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Warming climate in the Arctic is leading to an increase in isoprene emission from ecosystems. We assessed the influence of temperature on isoprene emission from Arctic willows with laboratory and field measurements. Our findings indicate that the hourly temperature response curve ofSalixspp., the dominant isoprene emitting shrub in the Arctic, aligns with that of temperate plants. In contrast, the isoprene capacity of willows exhibited a more substantial than expected response to the mean ambient temperature of the previous day, which is much stronger than the daily temperature response predicted by the current version of the Model of Emissions of Gases and Aerosols from Nature (MEGAN). With a modified algorithm from this study, MEGAN predicts 66% higher isoprene emissions for Arctic willows during an Arctic heatwave. However, despite these findings, we are still unable to fully explain the high temperature sensitivity of isoprene emissions from high latitude ecosystems.

 

Abstract Nonstructural carbohydrates (NSCs) play a critical role in plant physiology and metabolism, yet we know little about their distribution within individual organs such as the stem. This leaves many open questions about whether reserves deep in the stem are metabolically active and available to support functional processes. To gain insight into the availability of reserves, we measured radial patterns of NSCs over the course of a year in the stemwood of temperate trees with contrasting wood anatomy (ring porous vs diffuse porous). In a subset of trees, we estimated the mean age of soluble sugars within and between different organs using the radiocarbon (14C) bomb spike approach. First, we found that NSC concentrations were the highest and most seasonally dynamic in the outermost stemwood segments for both ring-porous and diffuse-porous trees. However, while the seasonal fluctuation of NSCs was dampened in deeper stemwood segments for ring-porous trees, it remained high for diffuse-porous trees. These NSC dynamics align with differences in the proportion of functional sapwood and the arrangement of vessels between ring-porous and diffuse-porous trees. Second, radial patterns of 14C in the stemwood showed that sugars became older when moving toward the pith. The same pattern was found in the coarse roots. Finally, when taken together, our results highlight how the radial distribution and age of NSCs relate to wood anatomy and suggest that while deeper, and likely older, reserves in the stemwood fluctuated across the seasons, the deepest reserves at the center of the stem were not used to support tree metabolism under usual environmental conditions. 

In response to a strong El Niño, fires in Indonesia during September and October 2015 released a large amount of carbon dioxide and created a massive regional smoke cloud that severely degraded air quality in many urban centers across Southeast Asia. Although several lines of evidence indicate that peat burning was a dominant contributor to emissions in the region, El Niño-induced drought is also known to increase deforestation fires and agricultural waste burning in plantations. As a result, uncertainties remain with respect to partitioning emissions among different ecosystem and fire types. Here we measured the radiocarbon content (¹⁴C) of carbonaceous aerosol samples collected in Singapore from September 2014 through October 2015, with the aim of identifying the age and origin of fire-emitted fine particulate matter (particulate matter with an aerodynamic diameter less than or equal to 2.5 μm). The Δ¹⁴C of fire-emitted aerosol was −76 ± 51‰, corresponding to a carbon pool of combusted organic matter with a mean turnover time of 800 ± 420 y. Our observations indicated that smoke plumes reaching Singapore originated primarily from peat burning (∼85%), and not from deforestation fires or waste burning. Atmospheric transport modeling confirmed that fires in Sumatra and Borneo were dominant contributors to elevated PM_2.5in Singapore during the fire season. The mean age of the carbonaceous aerosol, which predates the Industrial Revolution, highlights the importance of improving peatland fire management during future El Niño events for meeting climate mitigation and air quality commitments.

 

Glacial runoff exports large amounts of carbon (C) to the oceans, but major uncertainty remains regarding sources, seasonality, and magnitude. We apportioned C exported by five rivers from glacial and periglacial sources in northwest Greenland by monitoring discharge, water sources (δ¹⁸O), concentration and composition of dissolved organic carbon (DOC), and ages (¹⁴C) of DOC and particulate organic C over three summers (2010–2012). We found that particulate organic C (F = 1.0366–0.2506) was generally older than DOC in glacial sourced rivers and likely sourced from the physical erosion of aged C pools. Most exported DOC showed strong seasonal variations in sources and discharge. In summer, mean DOC ages ranged from modern to 4,750 cal years BP (F = 1.0022–0.6291); however, the annual C flux from glacially sourced rivers was dominated by young, plant‐derived DOC (F = 0.9667–1.002) exported during the spring freshet. The most aged DOC (F = 0.6891–0.8297) was exported in middle to late summer at lower concentrations and was glacial in origin. Scaled to the whole of Greenland using model‐estimated runoff, we estimate a total riverine DOC flux of 0.29% to 0.45% ± 20% Tg C/year. Our flux results indicate that the highest C fluxes occur during the time of year when the majority of C is modern in age. However, higher melt rates from the Greenland ice sheet and longer growing seasons could result in increasing amounts of ancient C from the Greenland ice sheet and from the periglacial landscape to the ocean.

 

The magnitude of future emissions of greenhouse gases from the northern permafrost region depends crucially on the mineralization of soil organic carbon (SOC) that has accumulated over millennia in these perennially frozen soils. Many recent studies have used radiocarbon (¹⁴C) to quantify the release of this “old” SOC as CO₂or CH₄to the atmosphere or as dissolved and particulate organic carbon (DOC and POC) to surface waters. We compiled ~1,900¹⁴C measurements from 51 sites in the northern permafrost region to assess the vulnerability of thawing SOC in tundra, forest, peatland, lake, and river ecosystems. We found that growing season soil¹⁴C‐CO₂emissions generally had a modern (post‐1950s) signature, but that well‐drained, oxic soils had increased CO₂emissions derived from older sources following recent thaw. The age of CO₂and CH₄emitted from lakes depended primarily on the age and quantity of SOC in sediments and on the mode of emission, and indicated substantial losses of previously frozen SOC from actively expanding thermokarst lakes. Increased fluvial export of aged DOC and POC occurred from sites where permafrost thaw caused soil thermal erosion. There was limited evidence supporting release of previously frozen SOC as CO₂, CH₄, and DOC from thawing peatlands with anoxic soils. This synthesis thus suggests widespread but not universal release of permafrost SOC following thaw. We show that different definitions of “old” sources among studies hamper the comparison of vulnerability of permafrost SOC across ecosystems and disturbances. We also highlight opportunities for future¹⁴C studies in the permafrost region.