Wednesday, November 15, 2006

Nitrous Oxide Production in Riparian Soils.

Patrick Hunt, Terry Matheny, and Kyoung Ro. USDA-ARS, 2611 W Lucas St., 2611 W Lucas St., Florence, SC 29501-1241, United States of America

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 of 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 μg N kg-1 soil -1 h -1 (Std. Dev. ± 136) for all 283 soil samples from the entire watershed.  If no acetylene was added to block conversion of nitrous oxide to dinitrogen gas, only 15 μg N kg-1 soil -1 h -1 (Sta. 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 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.