Using Stable Carbon Isotopes of Seasonal Ecosystem Respiration to Determine Permafrost Carbon Loss

Abstract

High latitude warming and permafrost thaw will expose vast stores of deep soil organic carbon (SOC) to decomposition. Thaw also changes water movement causing either wetter or drier soil. The fate of deep SOC under different thaw and moisture conditions is unclear. We measured weekly growing‐season δ13C of ecosystem respiration (Recoδ13C) across thaw and moisture conditions (Shallow‐Dry; Deep‐Dry; Deep‐Wet) in a soil warming manipulation. Deep SOC loss was inferred from known δ13C signatures of plant shoot, root, surface soil, and deep soil respiration. In addition, a 2‐year‐old vegetation removal treatment (No Veg) was used to isolate surface and deep SOC decomposition contributions to Reco. In No Veg, seasonal Recoδ13C indicated that deep SOC loss increased as the soil column thawed, while in vegetated areas, root contributions appeared to dominate Reco. The Recoδ13C differences between Shallow‐Dry and Deep‐Dry were significant but surprisingly small. This most likely suggests that, under dry conditions, soil warming stimulates root and surface SOC respiration with a negative 13C signature that opposes the more positive 13C signal from increased deep SOC respiration. In Deep‐Wet conditions, Recoδ13C suggests reduced deep SOC loss but could also reflect altered diffusion or methane (CH4) dynamics. Together, these results demonstrate that frequent Recoδ13C measurements can detect deep SOC loss and that plants confound the signal. In future studies, soil profile δ13C measurements, vegetation removal across thaw gradients, and isotopic effects of CH4 dynamics could further deconvolute deep SOC loss via surface Reco.

Lower soil moisture and deep soil temperatures in thermokarst features increase old soil carbon loss after 10 years of experimental permafrost warming

“…Microbial metabolism, on the other hand, strongly favors the lighter isotope, 12 C (Trumbore, 2006). Soil organic matter becomes more decomposed and processed with depth, and the 13 C composition (δ 13 C) of bulk soil C can increase up to 6‰ from the surface down to ~85 cm (Mauritz et al, 2019; Pries et al, 2013; Schuur et al, 2003). Stable C isotope data can be complemented by radiocarbon ( 14 C) to provide further insight on R eco source contributions.…”

Carbon isotopic evidence for rapid methane clathrate release recorded in coals at the terminus of the Late Palaeozoic Ice Age

“…It is possible that this negative carbon isotopic shift is due to the release of methane clathrates (CH 4 ) into the atmosphere during terminal deglaciation. Furthermore, the contribution of deep soil organic carbon (SOC) loss and CH 4 from terrestrial permafrost may also have contributed to widespread δ 13 C perturbation 31 .…”

Author response to Reviewer Comments #3

“…Tarnocai et al, 2009;Ciais et al, 2012;Schneider et al, 2013;Eggleston et al, 2016;Ganopolski and Brovkin, 2017;Treat et al, 2019). The freezing and burial of organic matter across the glacial cycle may significantly imprint the terrestrial biosphere CO2 size and δ13C signature (Tarnocai et al, 2009;Ciais et al, 2012;Schneider et al, 2013;Eggleston et al, 2016;Ganopolski and Brovkin, 2017;Mauritz et al, 2018;Treat et al, 2019). Schneider et al (2013) and Eggleston et al (2016) both observed a permanent increase in atmospheric δ13C during the last glacial cycle, of âĹij0.4‰ and attributed its cause likely due to soil storage of carbon in peatlands which were buried or frozen as permafrost as the glacial cycle progressed.…”

“…Printer-friendly version Discussion paper ston et al, 2016;Mauritz et al, 2018;Treat et al, 2019), and were partially or wholly replaced by other soil stocks of carbon (e.g. Lindgren et al, 2018).…”

“…References added following this reviewers' comments: Watson et al, 2000Joos et al, 2004Tarnocai et al, 2009Ganopolski et al, 2010Brovkin et al, 2012Jaccard et al, 2013Schneider et al, 2013Menviel et al, 2015Yu et al, 2016Ganopolski and Brovkin, 2017Lindgren et al, 2018Mauritz et al, 2018Yamamoto et al, 2019Treat et al, 2019 Specific comments: RC: 1) Abstract: The first line does not make sense. Please reformulate.…”