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Abstract Purpose of ReviewWhile previously thought to be negligible, carbon emissions during the non-growing season (NGS) can be a substantial part of the annual carbon budget in the Arctic boreal zone (ABZ), which can shift the carbon balance of these ecosystems from a long-held annual carbon sink towards a net annual carbon source. The purpose of this review is to summarize NGS carbon dioxide (CO₂) flux research in the ABZ that has been published within the past 5 years. Recent FindingsWe explore the processes and magnitudes of CO₂fluxes, and the status of modeling efforts, and evaluate future directions. With technological advances, direct measurements of NGS fluxes are increasing at sites across the ABZ over the past decade, showing ecosystems in the ABZ are a large source of CO₂in the shoulder seasons, with low, consistent, winter emissions. SummaryEcosystem carbon cycling models are being improved with some challenges, such as modeling below ground and snow processes, which are critical to understanding NGS CO₂fluxes. A lack of representative in situ carbon flux data and gridded environmental data are leading limiting factors preventing more accurate predictions of NGS carbon fluxes. 

Abstract. Landscapes are often assumed to be homogeneous when interpreting eddy covariance fluxes, which can lead to biases when gap-filling and scaling up observations to determine regional carbon budgets. Tundra ecosystems are heterogeneous at multiple scales. Plant functional types, soil moisture, thaw depth, and microtopography, for example, vary across the landscape and influence net ecosystem exchange (NEE) of carbon dioxide (CO2) and methane (CH4) fluxes. With warming temperatures, Arctic ecosystems are changing from a net sink to a net source of carbon to the atmosphere in some locations, but the Arctic's carbon balance remains highly uncertain. In this study we report results from growing season NEE and CH4 fluxes from an eddy covariance tower in the Yukon–Kuskokwim Delta in Alaska. We used footprint models and Bayesian Markov chain Monte Carlo (MCMC) methods to unmix eddy covariance observations into constituent land-cover fluxes based on high-resolution land-cover maps of the region. We compared three types of footprint models and used two land-cover maps with varying complexity to determine the effects of these choices on derived ecosystem fluxes. We used artificially created gaps of withheld observations to compare gap-filling performance using our derived land-cover-specific fluxes and traditional gap-filling methods that assume homogeneous landscapes. We also compared resulting regional carbon budgets when scaling up observations using heterogeneous and homogeneous approaches. Traditional gap-filling methods performed worse at predicting artificially withheld gaps in NEE than those that accounted for heterogeneous landscapes, while there were only slight differences between footprint models and land-cover maps. We identified and quantified hot spots of carbon fluxes in the landscape (e.g., late growing season emissions from wetlands and small ponds). We resolved distinct seasonality in tundra growing season NEE fluxes. Scaling while assuming a homogeneous landscape overestimated the growing season CO2 sink by a factor of 2 and underestimated CH4 emissions by a factor of 2 when compared to scaling with any method that accounts for landscape heterogeneity. We show how Bayesian MCMC, analytical footprint models, and high-resolution land-cover maps can be leveraged to derive detailed land-cover carbon fluxes from eddy covariance time series. These results demonstrate the importance of landscape heterogeneity when scaling carbon emissions across the Arctic. 

The main impetus for this study is to answer the following questions: (i) What are the net late summer and autumn seasonal fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) across the North Slope and coastal Arctic Ocean? Using the van we will be able to measure fluxes in the late summer and autumn along the Dalton highway and connecting roads. While this will not be across the North Slope of Alaska from a longitudinal prospective, we will be able to sample most of the different ecotopes found there and sample into the foothills region. From this and surface ecotope maps we will still be able to derive a net flux and compare with other estimates in the literature. As a bonus, we will plan to acquire data on the transit drive between Boston and Alaska providing a cross-continental snapshot of emissions to compare with the emissions from permafrost. (ii) What are the primary surface land classes and associated mechanisms that contribute to these fluxes? Again, along the Dalton highway we will have access to sampling from rivers, wet sedge, mesic sedge, lakes, tussock tundra, dwarf shrub, among others. Though in extent we will sample less area, we will have better precision from the areas we do sample. We will also be able to operate in a fixed mode (parked vehicle) to increase our precision even further for isotopic measurements which are tied to mechanistic information. (iii) What remotely-sensed products are the best proxy for the physical and biological processes that regulate the net flux of methane and carbon dioxide and how do these vary regionally? This question we will be able to address in the same manner as proposed using our modeling capabilities and available remote products from satellites. (iv) How do the answers to the above questions vary depending on the scale of measurements used from local measurements up to regional scale inversion models? We will be able to answer this question in much the same way as proposed, but will only be able to scale from local to landscape scales. This dataset covers the Flux Observations of Carbon from an Automotive Laboratory (FOCAL) 2024 campaign. It includes two continuous eddy covariance towers set up on the North Slope south of Prudhoe Bay in 2023 and 2024. It also includes discontinuous eddy covariance measurements from 4 sites using a mobile tower van laboratory. The motivation of the campaign was to focus on late season carbon dioxide, methane, and nitrous oxide fluxes from various ecotypes along a North - South transect of the North Slope, AK. By measuring gas fluxes from different sites as the season progressed from summer to early winter we hoped to better understand how the permafrost is changing in terms of gas emissions. We also hoped to better quantify late season emissions which have been shown to be an important source of methane emissions from the Arctic region. We used eddy covariance to measure the gas fluxes combining sonic anemometers for the 3D wind with spectrometers tuned to the different gases of interest. The two towers used commercial instrumentation from LiCOR and Cambridge Scientific and the mobile tower used custom built spectrometers and a Gil Windmaster. 

Abstract. The continued warming of the Arctic could release vast stores of carbon into the atmosphere from high-latitude ecosystems, especially from thawingpermafrost. Increasing uptake of carbon dioxide (CO2) by vegetation during longer growing seasons may partially offset such release of carbon. However, evidence of significant net annual release of carbon from site-level observations and model simulations across tundra ecosystems has been inconclusive. To address this knowledge gap, we combined top-down observations of atmospheric CO2 concentration enhancements from aircraft and a tall tower, which integrate ecosystem exchange over large regions, with bottom-up observed CO2 fluxes from tundraenvironments and found that the Alaska North Slope is not a consistent net source nor net sink of CO2 to the atmosphere (ranging from −6 to+6 Tg C yr−1 for 2012–2017). Our analysis suggests that significant biogenic CO2 fluxes from unfrozen terrestrial soils, and likely inland waters, during the early cold season (September–December) are major factors in determining the net annual carbon balance of the North Slope, implying strong sensitivity to the rapidly warming freeze-up period. At the regional level, we find no evidence of the previously reported large late-cold-season (January–April) CO2 emissions to the atmosphere during the study period. Despite the importance of the cold-season CO2 emissions to the annual total, the interannual variability in the net CO2 flux is driven by the variability in growing season fluxes. During the growing season, the regional net CO2 flux is also highly sensitive to the distribution of tundra vegetation types throughout the North Slope. This study shows that quantification and characterization of year-round CO2 fluxes from the heterogeneous terrestrial and aquatic ecosystems in the Arctic using both site-level and atmospheric observations are important to accurately project the Earth system response to future warming.