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 (NO₃^-) 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, ¹⁵N/¹⁴N and ¹⁸O/¹⁶O (the "dual isotope" technique) have previously been used in biogeochemical studies of alpine and forested ecosystems to demonstrate that most of the NO₃^- 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 NO₃^- concentrations of bulk deposition and surface waters at three sites are very similar. While noting the constraints imposed by uncertainty in the precise δ¹⁸O value for microbial NO₃^-, however, we estimate that 79-98% of the annual mean NO₃^- has been microbially produced. Direct leaching of atmospheric NO₃^- is a minor component of catchment NO₃^- export, although greater than in many similar studies in forested watersheds. A greater proportion of atmospheric NO₃^- is seen in the two lake sites relative to their inflow streams, demonstrating the importance of direct NO₃^- deposition to lake surfaces in catchments where terrestrial ecosystems intercept a large proportion of deposited N. The dominance of microbial sources of NO₃^- in upland waters suggests that reduced and oxidised N deposition may have similar implications in terms of contributing to NO₃^- leaching.