Carbon (C) and nitrogen (N) cycles in streams are highly coupled because many N processes are mediated by dissolved organic matter (DOM) availability. Yet, our knowledge on how DOM stoichiometry influences in-stream C and N processing is far from complete. To assess the role of DOM stoichiometry (i.e. C:N ratio) as a driver of stream aerobic metabolism and N uptake, we performed constant-rate additions of resazurin and ammonium, with and without a co-release of acetate, in 17 headwater streams of five biomes. The streams showed contrasting concentrations of dissolved organic C (DOC; 0.5⎼30 mg C/L) and dissolved inorganic N (DON; 0.008⎼0.5 mg N/L), resulting in a wide range of DOC:DON ratios (from 5 to 201). For each release, we calculated in-stream ammonium and acetate uptake, and analyzed heterotrophic activity based on resazurin transformation. Under ambient DOC conditions, DOC:DON ratio was not related to global patterns of heterotrophic activity. However, DOM stoichiometry explained 32% of the variability in in-stream ammonium uptake velocity, with higher values observed in Arctic streams holding high DOC:DON ratios. When acetate was co-released, both heterotrophic activity and ammonium uptake generally augmented, with larger increases occurring in streams with higher acetate uptake rates. In this case, DOC:DON ratio explained 15% and 50% of the variability in heterotrophic activity and ammonium uptake, respectively. Overall, our results highlight that DOM stoichiometry is critical for understanding the coupling between C and N cycling in headwater streams.

Primary Presenter: Anna Lupon, CEAB-CSIC (anna.lupon@gmail.com)

Authors:

Anna Lupon, Center for Advanced Studies of Blanes (CEAB-CSIC)  (anna.lupon@gmail.com)

Dolly Kothawala, Uppsala University  (dolly.kothawala@ebc.uu.se)

Susana Bernal, Center for Advanced Studies of Blanes (CEAB-CSIC)  (sbernal@ceab.csic.es)

Xavier Peñarroya, Center for Advanced Studies of Blanes (CEAB-CSIC)  (xp.galceran@gmail.com)

Allison Herreid, University of New Hampshire  (Allison.Herreid@unh.edu)

Ryan Sponseller, Umeå University  (ryan.sponseller@umu.se)

Lluís Gómez-Gener, Centre for Research on Ecology and Forestry Application (CREAF)  (l.gomez@creaf.uab.cat)

Ada Pastor, University of Girona  (ada.pastor@udg.edu)

Matthew Cohen, University of Florida  (mjc@ufl.edu)

Hjalmar Laudon, Swedish University of Agricultural Sciences  (hjalmar.laudon@slu.se)

Eugènia Martí, Center for Advanced Studies of Blanes (CEAB-CSIC)  (eugenia@ceab.csic.es)