Differential response of carbon cycling to long-term nutrient input and altered hydrological conditions in a continental Canadian peatland

Abstract

Abstract. Peatlands play an important role in global carbon cycling, but their responses to long-term anthropogenically changed hydrologic conditions and nutrient infiltration are not well known. While experimental manipulation studies, e.g., fertilization or water table manipulations, exist on the plot scale, only few studies have addressed such factors under in situ conditions. Therefore, an ecological gradient from the center to the periphery of a continental Canadian peatland bordering a eutrophic water reservoir, as reflected by increasing nutrient input, enhanced water level fluctuations, and increasing coverage of vascular plants, was used for a case study of carbon cycling along a sequence of four differently altered sites. We monitored carbon dioxide (CO 2 ) and methane (CH 4 ) surface fluxes and dissolved inorganic carbon (DIC) and CH 4 concentrations in peat profiles from April 2014 through September 2015. Moreover, we studied bulk peat and pore-water quality and we applied δ 13 C-CH 4 and δ 13 C-CO 2 stable isotope abundance analyses to examine dominant CH 4 production and emission pathways during the growing season of 2015. We observed differential responses of carbon cycling at the four sites, presumably driven by abundances of plant functional types and vicinity to the reservoir. A shrub-dominated site in close vicinity to the reservoir was a comparably weak sink for CO 2 (in 1.5 years: −1093 ± 794, in 1 year: +135 ± 281 g CO 2 m −2 ; a net release) as compared to two graminoid-moss-dominated sites and a moss-dominated site (in 1.5 years: −1552 to −2260 g CO 2 m −2 , in 1 year: −896 to −1282 g CO 2 m −2 ). Also, the shrub-dominated site featured notably low DIC pore-water concentrations and comparably 13 C-enriched CH 4 (δ 13 C-CH 4 : −57.81 ± 7.03 ‰) and depleted CO 2 (δ 13 C-CO 2 : −15.85 ± 3.61 ‰) in a more decomposed peat, suggesting a higher share of CH 4 oxidation and differences in predominant methanogenic pathways. In comparison to all other sites, the graminoid-mossdominated site in closer vicinity to the reservoir featured a ∼ 30 % higher CH 4 emission (in 1.5 years: +61.4 ± 32, in 1 year: +39.86 ± 16.81 g CH 4 m −2 ). Low δ 13 C-CH 4 signatures (−62.30 ± 5.54 ‰) indicated only low mitigation of CH 4 emissions by methanotrophic activity here. Pathways of methanogenesis and methanotrophy appeared to be related to the vicinity to the water reservoir: the importance of acetoclastic CH 4 production apparently increased toward the reservoir, whereas the importance of CH 4 oxidation increased toward the peatland center. Plant-mediated transport was the prevailing CH 4 emission pathway at all sites even where graminoids were rare. Our study thus illustrates accelerated carbon cycling in a strongly altered peatland with consequences for CO 2 and CH 4 budgets. However, our results suggest that long-term excess nutrient input does not necessarily lead to a loss of the peatland carbon sink function.

Large impacts of small methane fluxes on carbon isotope values of soil respiration

“…The relatively positive  13 C value of CO2 (-10.13‰) in the Mollisol relative to C4 litter (-13.3‰) and soil organic C (-22.0‰) may be attributed to a small proportion of CO2 generated by the dissolution of carbonate (-2.08‰). In comparison, the more positive  13 C values of CO2 under the anaerobic phase (-9.05‰ for the for the Oxisol) coincided with higher CH4 percentages, as is commonly observed in consistently saturated wetland systems with much greater CH4 production (Corbett et al, 2013;Berger et al, 2018). Compared with the relatively stable  13 C value of CO2 in the control, the mean  13 C values of CH4 under the anaerobic phase (-43.97‰ for the Mollisol and -49.16‰ for the Oxisol) were consistent with a mixture of hydrogenotrophic and acetoclastic methanogenesis (Blaser and Conrad, 2016).…”

Zero to moderate methane emissions in a densely rooted, pristine Patagonian bog – biogeochemical controls as revealed from isotopic evidence

“…Then, slow diffusive transport along the concentration gradient from deep anoxic peat layers to the atmosphere leads to CH 4 release. In upper, unsaturated and oxic peat zones that typically extend only about a few decimetres (Whalen, 2005;Limpens et al, 2008), CH 4 might get consumed by methanotrophic microbes (Chasar et al, 2000;Whalen, 2005;Berger et al, 2018). Consequently, water table position and fluctuations strongly control the amount of emitted CH 4 (Blodau and Moore, 2003;Whalen, 2005).…”

“…While CH 4 oxidation suppresses CH 4 emissions in diffusion-dominated systems, ebullition by fast release of gas bubbles or plant-mediated transport by aerenchymatic roots can substantially increase CH 4 emissions (Fechner-Levy and Hemond, 1996;Joabsson et al, 1999, and references therein;Chasar et al, 2000;Colmer, 2003;Whalen, 2005;Knoblauch et al, 2015;Burger et al, 2016;Berger et al, 2018). With a high share of ebullitive fluxes, vegetated or open water pools in peatlands are considered to be strong CH 4 emitters (Hamilton et al, 1994;Blodau 2002;Burger et al, 2016) that have, however, received less attention than the vegetated surfaces (Pelletier et al, 2014). Pools can even turn the peatlands' C balance into a source (Pelletier et al, 2014), but examples of low-emission pools have also been reported (Knoblauch et al, 2015).…”

“…The majority of studies elucidating CH 4 dynamics in peatlands have been conducted on the Northern Hemisphere (e.g. Blodau, 2002;Limpens et al, 2008;Yu, 2012, and references in these), whereas only little research deals with CH 4 dynamics in bogs on the Southern Hemisphere (hereafter termed southern bogs) (Broder et al, 2015). Contrary to northern bogs, southern bogs are not only vegetated by Sphagnum mosses, dwarf shrubs or a few graminoids such as the rushlike species Tetroncium magellanicum, but also by vascular plants (Kleinebecker et al, 2007) that can densely root upper peat layers (Fritz et al, 2011;Knorr et al, 2015).…”

“…Despite the variety of bogs dominated by densely rooted aerenchymous plants in the Northern Hemisphere and Southern Hemisphere, there is some evidence from the latest research that very low CH 4 pore water concentrations could be a more common phenomenon in those ecosystems (Knorr et al, 2015;Dullo et al, 2017;Agethen et al, 2018). In general, promotion or suppression of CH 4 production should be determined by the ratio of root density and activity associated with O 2 release versus presence of labile root organic matter or exudates accompanied by O 2 consumption (Blodau, 2002;Agethen et al, 2018).…”