Gareth K. Phoenix’s research while affiliated with The University of Sheffield and other places

Thawing permafrost soils are predicted to release substantial amounts of carbon by 2100. In addition to this, warming-induced active-layer deepening and increased rooting depth may result in further carbon losses from previously-frozen soil by stimulating microbial communities through fresh carbon inputs inducing positive rhizosphere priming. While models based on temperate data predict significant permafrost carbon loss through rhizosphere priming, data from permafrost soils are lacking. Here, we provide direct evidence of live plant-induced positive rhizosphere priming in permafrost and active-layer soils across diverse soil types from Arctic and Subarctic Canada. By ¹³CO2 labelling plants in a controlled environment, we show that root activity increases carbon loss from previously frozen soils by 31%. This rhizosphere priming effect persists longer in permafrost than in active-layer soils, suggesting greater vulnerability of permafrost carbon. These findings underscore the urgency of incorporating plant–soil–microbe interactions into models predicting greenhouse gas emissions from thawing permafrost.

Arctic landscapes occupy a nexus of environmental change processes, globally significant soil carbon stores, wildlife populations, and subsistence-based human societies. In response to rapid climate warming, tundra ecosystems are experiencing widespread changes to vegetation and underlying permafrost, coupled with an array of ecological disturbances that are expected to intensify in the future. Declines in the extent of the cryosphere on land (permafrost and seasonal snow) and offshore (sea-ice) raise the question of whether and for how long warmer portions of the Low Arctic will fit established concepts of “what is Arctic,” given the influence the cryosphere has historically had on tundra ecosystem structure and function. The era of spaceborne observation of circumpolar tundra greenness, in the form of the Normalized Difference Vegetation Index (NDVI), has entered its fifth decade and provides foundational information concerning ecosystem conditions and responses to climatic trends, variability, ecological disturbance, and successional processes. Here we review the evolving story of Arctic greening, and synthesize long-term spaceborne records of NDVI, climatic data, field observations, and the knowledge base of Arctic residents to place the last four decades of Arctic environmental change in context, and establish expectations and research priorities for the coming decade. Greenness dynamics display high spatio-temporal variability, reflecting complex interactions of climatic warming and variability, landscape history, ecological disturbance, and other factors. Nonetheless, long-term increases in NDVI—commonly known as “the greening of the Arctic”—remain prominent across large areas in all available long-term spaceborne datasets and align with long-term shifts in vegetation structure documented in disparate Arctic regions. Common shifts reported from the Low Arctic, such as shrubification, generally portend declines in floristic diversity, and shifts in fauna that favor boreal forest species. Despite lingering uncertainties regarding trend attribution and sources of interannual variability, the sequence of record-high circumpolar tundra greenness values observed since 2020 provides strong evidence that Arctic tundra ecosystems have entered a state without historic precedent on timescales approaching a millennium.

Arctic ecosystems are experiencing extreme climatic, biotic and physical disturbance events that can cause substantial loss of plant biomass and productivity, sometimes at scales of >1000 km². Collectively known as browning events, these are key contributors to the spatial and temporal complexity of Arctic greening and vegetation dynamics. If we are to properly understand the future of Arctic terrestrial ecosystems, their productivity, and their feedbacks to climate, understanding browning events is essential. Here we bring together understanding of browning events in Arctic ecosystems to compare their impacts and rates of recovery, and likely future changes in frequency and distribution. We also seek commonalities in impacts across these contrasting event types. We find that while browning events can cause high levels of plant damage (up to 100% mortality), ecosystems have substantial capacity for recovery, with biomass largely re-established within five years for many events. We also find that despite the substantial loss of leaf area of dominant species, compensatory mechanisms such as increased productivity of undamaged subordinate species lessen the impacts on carbon sequestration. These commonalities hold true for most climatic and biotic events, but less so for physical events such as fire and abrupt permafrost thaw, due to the greater removal of vegetation. Counterintuitively, some events also provide conditions for greater productivity (greening) in the longer-term, particularly where the disturbance exposes ground for plant colonisation. Finally, we find that projected changes in the causes of browning events currently suggest many types of events will become more frequent, with events of tundra fire and abrupt permafrost thaw expected to be the greatest contributors to future browning due to their severe impacts and occurrence in many Arctic regions. Overall, browning events will have increasingly important consequences for ecosystem structure and function, and for feedback to climate.

Changes in soil carbon (C) stocks are largely driven by rhizosphere processes forming new soil organic matter (SOM) or stimulating SOM decomposition by rhizosphere priming effects (RPEs). Quantifying these changes is challenging and requires high spatial sampling densities or plant–soil experiments with highly distinct C isotopic signatures for plants and soils. Current methods for quantifying new SOM formation and RPEs rely on low labelling intensities, which introduces high levels of uncertainty. Here, we describe the design and operation of an experimental laboratory facility—BLOSOM (Botanical Labelling Observatory for Soil Organic Matter)—optimised for continuous ¹³ C labelling of plants at high labelling intensities (> 500‰) to quantify new SOM formation and RPEs in temperature‐controlled soils from 216 experimental units. Throughout a > 6‐month experimental period, independent control of soil and air temperature was achieved across diurnal cycles averaging at 5.24°C ± 0.05°C and 21.4°C ± 1.2°C, respectively. BLOSOM can maintain stable CO 2 concentrations and δ ¹³ C isotopic composition within 5% of setpoints (CO 2 : 440 ppm, δ ¹³ C: 515‰) across a > 6‐month period. This high‐precision control on atmospheric enrichment enables the detection of new SOM formation with a total uncertainty of ±39% to ±3% for a theoretical range of 0.5%–10% new SOM formation, respectively. BLOSOM has the potential improve quantification and mechanistic understanding of new SOM formation and RPEs across many different combinations of plants, soils and simulated climatic conditions to mimic a wide range of ecosystems and climate scenarios.

Arctic observations in 2023 provided clear evidence of rapid and pronounced climate and environmental change, shaped by past and ongoing human activities that release greenhouse gases into the atmosphere and push the broader Earth system into uncharted territory. This chapter provides a snapshot of 2023 and summarizes decades-long trends observed across the Arctic, including warming surface air and sea-surface temperatures, decreasing snow cover, diminishing sea ice, thawing permafrost, and continued mass loss from the Greenland Ice Sheet and Arctic glaciers. These changes are driving a transition to a wetter, greener, and less frozen Arctic, with serious implications for Arctic peoples and ecosystems, as well as for low- and midlatitudes.

Globally pervasive increases in atmospheric CO 2 and nitrogen (N) deposition could have substantial effects on plant communities, either directly or mediated by their interactions with soil nutrient limitation. While the direct consequences of N enrichment on plant communities are well documented, potential interactions with rising CO 2 and globally widespread phosphorus (P) limitation remain poorly understood. We investigated the consequences of simultaneous elevated CO 2 (eCO 2) and N and P additions on grassland biodiversity, community and functional composition in P-limited grasslands. We exposed soil-turf monoliths from limestone and acidic grasslands that have received >25 years of N additions (3.5 and 14 g m −2 year −1) and 11 (limestone) or 25 (acidic) years of P additions (3.5 g m −2 year −1) to eCO 2 (600 ppm) for 3 years. Across both grasslands, eCO 2 , N and P additions significantly changed community composition. Limestone communities were more responsive to eCO 2 and saw significant functional shifts resulting from eCO 2-nutrient interactions. Here, legume cover tripled in response to combined eCO 2 and P additions, and combined eCO 2 and N treatments shifted functional dominance from grasses to sedges. We suggest that eCO 2 may disproportionately benefit P acquisition by sedges by subsidising the carbon cost of locally intense root exudation at the expense of co-occurring grasses. In contrast, the functional composition of the acidic grassland was insensitive to eCO 2 and its interactions with nutrient additions. Greater diversity of P-acquisition strategies in the limestone grassland, combined with a more functionally even and diverse community, may contribute to the stronger responses compared to the acidic grassland. Our work suggests we may see large changes in the composition and biodiversity of P-limited grasslands in response to eCO 2 and its interactions with nutrient loading, particularly where these contain a high diversity of P-acquisition strategies or developmentally young soils with sufficient bioavailable mineral P. K E Y W O R D S elevated CO 2 , grasslands, nitrogen deposition, phosphorus limitation, plant communities This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Rapid warming due to human-caused climate change is reshaping the Arctic, enhanced by physical processes that cause the Arctic to warm more quickly than the global average, collectively called Arctic amplification. Observations over the past 40+ years show a transition to a wetter Arctic, with seasonal shifts and widespread disturbances influencing the flora, fauna, physical systems, and peoples of the Arctic.

Rising atmospheric CO2 has stimulated plant productivity, with terrestrial ecosystems currently absorbing nearly one-third of anthropogenic CO2 emissions. Increases in photosynthesis can subsequently lead to increased carbon (C) storage in plants and soil. However, there is growing evidence that nitrogen (N) availability constrains elevated CO2 (eCO2) responses, yet we know much less about the role of phosphorus (P) limitation on productivity under eCO2. This is important because P-limited ecosystems are globally widespread, and the biogeochemical cycles of N and P differ fundamentally. In the Peak District National Park of northern England, we conducted a free-air CO2 enrichment (FACE) experiment for three years on two contrasting P-limited grasslands under long-term nutrient manipulation. Here we show that competition between plants and microbes for P can determine plant productivity responses to eCO2. In a limestone grassland, aboveground productivity increased (16%) and microbial biomass P remained unchanged, whereas in an acidic grassland, aboveground productivity and P uptake declined (11% and 20%, respectively), but P immobilization into microbial biomass increased (36%). Our results demonstrate that strong competition with microbes can cause plant P uptake to decline under eCO2, with implications for the future productivity of P-limited ecosystems in response to climate change.

The article by Hutsemékers and colleagues in this issue reveals the influences of air quality and climate change on biodiversity of epiphytic bryophytes. Biodiversity of this flora has tracked the improving air quality well since the 1980s, and air quality has improved enough so that the influence of current climate can now be seen in species distributions.