Dominance of biologically produced nitrate in upland waters of Great Britain indicated by stable isotopes

Atmospheric deposition of nitrogen (N) compounds is the major source of anthropogenic N to most upland ecosystems, where leaching of nitrate (NO) into surface waters contributes to eutrophication and acidification as well as indicating an excess of N in the terrestrial catchment ecosystems. Natural abundance stable isotopes ratios, N/N and O/O (the "dual isotope" technique) have previously been used in biogeochemical studies of alpine and forested ecosystems to demonstrate that most of the NO in upland surface waters has been microbially produced. Here we present an application of the technique to four moorland catchments in the British uplands including a comparison of lakes and their stream inflows at two sites. The NO concentrations of bulk deposition and surface waters at three sites are very similar. While noting the constraints imposed by uncertainty in the precise δO value for microbial NO, however, we estimate that 79-98% of the annual mean NO has been microbially produced. Direct leaching of atmospheric NO is a minor component of catchment NO export, although greater than in many similar studies in forested watersheds. A greater proportion of atmospheric NO is seen in the two lake sites relative to their inflow streams, demonstrating the importance of direct NO deposition to lake surfaces in catchments where terrestrial ecosystems intercept a large proportion of deposited N. The dominance of microbial sources of NO in upland waters suggests that reduced and oxidised N deposition may have similar implications in terms of contributing to NO leaching. © 2011 Springer Science+Business Media B.V.