Mobilization of soil/sediment organic carbon into inland waters constitutes a substantial, but poorly-constrained, component of the global carbon cycle. Radiocarbon ( 14 C) analysis has proven a valuable tool in tracing the sources and fate of mobilized carbon, but aquatic 14 C studies in permafrost regions rarely detect 'old' carbon (assimilated from the atmosphere into plants and soil prior to AD1950). The emission of greenhouse gases derived from old carbon by aquatic systems may indicate that carbon sequestered prior to AD1950 is being destabilized, thus contributing to the 'permafrost carbon feedback' (PCF). Here, we measure directly the 14 C content of aquatic CO 2 , alongside dissolved organic carbon, in headwater systems of the western Canadian Arctic-the first such concurrent measurements in the Arctic. Age distribution analysis indicates that the age of mobilized aquatic carbon increased significantly during the 2014 snow-free season as the active layer deepened. This increase in age was more pronounced in DOC, rising from 101-228 years before sampling date (a 120%-125% increase) compared to CO 2 , which rose from 92-151 years before sampling date (a 59%-63% increase). 'Pre-industrial' aged carbon (assimilated prior to ∼AD1750) comprised 15%-40% of the total aquatic carbon fluxes, demonstrating the prevalence of old carbon to Arctic headwaters. Although the presence of this old carbon is not necessarily indicative of a net positive PCF, we provide an approach and baseline data which can be used for future assessment of the PCF.

Elevated nitrate (NO 3 − ) concentrations can cause eutrophication, which may lead to harmful algal blooms, loss of habitat and reduction in biodiversity. Denitrification, a dissimilatory process that removes NO 3 − mainly as dinitrogen gas (N 2 ), is believed to be the dominant NO 3 − removal pathway in aquatic ecosystems. Evidence suggests that a less well-studied process, dissimilatory nitrate reduction to ammonium (DNRA), which retains nitrogen (N) in the system, may also be important under favorable conditions. Using stable isotope tracers in sealed microcosms, we measured the potential for NO 3 − losses due to DNRA and denitrification in an oligotrophic freshwater ecosystem. We took sediment and water samples at runoff and baseflow, across several ecotypes. Our objective was to quantify the relative importance of DNRA compared to denitrification with changes in ecotype and season. Potential denitrification rates ranged from 0 to 0.14 ± 0.03 µgN gAFDM −1 d −1. Potential DNRA rates ranged from 0 to 0.0051 ± 0.0008 µgN gAFDM −1 d −1. Denitrification losses peaked at the inflow stream ecotype at 96.2% of total dissimilatory NO 3 − removal, whereas losses due to DNRA peaked in the lake ecotype at 34.4%. When averaged over the entire system, denitrification peaked at baseflow (31.2%), while DNRA peaked at runoff (2.9%). Although NO 3 − transformations due to denitrification were higher than DNRA in all ecotype and temporal comparisons, our results suggest that DNRA is also important under favorable conditions. KEY WORDS: DNRA · Denitrification · Nitrogen transformations · Ecotype · Season Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 65: [55][56][57][58][59][60][61][62][63][64] 2011 finally N 2 (Ye et al. 1995). The final reduction products, nitrous oxide (N 2 O), a potent greenhouse gas (Ramaswamy et al. 2001), and dinitrogen gas (N 2 ), are lost from the system into the atmosphere (Delwiche & Bryan 1976). In the presence of O 2 , most denitrifying bacteria will switch to the physiologically preferred process of aerobic respiration at the expense of NO 3 − reduction. (Megonigal et al. 2004). Denitrification may be diminished by the presence of free sulfides, which can inhibit the enzymes responsible for the final 2 stages of the process (Burgin & Hamilton 2007).DNRA is a microbial process that transforms NO 3 − to ammonium (NH 4 + ) via formation of NO 2 − in anaerobic or low O 2 environments. The final N form, NH 4 + , is bioavailable and readily immobilized by microbes and plants, or transformed by nitrification (Bengtsson et al. 2003). There are 2 DNRA pathways; fermentative and chemolithoautotrophic. Fermentative DNRA microbes reduce NO 3 − to NO 2 − to produce ATP. The subsequent reduction of NO 2 − to NH 4 + is an electron sink that allows re-oxidation of NADH (Tiedje 1988). Chemolithoautotrophic DNRA is the transformation of NO 3 − to NH 4 + , linked to oxidation of reduced sulfur (S) compounds. This sulfur-driven NO 3 − reduction leads to ...

Climate change poses a substantial threat to the stability of the Arctic terrestrial carbon (C) pool as warmer air temperatures thaw permafrost and deepen the seasonally-thawed active layer of soils and sediments. Enhanced water flow through this layer may accelerate the transport of C and major cations and anions to streams and lakes. These act as important conduits and reactors for dissolved C within the terrestrial C cycle. It is important for studies to consider these processes in small headwater catchments, which have been identified as hotspots of rapid mineralisation of C sourced from ancient permafrost thaw. In order to better understand the role of inland waters in terrestrial C cycling we characterised the biogeochemistry of the freshwater systems in a c. 14 km2  study area in the western Canadian Arctic. Sampling took place during the snow-free seasons of 2013 and 2014 for major inorganic solutes, dissolved organic and inorganic C (DOC and DIC, respectively), carbon dioxide (CO2) and methane (CH4) concentrations from three water type groups: lakes, polygonal pools and streams. These groups displayed differing biogeochemical signatures, indicative of contrasting biogeochemical controls. However, none of the groups showed strong signals of enhanced permafrost thaw during the study seasons. The mean annual air temperature in the region has increased by more than 2.5 °C since 1970, and continued warming will likely affect the aquatic biogeochemistry. This study provides important baseline data for comparison with future studies in a warming Arctic

Abstract. Nitrogen deposition was experimentally increased on a Scottish peatbog over a period of 13 years (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015). Nitrogen was applied in three forms, NH 3 gas, NH 4 Cl solution, and NaNO 3 solution, at rates ranging from 8 (ambient) to 64 kg N ha −1 yr −1 , and higher near the NH 3 fumigation source. An automated system was used to apply the nitrogen, such that the deposition was realistic in terms of rates and high frequency of deposition events. We measured the response of nitrous oxide (N 2 O) flux to the increased nitrogen input. Prior expectations, based on the IPCC default emission factor, were that 1 % of the added nitrogen would be emitted as N 2 O. In the plots treated with NH + 4 and NO − 3 solution, no response was seen, and there was a tendency for N 2 O fluxes to be reduced by additional nitrogen, though this was not significant. Areas subjected to high NH 3 emitted more N 2 O than expected, up to 8.5 % of the added nitrogen. Differences in the response are related to the impact of the nitrogen treatments on the vegetation. In the NH + 4 and NO − 3 treatments, all the additional nitrogen is effectively immobilised in the vegetation and top 10 cm of peat. In the NH 3 treatment, much of the vegetation was killed off by high doses of NH 3 , and the nitrogen was presumably more available to denitrifying bacteria. The design of the wet and dry experimental treatments meant that they differed in statistical power, and we are less likely to detect an effect of the NH + 4 and NO − 3 treatments, though they avoid issues of pseudo-replication.