In situ application of the 15NO3- isotope pairing technique to measure denitrification in sediments at the surface water-groundwater interface

There is a need to understand the hydro-ecological significance of surface-subsurface interactions, including denitrification, in sediments in permeable catchments. Measurement of denitrification in such sediments is complicated by the NO3- reduction zone being relatively deep in the sediment (a few to tens of centimeters), there being 2 significant sources of NO3- ( the overlying water and the underlying groundwater) and, in some environments, the impact of macrophytes such as Ranunculus. These factors negate the collection of sediment cores for measuring denitrification by traditional techniques. Instead, we have developed a minipore water probe system that can be used to identify NO3- reduction zones and measure in situ rates of denitrification based on spiking sediment pore waters with (NO3-)-N-15 followed by a short incubation time ( 15 min) and subsequent quantification of N-29(2) and N-30(2). Given the short incubation time, we suggest that the technique can be applied to pore waters with significant advective flow, and present results for fine sand sediments from the River Frome ( Dorset, England). The limitations of the technique, including a low depth resolution ( 2 cm) and intrusion of pore water from outside the sample target depth, are calculated and discussed. Accepting the disturbance caused by the initial insertion of the probes, which is transient, this approach enables denitrification to be quantified under close to in situ conditions without the use of inhibitors and without destruction or isolation of the sediment.