Behind the scenes: mechanisms regulating climatic patterns of dissolved organic carbon uptake in headwater streams

Large variability in dissolved organic carbon (DOC) uptake rates has been reported for headwater streams, but the causes of this variability are still not well understood. Here we assessed acetate uptake rates across 11 European streams comprising different ecoregions by using whole-reach pulse acetate additions. We evaluated the main climatic and biogeochemical drivers of acetate uptake during two seasonal periods. Our results show a minor influence of sampling periods but a strong effect of climate and dissolved organic matter (DOM) composition on acetate uptake. In particular, mean annual precipitation explained half of the variability of the acetate uptake velocities (VfAcetate) across streams. Temperate streams presented the lowest VfAcetate, together with humic-like DOM and the highest stream respiration rates. In contrast, higher VfAcetate were found in semiarid streams, with protein-like DOM, indicating a dominance of reactive, labile compounds. This, together with lower stream respiration rates and molar ratios of DOC to nitrate, suggests a strong C limitation in semiarid streams, likely due to reduced inputs from the catchment. Overall, this study highlights the interplay of climate and DOM composition and its relevance to understand the biogeochemical mechanisms controlling DOC uptake in streams.Theauthors were supported by the following funding: N. C. by a Generalitat de Catalunya-Beatriu de Pinós grant (BP2016–00215), E. E. by a predoctoral grant from the Basque Government, A. G. B. by a Generalitat de Catalunya-Beatriu de Pinós (BP-00385-2016), A. M. G. F. by a predoctoral research grant (BES-2013-065770) from the Spanish Ministry of Economy and Competitiveness, P. R. L. by a Ramón Areces Foundation Postdoctoral Scholarship, and A. L. by a Kempe Foundation stipend. DOMIPEX project was funded by the First Call of Collaborative Projects among Young Researchers of the Iberian Association of Limnology (AIL; 2013–2015)

Warming stimulates sediment denitrification at the expense of anaerobic ammonium oxidation.

Temperature is one of the fundamental environmental variables governing microbially mediated denitrification and anaerobic ammonium oxidation (anammox) in sediments. The GHG nitrous oxide (N2O) is produced during denitrification, but not by anammox, and knowledge of how these pathways respond to global warming remains limited. Here, we show that warming directly stimulates denitrification-derived N2O production and that the warming response for N2O production is slightly higher than the response for denitrification in subtropical sediments. Moreover, denitrification had a higher optimal temperature than anammox. Integrating our data into a global compilation indicates that denitrifiers are more thermotolerant, whereas anammox bacteria are relatively psychrotolerant. Crucially, recent summer temperatures in low-latitude sediments have exceeded the optimal temperature of anammox, implying that further warming may suppress anammox and direct more of the nitrogen flow towards denitrification and associated N2O production, leading to a positive climate feedback at low latitudes