Circumpolar assessment of permafrost C quality and its vulnerability over time using long‐term incubation data

Abstract

High-latitude ecosystems store approximately 1700 Pg of soil carbon (C), which is twice as much C as is currently contained in the atmosphere. Permafrost thaw and subsequent microbial decomposition of permafrost organic matter could add large amounts of C to the atmosphere, thereby influencing the global C cycle. The rates at which C is being released from the permafrost zone at different soil depths and across different physiographic regions are poorly understood but crucial in understanding future changes in permafrost C storage with climate change. We assessed the inherent decomposability of C from the permafrost zone by assembling a database of long-term (>1 year) aerobic soil incubations from 121 individual samples from 23 high-latitude ecosystems located across the northern circumpolar permafrost zone. Using a three-pool (i.e., fast, slow and passive) decomposition model, we estimated pool sizes for C fractions with different turnover times and their inherent decomposition rates using a reference temperature of 5 °C. Fast cycling C accounted for less than 5% of all C in both organic and mineral soils whereas the pool size of slow cycling C increased with C : N. Turnover time at 5 °C of fast cycling C typically was below 1 year, between 5 and 15 years for slow turning over C, and more than 500 years for passive C. We project that between 20 and 90% of the organic C could potentially be mineralized to CO2 within 50 incubation years at a constant temperature of 5 °C, with vulnerability to loss increasing in soils with higher C : N. These results demonstrate the variation in the vulnerability of C stored in permafrost soils based on inherent differences in organic matter decomposability, and point toward C : N as an index of decomposability that has the potential to be used to scale permafrost C loss across landscapes.

Temperature and moisture effects on greenhouse gas emissions from deep active-layer boreal soils

“…The C : N ratio was not a significant predictor of GHG fluxes in this study, although this ratio has been found to be B. Bond-Lamberty et al: Temperature and moisture effects on greenhouse gas emissions important in metanalyses (Sistla et al, 2012;Schädel et al, 2014). In situ respiration rates have also been shown to be negatively correlated with C : N at large spatial scales (Allaire et al, 2012).…”

“…The Q 10 values observed in this experiment were low (all less than 2.0, even when controlling for changes in soil moisture). Temperature sensitivities of ∼ 2 are more typical (Dutta et al, 2006;Schädel et al, 2016), although the temperature sensitivity of C release can change over time of incubation (Dutta et al, 2006) and vary between soil fractions cycling over different time horizons (Karhu et al, 2010;Schädel et al, 2014). Observed surface CO 2 fluxes at this CPCRW site exhibited a Q 10 of 5.1 ± 1.4 over a temperature range of 3.5-15 • C (C. Anderson, personal communication, 2016); however, these surface fluxes were measured over multiple months and include root respiration preventing any direct comparison.…”

“…This will have a wide variety of ecosystem effects (Alexander and Mack, 2016): in particular, rising temperatures and increasing fire will likely result in changes in soil temperature and permafrost degradation (Pastick et al, 2015;Zhang et al, 2015;Genet et al, 2013;Helbig et al, 2016), with subsequent hydrology changes that will influence soil greenhouse gas (GHG) fluxes to the atmosphere (Schädel et al, 2014). Such fluxes are a large component of the global C cycle and could result in a significant and positive climate feedback Koven et al, 2011;Schaefer et al, 2014).…”

Substrate potential of Eemian to Holocene permafrost organic matter for future microbial greenhouse gas production

“…This will enhance microbial production and the release of greenhouse 10 gasses such as carbon dioxide and methane to the atmosphere (Wagner et al, 2003;Schuur et al, 2008;McGuire et al, 2009;Knoblauch et al, 2013). Incubation experiments on permafrost samples of different ages revealed that permafrost thawing is accompanied by outgassing of greenhouse gases (Waldrop et al, 2010;Lee et al, 2012;Lipson et al, 2012;Knoblauch et al, 2013;Schadel et al, 2014;Walz et al, 2017). Former studies on samples from NE Siberian Holocene and Late Pleistocene (LP) Yedoma deposits, an ice-rich paleosol formation that is wide-spread in NE Siberia, have shown that 15 microbial degradability of the freeze-locked OM in permafrost seems to depend on the amount and quality of organic carbon (OC) rather than on the age of the deposits (Knoblauch et al, 2013;Strauss et al, 2015;Stapel et al, 2016).…”

Transient dynamics of terrestrial carbon storage: mathematical foundation and its applications

“…This assumption is built upon observations from litter and SOC decomposition. Analysis of data from nearly 300 studies of litter decomposition (Zhang et al, 2008), about 500 studies of soil incubation (Schädel et al, 2014;Xu et al, 2016), more than 100 studies of forest succession (Yang et al, 2011), and restoration (Matamala et al, 2008) almost all suggests that the first-order decay function captures macroscopic patterns of land C dynamics. Even so, its biological, chemical, and physical underpinnings need more study .…”