Winter Ecosystem Respiration and Sources of CO2 From the High Arctic Tundra of Svalbard: Response to a Deeper Snow Experiment

Currently, there is a lack of understanding on how the magnitude and sources of carbon (C) emissions from High Arctic tundra are impacted by changing snow cover duration and depth during winter. Here we investigated this issue in a graminoid tundra snow fence experiment on shale-derived gelisols in Svalbard from the end of the growing season and throughout the winter. To characterize emissions, we measured ecosystem respiration (Reco) along with its radiocarbon (14C) content. We assessed the composition of soil organic matter (SOM) by measuring its bulk-C and nitrogen (N), 14C content, and n-alkane composition. Our findings reveal that greater snow depth increased soil temperatures and winter Reco (25 mg C m−2 d−1 under deeper snow compared to 13 mg C m−2 d−1 in ambient conditions). At the end of the growing season, Reco was dominated by plant respiration and microbial decomposition of C fixed within the past 60 years (Δ14C = 62 ± 8‰). During winter, emissions were significantly older (Δ14C = −64 ± 14‰), and likely sourced from microorganisms decomposing aged SOM formed during the Holocene mixed with biotic or abiotic mineralization of the carbonaceous, fossil parent material. Our findings imply that snow cover duration and depth is a key control on soil temperatures and thus the magnitude of Reco in winter. We also show that in shallow Arctic soils, mineralization of carbonaceous parent materials can contribute significant proportions of fossil C to Reco. Therefore, permafrost-C inventories informing C emission projections must carefully distinguish between more vulnerable SOM from recently fixed biomass and more recalcitrant ancient sedimentary C sources

Respiration of aged soil carbon during fall in permafrost peatlands enhanced by active layer deepening following wildfire but limited following thermokarst

Permafrost peatlands store globally significant amounts of soil organic carbon (SOC) that may be vulnerable to climate change. Permafrost thaw exposes deeper, older SOC to microbial activity, but SOC vulnerability to mineralization and release as carbon dioxide is likely influenced by the soil environmental conditions that follow thaw. Permafrost thaw in peat plateaus, the dominant type of permafrost peatlands in North America, occurs both through deepening of the active layer and through thermokarst. Active layer deepening exposes aged SOC to predominately oxic conditions, while thermokarst is associated with complete permafrost thaw which leads to ground subsidence, inundation and soil anoxic conditions. Thermokarst often follows active layer deepening, and wildfire is an important trigger of this sequence. We compared the mineralization rate of aged SOC at an intact peat plateau (∼70 cm oxic active layer), a burned peat plateau (∼120 cm oxic active layer), and a thermokarst bog (∼550 cm anoxic peat profile) by measuring respired 14C-CO2. Measurements were done in fall when surface temperatures were near-freezing while deeper soil temperatures were still close to their seasonal maxima. Aged SOC (1600 yrs BP) contributed 22.1 ± 11.3% and 3.5 ± 3.1% to soil respiration in the burned and intact peat plateau, respectively, indicating a fivefold higher rate of aged SOC mineralization in the burned than intact peat plateau (0.15 ± 0.07 versus 0.03 ± 0.03 g CO2-C m−2 d−1). None or minimal contribution of aged SOC to soil respiration was detected within the thermokarst bog, regardless of whether thaw had occurred decades or centuries ago. While more data from other sites and seasons are required, our study provides strong evidence of substantially increased respiration of aged SOC from burned peat plateaus with deepened active layer, while also suggesting inhibition of aged SOC respiration under anoxic conditions in thermokarst bogs

Source signatures from combined isotopic analyses of PM2.5 carbonaceous and nitrogen aerosols at the peri-urban Taehwa Research Forest, South Korea in summer and fall.

Isotopes are essential tools to apportion major sources of aerosols. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM2.5 at Taehwa Research Forest (TRF) near Seoul Metropolitan Area (SMA) during August-October 2014. PM2.5, TC, and TN concentrations were 19.4 ± 10.1 μg m-3, 2.6 ± 0.8 μg C m-3, and 1.4 ± 1.4 μg N m-3, respectively. The δ13C of TC and the δ15N of TN were - 25.4 ± 0.7‰ and 14.6 ± 3.8‰, respectively. EC was dominated by fossil-fuel sources with Fff (EC) of 78 ± 7%. In contrast, contemporary sources were dominant for TC with Fc (TC) of 76 ± 7%, revealing the significant contribution of contemporary sources to OC during the growing season. The isotopic signature carries more detailed information on sources depending on air mass trajectories. The urban influence was dominant under stagnant condition, which was in reasonable agreement with the estimated δ15N of NH4+. The low δ15N (7.0 ± 0.2‰) with high TN concentration was apparent in air masses from Shandong province, indicating fossil fuel combustion as major emission source. In contrast, the high δ15N (16.1 ± 3.2‰) with enhanced TC/TN ratio reveals the impact of biomass burning in the air transported from the far eastern border region of China and Russia. Our findings highlight that the multi-isotopic composition is a useful tool to identify emission sources and to trace regional sources of carbonaceous and nitrogen aerosols

Convergence in Nitrogen Deposition and Cryptic Isotopic Variation Across Urban and Agricultural Valleys in Northern Utah

The extent to which atmospheric nitrogen (N) deposition reflects land use differences and biogenic versus fossil fuel reactive N sources remains unclear yet represents a critical uncertainty in ecosystem N budgets. We compared N concentrations and isotopes in precipitation-event bulk (wet + dry) deposition across nearby valleys in northern Utah with contrasting land use (highly urban versus intensive agriculture/low-density urban). We predicted greater nitrate (NO3−) versus ammonium (NH4+) and higher δ15N of NO3− and NH4+ in urban valley sites. Contrary to expectations, annual N deposition (3.5–5.1 kg N ha−1 yr−1) and inorganic N concentrations were similar within and between valleys. Significant summertime decreases in δ15N of NO3− possibly reflected increasing biogenic emissions in the agricultural valley. Organic N was a relatively minor component of deposition (~13%). Nearby paired wildland sites had similar bulk deposition N concentrations as the urban and agricultural sites. Weighted bulk deposition δ15N was similar to natural ecosystems (−0.6 ± 0.7‰). Fine atmospheric particulate matter (PM2.5) had consistently high values of bulk δ15N (15.6 ± 1.4‰), δ15N in NH4+ (22.5 ± 1.6‰), and NO3− (8.8 ± 0.7‰), consistent with equilibrium fractionation with gaseous species. The δ15N in bulk deposition NH4+ varied by more than 40‰, and spatial variation in δ15N within storms exceeded 10‰. Sporadically high values of δ15N were thus consistent with increased particulate N contributions as well as potential N source variation. Despite large differences in reactive N sources, urban and agricultural landscapes are not always strongly reflected in the composition and fluxes of local N deposition—an important consideration for regional-scale ecosystem models

Influence of production variables and starting material on charcoal stable isotopic and molecular characteristics

We present a systematic study on the effect of starting species, gas composition, temperature, particle size and duration of heating upon the molecular and stable isotope composition of high density (mangrove) and low density (pine) wood. In both pine and mangrove, charcoal was depleted in o13C relative to the starting wood by up to 1.6% and 0.8%, respectively. This is attributed predominantly to the progressive loss of isotopically heavier polysaccharides, and kinetic effects of aromatization during heating. However, the pattern of o13C change was dependant upon both starting species and atmosphere, with different structural changes associated with charcoal production from each wood type elucidated by Solid-State o13C Nuclear Magnetic Resonance Spectroscopy. These are particularly evident at lower temperatures, where variation in the oxygen content of the production atmosphere results in differences in the thermal degradation of cellulose and lignin. It is concluded that production of charcoal from separate species in identical conditions, or from a single sample exposed to different production variables, can result in significantly different o13C of the resulting material, relative to the initial wood. These results have implications for the use of charcoal isotope composition to infer past environmental change

Isotopes in pyrogenic carbon: a review

Pyrogenic carbon (PC; also known as biochar, charcoal, black carbon and soot) derived from natural and anthropogenic burning plays a major, but poorly quantified, role in the global carbon cycle. Isotopes provide a fundamental fingerprint of the source of PC and a powerful tracer of interactions between PC and the environment. Radiocarbon and stable carbon isotope techniques have been widely applied to studies of PC in aerosols, soils, sediments and archaeological sequences, with the use of other isotopes currently less developed. This paper reviews the current state of knowledge regarding (i) techniques for isolating PC for isotope analysis and (ii) processes controlling the carbon (<sup>13</sup>C and <sup>14</sup>C), nitrogen, oxygen, hydrogen and sulfur isotope composition of PC during formation and after deposition. It also reviews the current and potential future applications of isotope based studies to better understand the role of PC in the modern environment and to the development of records of past environmental change

Alkali extraction of archaeological and geological charcoal: evidence for diagenetic degradation and formation of humic acids

Charcoal forms a crucial source of archaeological and palaeoenvironmental data, providing a record of cultural activities, past climatic conditions and a means of chronological control via radiocarbon (<sup>14</sup>C) dating. Key to this is the perceived resistance of charcoal to post-depositional alteration, however recent research has highlighted the possibility for alteration and degradation of charcoal in the environment. An important aspect of such diagenesis is the potential for addition of exogenous 'humic acids' (HAs), to affect the accuracy of archaeological and palaeoenvironmental reconstructions based upon chemical analyses of HA-containing charcoal. However the release of significant quantities of HA from apparently pristine charcoals raises the question whether some HA could be derived via diagenetic alteration of charcoal itself. Here we address this question through comparison of freshly produced charcoal with samples from archaeological and geological sites exposed to environmental conditions for millennia using elemental (C/H/O) and isotopic (δ<sup>13</sup>C) measurements, Fourier Transform Infrared Spectroscopy (FTIR) and proton Liquid-State Nuclear Magnetic Resonance (<sup>1</sup>H NMR). The results of analyses show that the presence of highly carboxylated and aromatic alkali-extractable HA in charcoal from depositional environments can often be attributable to the effects of post-depositional processes, and that these substances can represent the products of post-depositional diagenetic alteration in charcoal

High temperature sensitivity of Arctic isoprene emissions explained by sedges.

It has been widely reported that isoprene emissions from the Arctic ecosystem have a strong temperature response. Here we identify sedges (Carex spp. and Eriophorum spp.) as key contributors to this high sensitivity using plant chamber experiments. We observe that sedges exhibit a markedly stronger temperature response compared to that of other isoprene emitters and predictions by the widely accepted isoprene emission model, the Model of Emissions of Gases and Aerosols from Nature (MEGAN). MEGAN is able to reproduce eddy-covariance flux observations at three high-latitude sites by integrating our findings. Furthermore, the omission of the strong temperature responses of Arctic isoprene emitters causes a 20% underestimation of isoprene emissions for the high-latitude regions of the Northern Hemisphere during 2000-2009 in the Community Land Model with the MEGAN scheme. We also find that the existing model had underestimated the long-term trend of isoprene emissions from 1960 to 2009 by 55% for the high-latitude regions