Editor's choice 

This study presents an interesting finding that, when climatic conditions are dry, the direction of soil evaporation from soil to sky can reverse where atmospheric vapor adsorbs downward into the soil. This increased soil moisture input from the atmosphere serves as a non-precipitation water source. The authors show evidence for this process with lysimeters and also demonstrate that this soil adsorption process shows up in commonly used eddy covariance measurements.

This paper describes an extensive Critical Zone Observatory in a unique Patagonian Coastal Rainforest. The authors make a compelling argument for studying the ecological, biogeological, and hydrological value of this Rainforest type and present a comprehensive measurement approach for quantifying water and trace gas fluxes and the environmental drivers to which they respond including disturbance regimes as measured in part by seismology.

This excellent publication uses a multiproxy approach consisting of benthic formaminifera, lipid biomarkers and stable isotopes to study changing redox conditions in the Gulf of Oman during the past 43 kyrs. In large detail, the authors reconstruct periods dominated by either oxygenated conditions or largely oxygen depleted conditions. This high-resolution reconstruction revealed dominantly oxygenated conditions during Marine Isotope Stage 3 with deoxygenation events dominating most of the warmer Dansgaard-Oeschger events.

Arctic greenhouse gas fluxes are key for climate feedback but the Arctic greenhouse gas balance is poorly constrained due to a limited understanding of the spatial variation in these fluxes. This study combines extensive chamber-based flux measurements and remote sensing data to develop a machine-learning model to predict greenhouse gas fluxes across a tundra landscape in Finland. The analysis revealed that the system was a net greenhouse gas sink and showed widespread CH4 uptake in upland vegetation types, almost surpassing the high wetland CH4 emissions at the landscape scale.

Reaching the Paris Agreement targets to limit global warming to 1.5 or 2°C implies not only reducing emissions, but also active carbon dioxide removal from the atmosphere. While land-based carbon dioxide removal or negative emission technologies have received most attention, ocean solutions are increasingly being considered. Ocean alkalinity enhancement (OAE), or alkalinization, is one promising ocean-based carbon dioxide technology. However, any new carbon dioxide removal technique needs thorough investigations for its effectiveness, longevity, benefits and lack of disbenefits, financial viability, social acceptance and governability. The paper by Bach is a nice illustration of the type of research that must be done if we are to consider large scale application of OAE. It focuses on the additionality problem of ocean alkalinity enhancement, specifically it investigates how the addition of alkalinity modifies the natural alkalinity cycle and in that way the efficiency of carbon dioxide sequestration.

This is an excellent review of gross primary production (GPP) and net community production (NCP) measurements in the world's oceans using a new fleet of autonomous ocean-going instruments called biogeochemical-Argo (BGC-Argo) floats. The number of BGC-Argo floats has recently been surging with their users, so this review is timely and helpful for many oceanographers and biogeochemists to understand the advantages and challenges in the measurements of GPP and NCP in the oceans with the recently emerged technology.

This important study evaluates the impacts of enhanced olivine weathering on globally relevant phytoplankton species. Results from the study indicate that olivine dissolution products are unlikely to negatively impact the six phytoplankton species tested and may even enhance growth under certain conditions. Studies examining the safety of ocean alkalinity enhancement are urgently needed as interest in deploying this strategy for carbon dioxide removal is increasing.

The findings of this study are interesting to a broader audience as it demonstrates that supposedly biodegradable mulch films used for agricultural purposes remain unchanged in lake sediments over several months. The results indicate that lake sediments are a place of long-term storage rather than a place of degradation for this form of plastic.

This study documents the formation of Fe, Mn and Si amorphous mineral phases by very small cyanobacteria living in oxic, alkaline lakes. This finding has implications for using sedimentary Fe/Mn enrichments as a proxy for redox cycling in the past and extends the documented evidence for biomineralization of Si-rich phases.

Lessmann et al. describes how reservoir flushing could represent a novel way to manage methane emissions from freshwater reservoirs. Novel methods to reduce greenhouse gas emissions are urgently needed as the IPCC recently stated we will need to actively remove carbon from the atmosphere on top of reducing emissions in order to reach important climate goals. This study can help the development of diverse management strategies for mitigating global greenhouse gas emissions.

This study presents element/Ca data collected on Nodosariata, a particular group of foraminifera. The results of this work can potentially expand the applicability of foraminiferal proxies to the Permian, with important implication for the pale oceanographic and modeling communities.

The study covers a relevant contemporary topic (wildfire) and focusses on the recent major European summer of wildfires. The authors demonstrate that the assessment of burned area alone is not enough to fully characterize the impact of fires. Novel methods to assess burned area and above ground biomass losses are described to provide a more informative impact-based characterization of fires.

Nitrite is a nexus in nitrogen turnover, especially in oxygen minimum zones. This study works out new details of its specific role and adds new evidence for anaerobic nitrite oxidation. Furthermore, it shows that nitrogen recycling (nitrate reduction and nitrite oxidation) is quantitatively more important than loss processes.

Marine warming and deoxygenation are projected to intensify and drive a relative decrease in global habitat viability penetrating to all depths with warming dominating at the surface and deoxygenation becomes increasingly important with depth. In a 2°C scenario of global warming, epipelagic species' habitat losses are generally in the order of 0.1-0.5 million km3, while mesopelagic habitat losses are 0.01-0.15 million km3 and demersal losses are in the order of about 0.00025 million km3.

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 development of hyperspectral core scanning has facilitated high spatial/temporal pigment reconstructions indicative of changes in bioproductivity and was so far limited to those bacteriochlorophylls derived from Chromatiaceae (purple sulfur bacteria) to reconstruct changes in water column redox conditions and meromixis. This is the first study that reconstructs and quantifies Chlorobi-derived bacteriochlorophylls and its derivatives at high resolution in lake sediments. Therefore, this paper yields more insight into temporal variations of oxygenic and anoxygenic phototrophic communities, and provides a non-destructive tool for more detailed reconstructions of photic zone euxinia in meromictic lakes.

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 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.

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.

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.

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.

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.

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.

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.

The Arctic is a hot spot of warming triggering methane releases from thawing permafrost. This study by Castro-Morales and Co-Workers provides new insights into the transport of methane from soil to rivers. This work makes an essential contribution to comprehending the magnitude of methane emissions in Arctic rivers and potential contribution to the global methane budget under climate change.

Carbonne et al. identified the concurrent impact of Ocean Acidification as well as ocean warming on larval survivorship of corals. The study focuses on more temperate corals and also demonstrates the impact of lowered ocean pH on larval settlement and growth afterwards. This nicely designed experiment and the conclusive results advance our understanding of human's biogeochemical footprint on important taxa in marine systems.

Accurate constraining the ocean carbon sink is of utmost importance to improve our understanding of the fate of anthropogenic carbon and to better project anthropogenic carbon uptake in the coming decades. This study combines the outcomes of a suite of earth-system models with three well-documented observations (sea surface salinity in the Southern Ocean, strength of Atlantic Overturning and Revelle factor) to reduce bias and uncertainty in the global ocean carbon sink. The results suggest that the ocean carbon sink is about 10% larger than estimated before. This has implications for the global carbon budget.

This study significantly contributes to alleviating the data scarcity of drought legacy effects on ecosystem photosynthesis in temperate deciduous forests with a novel machine learning approach. This is probably the first time that drought legacies on ecosystem carbon fluxes are quantified using eddy-covariance data. The study reported that the reduction in photosynthesis due to drought legacy effects was of comparable magnitude to the concurrent drought effects at the studied sites. This study thus emphasizes the importance of drought legacy effects and provides a novel analytical method to quantify legacy effects elsewhere.