Temperature and peat type control CO₂ and CH₄ production in Alaskan permafrost peats

Abstract

Controls on the fate of ~277 Pg of soil organic carbon (C) stored in permafrost peatland soils remain poorly understood despite the potential for a significant positive feedback to climate change. Our objective was to quantify the temperature, moisture, organic matter, and microbial controls on soil organic carbon (SOC) losses following permafrost thaw in peat soils across Alaska. We compared the carbon dioxide (CO2 ) and methane (CH4 ) emissions from peat samples collected at active layer and permafrost depths when incubated aerobically and anaerobically at -5, -0.5, +4, and +20 °C. Temperature had a strong, positive effect on C emissions; global warming potential (GWP) was >3× larger at 20 °C than at 4 °C. Anaerobic conditions significantly reduced CO2 emissions and GWP by 47% at 20 °C but did not have a significant effect at -0.5 °C. Net anaerobic CH4 production over 30 days was 7.1 ± 2.8 μg CH4 -C gC(-1) at 20 °C. Cumulative CO2 emissions were related to organic matter chemistry and best predicted by the relative abundance of polysaccharides and proteins (R(2) = 0.81) in SOC. Carbon emissions (CO2 -C + CH4 -C) from the active layer depth peat ranged from 77% larger to not significantly different than permafrost depths and varied depending on the peat type and peat decomposition stage rather than thermal state. Potential SOC losses with warming depend not only on the magnitude of temperature increase and hydrology but also organic matter quality, permafrost history, and vegetation dynamics, which will ultimately determine net radiative forcing due to permafrost thaw.

Responses of peat carbon at different depths to simulated warming and oxidizing

“…At the same time, the difference of Q 10 values under AE and AN conditions also demonstrated that soil carbon became more liable to microbes under O 2 abundant environment (Lee et al, 2012;Wang et al, 2013). Though the effect of temperature and O 2 on Rs had been well studied in other peatlands (Knoblauch et al, 2013;Moore and Dalva, 1997;Szafranek-Nakonieczna and Stêpniewska, 2014;Treat et al, 2014;Wang et al, 2013), our results from an area of midlatitude and high altitude were different from others. Rs increase with oxidizing or 10°C warming was much lower in our study than in other incubation studies (85.8-226.1% and 128.9-200%) (Table S1), indicating the lower sensitivity of peat in this area to warming and oxidizing, which can be explained by the different climate (temperature and precipitation) and vegetation in our study area from those of the others (Table S1).…”

“…the increment at each depth was unequable and differed much with depth, which may reflect the variation of soil substrate among all profiles (Treat et al, 2014). The fact that the increments at 0-20 cm and 60-80 cm were higher than other layers may demonstrate that the carbon at these two layers is intrinsically labile and has higher potential decomposability (Oechel et al, 1998).…”

“…Biasi et al (2005) reported that temperature could influence the utilization of carbon source by microbes or microbial metabolism leading to Rs rate changes. Treat et al (2014) found that warmer and drier climate could result in substantial increase in carbon loss (Biasi et al, 2005;Dutta et al, 2006;Treat et al, 2014). DOC also was driven by temperature rising (Freeman et al, 2001b).…”

“…Just like soil characters that are determined by vegetation and vary markedly with climate (Chu et al, 2010;Zak and Kling, 2006), peat as the remains of vegetation may also differ among the whole depth profile as a result of climate change during peat development (Moore and Dalva, 1997) and cryoturbation (O'Donnell et al, 2012;Rinkes et al, 2013;Treat et al, 2014;Wickings et al, 2012). Under a warm and humid climate, peat mainly derives from Cyperaceae and Equisetaceae; under a cool and dry climate, however, it mainly consists of Picea remains (Guo et al, 2013).…”

“…Water table drawdown may destroy the stable environment in peatlands, exposing the preserved carbon to aerobic (AE) environment (Oechel et al, 1998), and together with the optimum soil moisture creating an available environment for microbes (Serreze et al, 2000;Yan et al, 2014;Zimov et al, 2006). Water table drawdown also shifts soil respiration (Rs) pathway from anaerobic (AN) to aerobic environment, which responds to climate change differently (Knoblauch et al, 2013;Lee et al, 2012;Schuur et al, 2008;Treat et al, 2014;Yang et al, 2014). Studies showed that the stored old carbon in deep soil of degraded peatlands had participated in modern carbon cycle (McCallister and Del Giorgio, 2012;Schuur et al, 2009;Singer et al, 2012).…”