Understanding pathways of dissimilatory and assimilatory dissolved inorganic nitrogen uptake in streams

Ribot, MiquelVon Schiller, DanielMartí, Eugènia. Limnology and Oceanography : doi: 10.1002/lno.10493 (2017) DIGITAL CSIC

Dissolved inorganic nitrogen (DIN) in streams is mostly available as two different species: nitrate (NO2 3 ) and ammonium (NH1 4 ). These two DIN species undergo specific dissimilatory uptake pathways and show distinct preference during biological assimilation. These differences ultimately dictate how DIN is cycled within streams and its further export to downstream ecosystems. Here, we provide a synthesis analysis of the uptake of NO2 3 and NH1 4 at the reach scale and on the contribution of dissimilatory and assimilatory uptake pathways. We combined 15N-tracer experiments in a single stream with compiled results of NO2 3 and NH1 4 uptake from the literature. As expected, streams were more efficient in processing NH1 4 than NO2 3 at the reach scale. These results were partially explained by the fact that, on average, dissimilatory NO2 3 uptake (i.e., denitrification and DNRA) had a low incidence on total NO2 3 uptake, whereas dissimilatory NH1 4 uptake (i.e., nitrification) contributed to a high proportion of total NH1 4 uptake thereby increasing in-stream NO2 3 concentration. Furthermore, assimilatory uptake by in-stream biotic compartments dominated the total uptake of the two DIN species and was generaly higher for NH1 4 than for NO2 3 . Overall, results from this study indicate that assimilatory uptake by biotic compartments rather than permanent removal dominates total NO2 3 uptake in streams. In contrast, both assimilatory and dissimilatory uptake can contribute similarly to total NH1 4 uptake. Our findings have strong implications for a better understanding of N cycling within the context of widespread increases in DIN concentration and changes in the NO2 3 : NH1 4 ratio driven by human activities.