This special issue arises from work conducted by the AMAP Expert Group on short-lived climate forcers for an assessment that will be finalized in 2021. It will include only invited papers.

The special issue will present results from the MOSES/REEBUS Eddy Study on the mesoscale and sub-mesoscale dynamics associated with ocean eddies of West Africa which was carried out on the German R/V Meteor cruises M156 and M160 in 2019.

The scope of the study included physical, chemical, biological and geological research questions around the physical-chemical-biological coupling and the overall role of ocean eddies in an ocean area in the vicinity of the Canary Current System, a major eastern boundary upwelling system. The observational concept featured a wide range of platforms (research vessel, glider airplane, Saildrones, wave gliders, gliders, BGC Argo floats, drifters, moorings, AUVs, bottom landers, bottom crawler), observation and sampling approaches (towed, winched, free-falling and drifting instruments, multinet, rhodamine dye release experiment), instruments, and observed variables (in situ, underway, discrete samples) and specifically addressed the mesoscale and sub-mesoscale variability associated with ocean eddies.

The Peruvian upwelling system is one of the most productive ecosystems in the ocean. Continuous upwelling of nutrient-rich water generates high primary production, supporting an efficient food web and leading to high fishery yields. An extensive oxygen minimum zone underlying the productive surface waters gives rise to high losses of fixed nitrogen via denitrification and anaerobic ammonium oxidation. This results in nutrient ratios of upwelled waters deviating strongly from Redfield stoichiometry, with a high N deficit and excess P. How this unusual nutrient stoichiometry affects plankton community production and succession and how this feeds back on biogeochemical cycling are still not well understood. To address these questions, an in situ mesocosm experiment was conducted in the coastal upwelling system off Peru in the framework of the Collaborative Research Center 754 "Climate-Biogeochemistry Interactions in the Tropical Ocean". The field experiment took place from January to April 2017 during a coastal El Niño, a rare event of El Niño conditions confined to Peruvian coastal waters. Key objectives of this mesocosm study, which involved 57 researchers from 10 nations, were to unravel the interplay of pelagic ecology and biogeochemistry under the unique environmental conditions of the Peruvian upwelling system. This special issue in Biogeosciences is intended as a collective outlet for publications resulting from this study, covering the broad range of research topics from seawater chemistry to plankton ecology and pelagic biogeochemistry.

Climate change is exposing terrestrial ecosystems to warmer temperatures, increased CO₂ levels, and more frequent and intense drought and rainfall events. How different ecosystems respond to changing climatic conditions depends on a multitude of biological and geochemical processes which interact at different temporal and spatial scales.

On the one hand, plants assimilate carbon from the atmosphere and control carbon inputs into soil via plant detritus and rhizodeposition. On the other hand, soil fauna and microorganisms govern the decomposition of plant detritus and soil organic matter, which determines the cycling of carbon and of the nutrients necessary for plant growth. These interactions are further shaped by local environmental properties, such as vegetation type, relief, soil mineralogy, and physical structure, as well as the composition of plants and soil biota. Since soils represent the largest stock of organic carbon in terrestrial ecosystems and regulate carbon fluxes for about 10 times the current anthropogenic emissions, changes in the above-mentioned processes can have strong implications for global carbon cycling. Furthermore, the understanding of soil functioning is critical for improving predictions on the resistance and resilience of terrestrial ecosystems to climate change.

In this special issue, we seek to highlight research on biotic and abiotic interactions underlying terrestrial biogeochemical responses to climate, with a specific emphasis on elevated atmospheric CO₂, warming, drought, and drying–rewetting events. We invite contributions covering field and laboratory experiments, conceptual development, and modelling studies.

Remote sensing observations in the microwave frequencies have been intensively used to map and investigate water dynamics at the land surface. Microwave observations are also increasingly used to observe changes in vegetation structure and function and to study interactions between water and vegetation. For example, microwave observables such as brightness temperature, radar backscatter, polarization, SAR interferometry, GNSS reflectometry, and variables retrieved from them such as vegetation optical depth and soil moisture allow one to map ecosystem states. All these variables can be used to study processes, states, and temporal changes in vegetation–water interactions such as in vegetation phenology, canopy structure, biomass, photosynthesis, evapotranspiration, interception, dew formation, vegetation water content, water stress, surface and root-zone soil moisture, and irrigation.

The special issue aims to advance our understanding on existing and new techniques, methods, and applications of microwave remote sensing for vegetation–water interactions. We welcome studies that use active or passive microwave satellite, airborne, tower, and field observations to monitor processes, states, and temporal dynamics of vegetation–water interactions by

1. developing and evaluating microwave radiative transfer models for vegetation and water;
2. retrieving land surface variables through radiative transfer model inversion, machine learning, or hybrid approaches;
3. evaluating and constraining land surface, ecosystem, crop, forest, and hydrological models with microwave observations; and
4. assessing field experiments on the impacts of plant water dynamics on microwave observations.

This special issue is an outcome of sessions BG3.20 (Global Earth observation for improved understanding of terrestrial ecosystem dynamics) and HS6.4 (Remote sensing of interactions between vegetation and hydrology) held at the General Assembly 2020 of the European Geoscience Union and of the related precursor sessions in previous years. However, the special issue is open for all submissions within its scope.

The Weddell Sea and the ocean off Dronning Maud Land, constituting the Weddell Gyre, are representative of the high-latitude Southern Ocean due to its pronounced seasonality, circumpolar currents, deep-water formation and seasonal sea-ice cover. The Weddell Gyre connects water masses from the Antarctic Circumpolar Current with those shaped by the large ice shelves and sea-ice formation in the southern perimeter of the Weddell Gyre. Biological and biogeochemical processes are strongly influenced by these conditions and contribute to a unique Weddell Sea ecosystem.

A portion of the western Weddell Sea is already experiencing consequences of climate change, including the acceleration of mass loss from ice shelves, enhanced ocean warming and freshening, rising air temperatures, and changes in wind patterns. This likely has an emerging influence on the local biological and biogeochemical processes. The eastern part of the Weddell Gyre, off Dronning Maud Land, however, appears relatively stable but is expected to experience warming, sea-ice retreat and ice-shelf loss in the course of this century. The re-emergence of the Maud Rise Polynya, with a potentially significant impact on regional water masses and biogeochemistry, underscores the importance of observing and understanding this region. Establishing knowledge of the present biological and biogeochemical processes and coupling with physics prior to major changes in this region is essential.

This special issue emerged from an October 2020 workshop organized by the Southern Ocean Observing System (SOOS) Weddell Sea and Dronning Maud Land Regional Working Group that brought together numerous scientists across multiple disciplines. However, the issue is open to anyone and we welcome additional contributions.

Fire is an essential feature of terrestrial ecosystems and plays an important role in the Earth system. Fires are regulated by climate, vegetation characteristics, and human activity and also feedback to them in multiple ways. For example, fires can shape vegetation composition and structure; adjust land carbon, nutrient, water, and energy cycles; change atmospheric composition, chemistry, and physics; and affect air quality and human health. Elevated fire activity in 2019 across the arctic, Amazon, and other regions underscores the urgency of a quantitative understanding of fire as an Earth system process that interacts with vegetation, climate, and humans. The EGU2020 B3.17 session on the role of fire in the Earth system received the most abstracts in the Biogeosciences division this year and was highly attended during the live chat. The abstracts cover several aspects of interactions between fire and the biosphere, atmosphere, and humans across various temporal and spatial scales using modelling, field and laboratory observations, and remote sensing. Given that the topic is highly interdisciplinary and overarching, we propose an inter-journal (ESD/BG/ACP/GMD/NHESS) special issue for this session with ESD as the lead journal, and we also encourage the submission of topic-related studies outside the EGU fire session. The special issue will bring together new advances in understanding the feedbacks and interactions between fire and other components of the Earth system at all temporal and spatial scales using various methods, including (1) impacts of fire on weather, climate, and atmospheric chemistry; (2) interactions between fire, the biogeochemical cycle, vegetation composition and structure, and land water and energy budgets; (3) influence of humans on fire and vice versa; (4) fire characteristics (e.g., fire duration, emission factor, emission height, smoke transport); (5) spatial and temporal changes of fires in the past, present, and future; (6) fire products and models, their validation, and error/bias assessment; and (7) analytical tools designed to enhance situational awareness among fire practitioners and early warning systems, addressing specific needs of operational fire behaviour modelling.

The coastal ocean (i.e. regions that are directly influenced by their watershed, including estuaries) is under increasing pressures that affect its functioning and health, and compromise the provision of services to society. The implementation of the Blue Growth strategy has led to the multiplication of marine and maritime activities (e.g. dredging, offshore wind farms, eutrophication, fishing, pollution) with consequences for biodiversity and biogeochemistry that are still poorly known, in particular, at ecosystem scales. All these activities are developing in a context of global change (e.g. warming, acidification, deoxygenation, global sea level rise), adding additional complexity to the challenge of unravelling the intricate interplay between multiple stressors that may act synergistically or antagonistically.
In this special issue, we invite contributions to overview progress in our capabilities to understand, monitor, and forecast the impact of human activities on coastal marine environments to support the set-up of an ecosystem-based management scheme for the marine ecosystem. We welcome contributions linking science and policy, taking account of selecting useful and communicable indicators for ecosystem health, targeting ecosystem services, and making use of novel analytical tools acknowledging the complexity of driver–pressure–stressor–impact–response interactions.

This special issue is an output of the 52nd international Ocean Liège Colloquium on “Towards an understanding and assessment of human impact on coastal marine environments” that was organized online from 17–22 May 2021.

Climate change is exposing terrestrial ecosystems to warmer temperatures, increased CO₂ levels, and more frequent and intense drought and rainfall events. How different ecosystems respond to changing climatic conditions depends on a multitude of biological and geochemical processes which interact at different temporal and spatial scales.

On the one hand, plants assimilate carbon from the atmosphere and control carbon inputs into soil via plant detritus and rhizodeposition. On the other hand, soil fauna and microorganisms govern the decomposition of plant detritus and soil organic matter, which determines the cycling of carbon and of the nutrients necessary for plant growth. These interactions are further shaped by local environmental properties, such as vegetation type, relief, soil mineralogy, and physical structure, as well as the composition of plants and soil biota. Since soils represent the largest stock of organic carbon in terrestrial ecosystems and regulate carbon fluxes for about 10 times the current anthropogenic emissions, changes in the above-mentioned processes can have strong implications for global carbon cycling. Furthermore, the understanding of soil functioning is critical for improving predictions on the resistance and resilience of terrestrial ecosystems to climate change.

In this special issue, we seek to highlight research on biotic and abiotic interactions underlying terrestrial biogeochemical responses to climate, with a specific emphasis on elevated atmospheric CO₂, warming, drought, and drying–rewetting events. We invite contributions covering field and laboratory experiments, conceptual development, and modelling studies.

The special issue will present results from the MOSES/REEBUS Eddy Study on the mesoscale and sub-mesoscale dynamics associated with ocean eddies of West Africa which was carried out on the German R/V Meteor cruises M156 and M160 in 2019.

The scope of the study included physical, chemical, biological and geological research questions around the physical-chemical-biological coupling and the overall role of ocean eddies in an ocean area in the vicinity of the Canary Current System, a major eastern boundary upwelling system. The observational concept featured a wide range of platforms (research vessel, glider airplane, Saildrones, wave gliders, gliders, BGC Argo floats, drifters, moorings, AUVs, bottom landers, bottom crawler), observation and sampling approaches (towed, winched, free-falling and drifting instruments, multinet, rhodamine dye release experiment), instruments, and observed variables (in situ, underway, discrete samples) and specifically addressed the mesoscale and sub-mesoscale variability associated with ocean eddies.

The coastal ocean (i.e. regions that are directly influenced by their watershed, including estuaries) is under increasing pressures that affect its functioning and health, and compromise the provision of services to society. The implementation of the Blue Growth strategy has led to the multiplication of marine and maritime activities (e.g. dredging, offshore wind farms, eutrophication, fishing, pollution) with consequences for biodiversity and biogeochemistry that are still poorly known, in particular, at ecosystem scales. All these activities are developing in a context of global change (e.g. warming, acidification, deoxygenation, global sea level rise), adding additional complexity to the challenge of unravelling the intricate interplay between multiple stressors that may act synergistically or antagonistically.
In this special issue, we invite contributions to overview progress in our capabilities to understand, monitor, and forecast the impact of human activities on coastal marine environments to support the set-up of an ecosystem-based management scheme for the marine ecosystem. We welcome contributions linking science and policy, taking account of selecting useful and communicable indicators for ecosystem health, targeting ecosystem services, and making use of novel analytical tools acknowledging the complexity of driver–pressure–stressor–impact–response interactions.

This special issue is an output of the 52nd international Ocean Liège Colloquium on “Towards an understanding and assessment of human impact on coastal marine environments” that was organized online from 17–22 May 2021.

The Weddell Sea and the ocean off Dronning Maud Land, constituting the Weddell Gyre, are representative of the high-latitude Southern Ocean due to its pronounced seasonality, circumpolar currents, deep-water formation and seasonal sea-ice cover. The Weddell Gyre connects water masses from the Antarctic Circumpolar Current with those shaped by the large ice shelves and sea-ice formation in the southern perimeter of the Weddell Gyre. Biological and biogeochemical processes are strongly influenced by these conditions and contribute to a unique Weddell Sea ecosystem.

A portion of the western Weddell Sea is already experiencing consequences of climate change, including the acceleration of mass loss from ice shelves, enhanced ocean warming and freshening, rising air temperatures, and changes in wind patterns. This likely has an emerging influence on the local biological and biogeochemical processes. The eastern part of the Weddell Gyre, off Dronning Maud Land, however, appears relatively stable but is expected to experience warming, sea-ice retreat and ice-shelf loss in the course of this century. The re-emergence of the Maud Rise Polynya, with a potentially significant impact on regional water masses and biogeochemistry, underscores the importance of observing and understanding this region. Establishing knowledge of the present biological and biogeochemical processes and coupling with physics prior to major changes in this region is essential.

This special issue emerged from an October 2020 workshop organized by the Southern Ocean Observing System (SOOS) Weddell Sea and Dronning Maud Land Regional Working Group that brought together numerous scientists across multiple disciplines. However, the issue is open to anyone and we welcome additional contributions.

This special issue arises from work conducted by the AMAP Expert Group on short-lived climate forcers for an assessment that will be finalized in 2021. It will include only invited papers.

Remote sensing observations in the microwave frequencies have been intensively used to map and investigate water dynamics at the land surface. Microwave observations are also increasingly used to observe changes in vegetation structure and function and to study interactions between water and vegetation. For example, microwave observables such as brightness temperature, radar backscatter, polarization, SAR interferometry, GNSS reflectometry, and variables retrieved from them such as vegetation optical depth and soil moisture allow one to map ecosystem states. All these variables can be used to study processes, states, and temporal changes in vegetation–water interactions such as in vegetation phenology, canopy structure, biomass, photosynthesis, evapotranspiration, interception, dew formation, vegetation water content, water stress, surface and root-zone soil moisture, and irrigation.

The special issue aims to advance our understanding on existing and new techniques, methods, and applications of microwave remote sensing for vegetation–water interactions. We welcome studies that use active or passive microwave satellite, airborne, tower, and field observations to monitor processes, states, and temporal dynamics of vegetation–water interactions by

1. developing and evaluating microwave radiative transfer models for vegetation and water;
2. retrieving land surface variables through radiative transfer model inversion, machine learning, or hybrid approaches;
3. evaluating and constraining land surface, ecosystem, crop, forest, and hydrological models with microwave observations; and
4. assessing field experiments on the impacts of plant water dynamics on microwave observations.

This special issue is an outcome of sessions BG3.20 (Global Earth observation for improved understanding of terrestrial ecosystem dynamics) and HS6.4 (Remote sensing of interactions between vegetation and hydrology) held at the General Assembly 2020 of the European Geoscience Union and of the related precursor sessions in previous years. However, the special issue is open for all submissions within its scope.

Fire is an essential feature of terrestrial ecosystems and plays an important role in the Earth system. Fires are regulated by climate, vegetation characteristics, and human activity and also feedback to them in multiple ways. For example, fires can shape vegetation composition and structure; adjust land carbon, nutrient, water, and energy cycles; change atmospheric composition, chemistry, and physics; and affect air quality and human health. Elevated fire activity in 2019 across the arctic, Amazon, and other regions underscores the urgency of a quantitative understanding of fire as an Earth system process that interacts with vegetation, climate, and humans. The EGU2020 B3.17 session on the role of fire in the Earth system received the most abstracts in the Biogeosciences division this year and was highly attended during the live chat. The abstracts cover several aspects of interactions between fire and the biosphere, atmosphere, and humans across various temporal and spatial scales using modelling, field and laboratory observations, and remote sensing. Given that the topic is highly interdisciplinary and overarching, we propose an inter-journal (ESD/BG/ACP/GMD/NHESS) special issue for this session with ESD as the lead journal, and we also encourage the submission of topic-related studies outside the EGU fire session. The special issue will bring together new advances in understanding the feedbacks and interactions between fire and other components of the Earth system at all temporal and spatial scales using various methods, including (1) impacts of fire on weather, climate, and atmospheric chemistry; (2) interactions between fire, the biogeochemical cycle, vegetation composition and structure, and land water and energy budgets; (3) influence of humans on fire and vice versa; (4) fire characteristics (e.g., fire duration, emission factor, emission height, smoke transport); (5) spatial and temporal changes of fires in the past, present, and future; (6) fire products and models, their validation, and error/bias assessment; and (7) analytical tools designed to enhance situational awareness among fire practitioners and early warning systems, addressing specific needs of operational fire behaviour modelling.

The Peruvian upwelling system is one of the most productive ecosystems in the ocean. Continuous upwelling of nutrient-rich water generates high primary production, supporting an efficient food web and leading to high fishery yields. An extensive oxygen minimum zone underlying the productive surface waters gives rise to high losses of fixed nitrogen via denitrification and anaerobic ammonium oxidation. This results in nutrient ratios of upwelled waters deviating strongly from Redfield stoichiometry, with a high N deficit and excess P. How this unusual nutrient stoichiometry affects plankton community production and succession and how this feeds back on biogeochemical cycling are still not well understood. To address these questions, an in situ mesocosm experiment was conducted in the coastal upwelling system off Peru in the framework of the Collaborative Research Center 754 "Climate-Biogeochemistry Interactions in the Tropical Ocean". The field experiment took place from January to April 2017 during a coastal El Niño, a rare event of El Niño conditions confined to Peruvian coastal waters. Key objectives of this mesocosm study, which involved 57 researchers from 10 nations, were to unravel the interplay of pelagic ecology and biogeochemistry under the unique environmental conditions of the Peruvian upwelling system. This special issue in Biogeosciences is intended as a collective outlet for publications resulting from this study, covering the broad range of research topics from seawater chemistry to plankton ecology and pelagic biogeochemistry.