) was 90 ug N/kg soil/h (Std. Dev. ± 146) for all 225 soil samples from the entire watershed. If no acetylene was added to block conversion of nitrous oxide to dinitrogen gas, only 16 ug N/kg soil/h (Sta. Dev. ± 41) was accumulated. The median value of DEA was 48 ug N/kg soil/h which was about half of the mean value. However, the median value for the nitrous oxide accumulated without addition of acetylene was 0 ug N/kg soil/h. This median of zero indicated that the incomplete denitrification was very unevenly distributed in the watershed. Half of the samples accumulated no nitrous oxide. Yet, some were accumulating substantial amounts of 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. This ratio of 25 was a threshold, and significant nitrous oxide accumulations only occurred in soils with lower C/N ratios, particularly below 20. Similar suppression of nitrous oxide emissions from soils has been recently reported in forested histosols of northern Europe. Thus, 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.