Short and Long‐Term Controls on Active Layer and Permafrost Carbon Turnover Across the Arctic

Abstract

Decomposition of soil organic matter (SOM) in permafrost terrain and the production of greenhouse gases is a key factor for understanding climate change‐carbon feedbacks. Previous studies have shown that SOM decomposition is mostly controlled by soil temperature, soil moisture, and carbon‐nitrogen ratio (C:N). However, focus has generally been on site‐specific processes and little is known about variations in the controls on SOM decomposition across Arctic sites. For assessing SOM decomposition, we retrieved 241 samples from 101 soil profiles across three contrasting Arctic regions and incubated them in the laboratory under aerobic conditions. We assessed soil carbon losses (Closs) five times during a 1 year incubation. The incubated material consisted of near‐surface active layer (ALNS), subsurface active layer (ALSS), peat, and permafrost samples. Samples were analyzed for carbon, nitrogen, water content, δ13C, δ15N, and dry bulk density (DBD). While no significant differences were observed between total ALSS and permafrost Closs over 1 year incubation (2.3 ± 2.4% and 2.5 ± 1.5% Closs, respectively), ALNS samples showed higher Closs (7.9 ± 4.2%). DBD was the best explanatory parameter for active layer Closs across sites. Additionally, results of permafrost samples show that C:N ratio can be used to characterize initial Closs between sites. This data set on the influence of abiotic parameter on microbial SOM decomposition can improve model simulations of Arctic soil CO2 production by providing representative mean values of CO2 production rates and identifying standard parameters or proxies for upscaling potential CO2 production from site to regional scales.

Lability classification of soil organic matter in the northern permafrost region

“…Upper permafrost mineral soil samples (<3 %C) from Kylatyk in NE Siberia, incubated at 2 ºC directly after field collection and thawing (measurement after 20-30 hr, following 3 days of pre-incubation), show median C release rates of c. 750 μgC-CO2 gC -1 d -1 (Weiss et al, 2016), compared to c. 1750 μgC-CO2 gC -1 d -1 in the same class of CryoCarb 1-Kolyma samples. Median C release rates in upper permafrost mineral soil samples of the PAGE21 experiment (Faucherre et al, 2018) decrease from c. 170 on day 8 to c. 35 μgC-CO2 gC -1 d -1 on day 343 since start of incubation. It is obvious from these results that there is a rapid decline in C release rates over time of incubation.…”

“…displayed a very low C-CO2 production rate per gC. The PAGE21 experiment also revealed that peat samples mineralized a smaller fraction of C over the one year of incubation compared to mineral soil samples (Faucherre et al, 2018).…”

“…Peat samples are available from a fourth PAGE21 study area. In total c. 240 soil samples from four study areas across the northern permafrost region (Ny Ålesund and Adventdalen, Svalbard; Stordalen Mire, N Sweden; Lena Delta, N Siberia) were incubated in one and the same experiment (Faucherre et al, 2018).…”

“…This is also the reason why expressing C release as a function of gram dry weight (gdw) is more straightforward than against gram C (gC). The latter would have the benefit of expressing C release directly as a function of C stock, but the relationship is complex with recent studies showing high initial C release rates per gC at low %C values (Weiss et al, 2016;Faucherre et al, 2018). In this study DBD is available for all samples, and we can also express C release as a function of soil volume (cm 3 ).…”

“…6a and to the presence of a highly labile C pool (e.g. DOC, plant roots) in the mineral soil layers that is quickly decomposed (see Weiss et al, 2016;Faucherre et al, 2018). However, rates from active layer and permafrost layer C-enriched pockets are significantly lower than those from adjacent, non C-enriched, mineral soil samples.…”