Open-channel measurement of denitrification in a large lowland river

Abstract

Denitrification is considered to be the most important pathway removing nitrogen from terrestrial and freshwater ecosystems. However, field studies that quantify this process under in situ conditions are sparse, especially in large rivers. Here, we measured N2, the end product of denitrification, directly in the water column of a large 8th order lowland river (Elbe, Germany) using N2/Ar ratios measured by Membrane Inlet Mass Spectrometry (MIMS). Denitrification was calculated according to the open-channel two-station approach based on Lagrangian sampling along a 580 km long, mostly free flowing river section. Gas exchange was computed by several empirical equations to bound uncertainty in air–water exchange and the resulting fluxes were used to estimate ranges in N2-production. In summer 2011 and spring 2012, we found slight but distinct N2 super saturations in the river water averaging 2.8 and 3.5 µM, respectively. Denitrification rates averaged 18 and 13 mg N m− 2 h− 1 for summer 2011 and spring 2012, respectively. On an annual cycle this corresponds to a nitrogen removal of 10,000 t N year− 1 that is 10% of the total N inputs along the studied river section. These results show that large rivers can remove large amounts of nitrogen during downstream transport and demonstrate that the open-channel N2 method provides a valuable tool to study in situ denitrification not only in small, but also in large rivers.