Carbon degradation in Subarctic organic permafrost (peat plateaus) after thawing – what constraints CO2 and CH4 production?

<p>Rapid warming in Subarctic areas releases large amounts of frozen carbon which can potentially result in large CO<sub>2</sub> and CH<sub>4</sub> emissions to the atmosphere. In Northern Norway vast amount of carbon are stored in peat plateaus, but these landscape elements have been found to decrease laterally since at least the 1950s. Peat plateaus are very sensitive to climate change as the permafrost is relatively warm compared to permafrost found in the arctic. So far, only limited information is available about potential degradation kinetics of organic carbon in these ecosystems. We sampled organic matter from depth profiles along a well-documented chronosequence of permafrost degradation in Northern Norway. After thawing over-night, we incubated permafrost and active layer for up to 3 months at 10°C. To determine factors constraining degradation, we measured gas kinetics (O<sub>2</sub>, CO<sub>2</sub>, CH<sub>4</sub>) under different conditions (oxic/anoxic, loosely packed/stirred suspensions in water, with altered DOC content and nutrient amendments) and related them to pH, DOC, element (C, N, P, S) and δ<sup>13</sup>C and δ<sup>15</sup>N signatures of the peat. Organic matter degradation was strongly inhibited in the absence of oxygen. By contrast, CH<sub>4</sub> production or release seemed to be related to soil depth rather than incubation conditions and was found to be highest in samples from the transition zone between active layer and permafrost. Degradation rates and their dependencies on peat characteristics will be compared with permafrost characteristics along the chronosequence and additional experiments exploring the role of O<sub>2</sub>, DOC and other nutrients for carbon degradation will be discussed.</p>

Watershed dissolved organic carbon transport: a modeling approach combining water travel times and reactivity continuum

<p>Although their contribution was neglected in the past, inland waters play a significant role in the carbon cycle and affect CO<sub>2</sub> global balance. Streams and rivers are now considered not only as pipelines but as active reactors able to collect and transform carbon from terrestrial ecosystems trough drainage, erosion, deposition and respiration. Quantifying the transfer of carbon from the terrestrial to the riverine ecosystems is thus of crucial importance to fully appreciate carbon cycle at the watershed, regional and global scales. Such transfer is largely controlled by the processes occurring in the critical zone where the carbon and water cycles are tightly coupled. Previous studies investigated how hydrological drivers can affect Dissolved Organic Carbon (DOC) concentration in streams highlighting an hysteretic and unsteady behavior for the DOC-discharge relationship. In this study, we focus on the drainage flux from hillslopes to stream and river networks during rainfall events combining a transport model for water and a model of carbon degradation in soil. Using high-frequency records of chloride and DOC in Plynlimon catchments (UK), we employ the recently developed StorAge Selection (SAS) theory to evaluate water travel time and its partition as evapotranspiration, discharge and storage. We combine this approach with the reactivity continuum  theory to model  carbon degradation along the flow paths using a gamma-distribution as probability density function of the quality. The developed model can thus predict not only the flux of DOC released from hillslopes but also its quality (i.e. lability). We also show how the variability of the DOC-discharge relationship can partially be explained by hydrological fluctuations.</p>

Is terrestrial carbon degradation in stream hyporheic zones stimulated by nutrients?

<p>The stream hyporheic zone (HZ) represents the interface between streams and groundwater. Due to the mixing of organic matter and nutrients from groundwater and surface waters it is a hot spot of microbial activities and carbon processing within a stream network. The magnitude of terrestrial carbon degradation by microorganisms in the HZ influences the quantity and biochemical quality of terrestrial carbon as well as greenhouse gas concentrations in streams. One of the factors controlling microbial activities and terrestrial carbon degradation in the HZ are nutrients. However, major knowledge gaps exist regarding the control of nutrients on terrestrial carbon processing in the HZ among different streams.</p><p>We investigated the role of algal DOM (DOM<sub>alg</sub>) and phosphorus (P) on the degradation of soil DOM (DOM<sub>soil</sub>) by hyporheic microorganisms in a lab- and a field-based experiment. In the lab-based experiment, we focused on the influence of different DOM<sub>soil</sub>:DOM<sub>alg</sub> ratios on the DOM degradation at similar carbon concentrations in microcosms mimicking the HZ. One batch was incubated at ambient P concentrations and a second batch at increased P concentrations adapted to the highest levels found in the pure DOM<sub>alg</sub>. We assessed microbial respiration and changes in DOM optical properties to examine quantitative and qualitative changes of the DOM pool. In the field-based experiment, we determined microbial respiration rates of HZ-sediments from 20 streams in Austria with differing ambient nutrient and organic carbon concentrations. The sediments were incubated with DOM<sub>soil</sub>, with and without additional P.</p><p>Results from the lab-based experiment show that microbial respiration in the HZ decreased with increasing soil DOM fractions. When P levels were adapted to DOM<sub>alg</sub> concentrations, microbial respiration rates were comparable between the different DOM mixtures and DOM<sub>soil</sub> was degraded. However, in the field-based experiment, P addition only stimulated microbial respiration rates in one out of 20 HZ-sediments, suggesting that microbial respiration rates are not solely controlled by P.</p><p>In conclusion, nutrient pulses can stimulate microbial activities and thus terrestrial carbon degradation in the HZ. However, when using different stream HZ-sediments, it becomes evident that the nutrient stimulation is not a ubiquitous mechanism and terrestrial carbon degradation in the HZ is controlled by a multitude of factors.</p>