2006 Annual Report 4d.Progress report. This report serves to document research conducted under a Specific Cooperative Agreement between the ARS-Hydrology and Remote Sensing Laboratory and the Smithsonian Environmental Research Center. Additional details of research can be found in the report for the parent project 1265-12130-002-00D entitled, "ASSESSING CLIMATE, SOIL AND LANDSCAPE PROCESSES AFFECTING AGRICULTURAL ECOSYSTEMS." As denitrification proceeds, the unconsumed nitrate becomes enriched in 15N and 18O because lighter isotopes of nitrate are consumed preferentially. Monitoring isotopic composition of nitrate in stream and ditch water can provide information on the extent of nitrate processing (denitrification) at landscape scales. Research at the Smithsonian Environmental Research Center is testing this concept. The research in progress will validate isotopic approach by use of dissolved gas analysis of groundwater to directly measure the extent of denitrification. Shallow groundwater was sampled at borders between croplands and riparian buffers and with the interior of the buffers in one study area which contains both a relatively young CREP buffer, and an older CRP buffer with mature loblolly pine. Groundwater nitrate concentrations were lower inside the buffers than at the edges of the croplands, suggesting that denitrification, dilution or plant uptake occurred within the buffer. Isotopic patterns indicated occurrence of denitrification. The project monitored the isotopic composition of nitrate at base flow in streams associated with the 15 subwatersheds. The subwatersheds with lower nitrate concentrations than would be expected from the proportion of agricultural land had higher 18O composition in the nitrate. This is consistent with the hypothesis that denitrification decreases nitrate concentrations to different extents in different subwatersheds.