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The study by Franz et al. provides exciting new insights into deep biosphere processes during the 'boring billion', indicating that fungi-like eukaryotic organisms developed before 1 Ga and that a deep continental biosphere was already present in the Early Mesoproterozoic/Late Paleoproterozoic.

The carbon accumulation rates of the described salt marsh soils are approximately 2-7 times greater than net C uptake rates of Canadian boreal forests, which highlights their potential importance as C reservoirs and the need to consider their C accumulation capacity as a climate mitigation co-benefit when conserving for other salt marsh ecosystem services.

By using an experimental approach to study how upwelling deep water modulates chemistry and biology response of surface plankton communities the authors address a highly relevant scientific topic as expected climate change will significantly alter ocean currents and upwelling patterns. This is particularly important as stimulation of highly productive phytoplankton blooms by upwelling water are generally attributed to the inputs of water rich in inorganic nutrients into the sunlit surface ocean. By using an experimental approach the authors could disentangle the different drivers of phytoplankton bloom dynamics in the highly productive northern Humboldt Upwelling System. Their results indicate that upwelling modifies phytoplankton succession after upwelling by additional factors and not just the physical supply of inorganic nutrients with the deep water. Accordingly, the most influential drivers were the stoichiometry of the inorganic nutrients added and deep water microbial communities, which can cause heterogeneity in phytoplankton bloom succession and control stoichiometry of residual inorganic nutrients during phytoplankton bloom brake down. Thus, deep water community composition are a major control of phytoplankton bloom dynamics in surface waters following upwelling events which have profound implications for ocean biodiversity, productivity and biochemistry which need to be taken into account in ocean models.

Yedema et al. used multiple biomarkers and palynological proxies to track the fate of plant-derived, soil-microibal, fluvial, and marine-produced organic matter in the coastal sediments of the northern Gulf of Mexico. The key findings show plant-derived materials reaching the deepest waters in contrast to fluvial organic matter confined to the proximity of the river mouth. This source-specific difference in the coastal distribution and storage of organic matter can greatly contribute to elucidating the role of terrestrial organic matter in coastal carbon sequestration.

Vast areas of permafrost are being degraded by climate change, which can release substantial quantities of nutrients into rivers and the ocean. This study shows how nitrogen isotopes can be used to determine how climate change affects the fluxes of nitrogen to the Arctic Ocean through permafrost melt.

Tropical cyclones are typically assumed to be associated with ecosystem damage. This study challenges this assumption and suggests that instead of reducing plant biomass, cyclones in East Asia may increase biomass production by alleviating water stress.

This paper advances our understanding of forest responses to elevated CO2 by considering implications for nutrient availability across the soil profile and the particular role of nutrient recycling in the rhizosphere versus soil organic matter mineralization.

This study is the first to quantify land carbon cycle feedbacks under decreasing atmospheric CO2 concentration following negative CO2 emissions and compare them to feedbacks under positive emissions. The novel approach presented here reduces the carbon cycle inertia in the phase where atmospheric CO2 concentration decreases in order to improve the quantification of carbon cycle feedbacks under negative emissions. This approach reduced the effectivity of negative emissions in reducing atmospheric CO2, due to larger concentration-carbon and climate-carbon feedbacks.

The paper by Turner and co-authors tackles the very timely question on how well we can reconstruct carbon inventories given the sparse observations. Using an Ensemble Optimal Interpolation approach and synthetic observations, the authors show that a large fraction of ocean carbon and its variability can be reconstructed using temperature and salinity measurements in the top 100 meter, however, reconstruction skill decreases when the top 2000 meters are considered. The authors propose a new way to use sparse observations to better understand historical carbon cycle changes, i.e., an important quantity in light of future changes driven by man-made emissions.

While the source and fate of particulate organic matter (POM) are better understood in riverine, deltaic, and lake systems, this paper provides an insight on POM dynamics in a permafrost catchment, while combining a multiple proxy approach (particulate organic carbon - POC, total suspended matter - TSM, carbon isotopes - d13C and d14C - in POC). Combined with mixing models, these findings help decipher the potential sources for authochtonous and allochtnonous organic matters, and hence provide important insights in the role of permafrost for the organic carbon mobilization in the Arctic region.

Schiferl and colleagues combine top-down and bottom-up estimates of carbon dioxide emissions from the North Slope of Alaska to find that CO2 efflux from terrestrial and aquatic ecosystems during the early cold season (September – December) are critical for explaining its carbon balance. Fluxes during the late cold season (January through April) were muted in comparison. Despite the importance of cold-season efflux, growing season CO2 uptake and release processes dominated its interannual variability. This study helps clarify how the carbon cycle of complex tundra ecosystems across large Arctic regions respond to ongoing climate changes.