Biogeosciences 

This special issue brings together a collection of articles on ecosystem manipulation experiments, which have the potential to inform mechanistic ecosystem models on the effects of global changes. Terrestrial ecosystem dynamics and the interplay of multiple biogeochemical cycles are responsible for unresolved uncertainties in Earth system change projections. The advancement of ecosystem experiments and mechanistic modelling has great potential for addressing this research challenge. However, finding answers requires diverse theoretical and empirical research to be integrated. Ecosystem manipulation experiments allow for empirical testing of theoretical hypotheses, model predictions, and their assumptions. However, results from manipulation experiments can only contribute to advance our ability to improve ecosystem–climate feedback if these are informed by and integrated into mechanistic models.

The scope of this special issue is centred around terrestrial ecosystem experiments. Papers contain an explicit theoretical component for evaluating results or generating hypotheses, address identified open challenges in modelling ecosystem responses to various experimental treatments, or are centred around a model–data synthesis, using results from ecosystem manipulation experiments. Manipulation experiments is to be understood in a broad sense, including (but not restricted to) nutrient, CO₂, and/or water manipulation; isotope labelling studies; and natural perturbations, ranging from ecosystem to planetary scale.

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 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.