Grazing enhances carbon cycling but reduces methane emission during peak growing season in the Siberian Pleistocene Park tundra site

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Fischer, Wolfgang ; Thomas, Christoph ; Zimov, Nikita ; Göckede, Mathias:
Grazing enhances carbon cycling but reduces methane emission during peak growing season in the Siberian Pleistocene Park tundra site.
In: Biogeosciences. Bd. 19 (2022) Heft 6 . - S. 1611-1633.
ISSN 1726-4189
DOI: https://doi.org/10.5194/bg-19-1611-2022

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Abstract

Large-herbivore grazing has been shown to sub- stantially alter tundra soil and vegetation properties as well as carbon fluxes, yet observational evidence to quantify the im- pact of herbivore introduction into Arctic permafrost ecosys- tems remains sparse. In this study we investigated growing- season CO2 and CH4 fluxes with flux chambers on a former wet tussock tundra inside Pleistocene Park, a landscape ex- periment in northeast Siberia with a 22-year history of graz- ing. Reference data for an undisturbed system were collected on a nearby ungrazed tussock tundra. Linked to a reduction in soil moisture, topsoil temperatures at the grazed site reacted 1 order of magnitude faster to changes in air temperatures compared to the ungrazed site and were significantly higher, and the difference strongly decreased with depth. Overall, both GPP (gross primary productivity, i.e., CO2 uptake by photosynthesis) and Reco (ecosystem respiration, i.e., CO2 release from the ecosystem) were significantly higher at the grazed site with notable variations across plots at each site. The increases in CO2 component fluxes largely compensated for each other, leaving NEE (net ecosystem exchange) simi- lar across grazed and ungrazed sites for the observation pe- riod. Soil moisture and CH4 fluxes at the grazed site de- creased over the observation period, while in contrast the constantly waterlogged soils at the ungrazed site kept CH4 fluxes at significantly higher levels. Our results indicate that grazing of large herbivores may promote topsoil warming and drying, in this way effectively accelerating CO2 turnover while decreasing methane emissions in the summer months of peak ecosystem activity. Since we lack quantitative in- formation on the pre-treatment status of the grazed ecosys- tem, however, these findings need to be considered qualita- tive trends for the peak growing season, and absolute differ- ences between treatments are subject to elevated uncertainty. Moreover, our experiment did not include autumn and win- ter fluxes, and thus no inferences can be made for the annual NEE and CH4 budgets in tundra ecosystems.