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There are many sources of uncertainty in scenarios and models of socio-ecological systems, and understanding these uncertainties is critical in supporting informed decision-making about the management of natural resources. Here, we review uncertainty across the steps needed to create socio-ecological scenarios, from narrative storylines to the representation of human and biological processes in models and the estimation of scenario and model parameters. We find that socio-ecological scenarios and models would benefit from moving away from “stylized” approaches that do not consider a wide range of direct drivers and their dependency on indirect drivers. Indeed, a greater focus on the social phenomena is fundamental in understanding the functioning of nature on a human-dominated planet. There is no panacea for dealing with uncertainty, but several approaches to evaluating uncertainty are still not routinely applied in scenario modeling, and this is becoming increasingly unacceptable. However, it is important to avoid uncertainties becoming an excuse for inaction in decision-making when facing environmental challenges.

World agriculture needs to find the right balance to cope with the trilemma between feeding a growing population, reducing its impact on biodiversity and minimizing greenhouse gas (GHG) emissions. In this paper, we evaluate a broad range of scenarios that achieve 4.3 GtCO2,eq/year GHG mitigation in the Agriculture, Forestry and Other Land-Use (AFOLU) sector by 2100. Scenarios include varying mixes of three GHG mitigation policies: second-generation biofuel production, dietary change and reforestation of pasture. We find that focusing mitigation on a single policy can lead to positive results for a single indicator of food security or biodiversity conservation, but with significant negative side effects on others. A balanced portfolio of all three mitigation policies, while not optimal for any single criterion, minimizes trade-offs by avoiding large negative effects on food security and biodiversity conservation. At the regional scale, the trade-off seen globally between biodiversity and food security is nuanced by different regional contexts.

We examine issues to consider when reframing conservation science and practice in the context of global change. New framings of the links between ecosystems and society are emerging that are changing peoples’ values and expectations of nature, resulting in plural perspectives on conservation. Reframing conservation for global change can thus be regarded as a stage in the evolving relationship between people and nature rather than some recent trend. New models of how conservation links with transformative adaptation include how decision contexts for conservation can be reframed and integrated with an adaptation pathways approach to create new options for global‐change‐ready conservation. New relationships for conservation science and governance include coproduction of knowledge that supports social learning. New processes for implementing adaptation for conservation outcomes include deliberate practices used to develop new strategies, shift world views, work with conflict, address power and intergenerational equity in decisions, and build consciousness and creativity that empower agents to act. We argue that reframing conservation for global change requires scientists and practitioners to implement approaches unconstrained by discipline and sectoral boundaries, geopolitical polarities, or technical problematization. We consider a stronger focus on inclusive creation of knowledge and the interaction of this knowledge with societal values and rules is likely to result in conservation science and practice that meets the challenges of a postnormal world.

Fil: Díaz, Sandra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Diversidad Biológica y Ecología. Instituto Multidisciplinario de Biología Vegetal; Argentina. Fil: Díaz, Sandra. Consejo Nacional de Investigaciones Científicas y Técnica. Instituto Multidisciplinario de Biología Vegetal; Argentina. Fil: Zafra Calvo, Noelia. Future Earth; España. Fil: Purvis, Andy. Natural History Museum; Reino Unido. Fil: Verburg, Peter H. VU University Amsterdam. Institute for Environmental Studies: Países Bajos. Fil: Obura, David. Coastal Oceans Research and Development Indian Ocean; Kenia. Fil: Leadley, Paul. Université Paris-Sud. Ecologie Systématique Evolution; Francia. Fil: Chaplin Kramer, Rebecca. Stanford University. Natural Capital Project; Estados Unidos. Fil: De Meester, Luc. Leibniz Institut für Gewässerökologie und Binnenfischerei; Alemania. Fil: Dulloo, Ehsan. Bioversity International; Mauricio. Fil: Martín- López, Berta. Leuphana University of Lüneburg. Faculty of Sustainability; Alemania. Fil: Shaw, Rebecca. The World Wide Fund for Nature; Estados Unidos. Fil: Visconti, Piero. International Institute for Applied Systems Analysis. Ecosystem Services and Management Program; Austria. Fil: Broadgate, Wendy. Future Earth; Suecia. Fil: Bruford, Michael W. Cardiff University. School of Biosciences and Sustainable Places Institute; Reino Unido. Fil: Burgess, Neil D. UN Environment Programme World Conservation Monitoring Centre; Reino Unido. Fil: Cavender Bares, Jeannine. University of Minnesota. Department of Ecology, Evolution and Behavior; Estados Unidos. Fil: DeClerck, Fabrice. EAT Foundation; Noruega. Fil: Fernández Palacios, José María. Universidad de La Laguna. Instituto Universitario de Enfermedades Tropicales y Salud Pública de Canarias; España. Fil: Garibaldi, Lucas A. Universidad Nacional de Río Negro. Instituto de Investigaciones en Recursos Naturales, Agroecología y Desarrollo Rural. Río Negro, Argentina. Fil: Garibaldi, Lucas A. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones en Recursos Naturales, Agroecología y Desarrollo Rural. Río Negro, Argentina. Fil: Hill, Samantha L. L. UN Environment Programme World Conservation Monitoring Centre; Reino Unido. Fil: Isbell, Forest. University of Minnesota. Department of Ecology, Evolution and Behavior; Estados Unidos. Fil: Khoury, Colin K. International Center for Tropical Agriculture; Colombia. Fil: Krug, Cornelia B. University of Zurich. Department of Geography; Suiza. Fil: Liu, Jianguo. Michigan State University; Estados Unidos. Fil: Maron, Martine. The University of Queensland; Australia. Fil: McGowan, Philip J. K. Newcastle University. School of Natural and Environmental Sciences; Reino Unido. Fil: Pereira, Henrique M. German Centre for Integrative Biodiversity Research; Alemania. Fil: Reyes García, Victoria. Catalan Institution for Research and Advanced Studies; España. Fil: Rocha, Juan. Future Earth; Suecia. Fil: Rondinini, Carlo. Sapienza University of Rome. Department of Biology and Biotechnologies. Global Mammal Assessment Program; Italia. Fil: Shannon, Lynne. University of Cape Town. Department of Biological Sciences; Sudáfrica. Fil: Shin, Yunne-Jai. University of Cape Town. Department of Biological Sciences; Sudáfrica. Fil: Snelgrove, Paul V. R. Memorial University of Newfoundland. Departments of Ocean Sciences and Biology; Canada. Fil: Spehn, Eva M. Swiss Academy of Sciences. Swiss Biodiversity Forum; Suiza. Fil: Strassburg, Bernardo. Pontifical Catholic University. Department of Geography and the Environment. Rio Conservation and Sustainability Science Centre; Brasil. Fil: Subramanian, Suneetha M. United Nations University. Institute for the Advanced Study of Sustainability; Japón. Fil: Tewksbury, Joshua J. University of Colorado; Estados Unidos. Fil: Watson, James E. M. Wildlife Conservation Society; Estados Unidos. Fil: Zanne, Amy E. George Washington University. Department of Biological Sciences; Estados Unidos. Global biodiversity policy is at a crossroads. Recent global assessments of living nature (1, 2) and climate (3) show worsening trends and a rapidly narrowing window for action. The Convention on Biological Diversity (CBD) has recently announced that none of the 20 Aichi targets for biodiversity it set in 2010 has been reached and only six have been partially achieved (4). Against this backdrop, nations are now negotiating the next generation of the CBD's global goals [see supplementary materials (SM)], due for adoption in 2021, which will frame actions of governments and other actors for decades to come. In response to the goals proposed in the draft post-2020 Global Biodiversity Framework (GBF) made public by the CBD (5), we urge negotiators to consider three points that are critical if the agreed goals are to stabilize or reverse nature's decline. First, multiple goals are required because of nature's complexity, with different facets—genes, populations, species, deep evolutionary history, ecosystems, and their contributions to people—having markedly different geographic distributions and responses to human drivers. Second, interlinkages among these facets mean that goals must be defined and developed holistically rather than in isolation, with potential to advance multiple goals simultaneously and minimize trade-offs between them. Third, only the highest level of ambition in setting each goal, and implementing all goals in an integrated manner, will give a realistic chance of stopping—and beginning to reverse—biodiversity loss by 2050.

Recent assessment reports by the Intergovernmental Panel on Climate Change (IPCC) and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) have highlighted the risks to humanity arising from the unsustainable use of natural resources. Thus far, land, freshwater, and ocean exploitation have been the chief causes of biodiversity loss. Climate change is projected to be a rapidly increasing additional driver for biodiversity loss. Since climate change and biodiversity loss impact human societies everywhere, bold solutions are required that integrate environmental and societal objectives. As yet, most existing international biodiversity targets have overlooked climate change impacts. At the same time, climate change mitigation measures themselves may harm biodiversity directly. The Convention on Biological Diversity’s post-2020 framework offers the important opportunity to address the interactions between climate change and biodiversity and revise biodiversity targets accordingly by better aligning these with the United Nations Framework Convention on Climate Change Paris Agreement and the Sustainable Development Goals. We identify the considerable number of existing and proposed post-2020 biodiversity targets that risk being severely compromised due to climate change, even if other barriers to their achievement were removed. Our analysis suggests that the next set of biodiversity targets explicitly addresses climate change-related risks since many aspirational goals will not be feasible under even lower-end projections of future warming. Adopting more flexible and dynamic approaches to conservation, rather than static goals, would allow us to respond flexibly to changes in habitats, genetic resources, species composition, and ecosystem functioning and leverage biodiversity’s capacity to contribute to climate change mitigation and adaptation. We acknowledge the support by Rainer Krug, Hien Ngo, and Max Gueze in parts of the literature review. Pierre Lopez drew Fig. 1, using spatial data layers provided by Aidin Niamir (https://doi.org/10.5281/zenodo.3975694).

Scenario-based modelling is a powerful tool to describe relationships between plausible trajectories of drivers, possible policy interventions, and impacts on biodiversity and ecosystem services. Model inter-comparisons are key in quantifying uncertainties and identifying avenues for model improvement but have been missing among the global biodiversity and ecosystem services modelling communities. The biodiversity and ecosystem services scenario-based inter-model comparison (BES-SIM) aims to fill this gap. We used global land-use and climate projections to simulate possible future impacts on terrestrial biodiversity and ecosystem services using a variety of models and a range of harmonized metrics. The goal of this paper is to reflect on the steps taken in BES-SIM, identify remaining methodological challenges, and suggest pathways for improvement. We identified five major groups of challenges; the need to: 1) better account for the role of nature in future human development storylines; 2) improve the representation of drivers in the scenarios by increasing the resolution (temporal, spatial and thematic) of land-use as key driver of biodiversity change and including additional relevant drivers; 3) explicitly integrate species- and trait-level biodiversity in ecosystem services models; 4) expand the coverage of the multiple dimensions of biodiversity and ecosystem services; and finally, 5) incorporate time-series or one-off historical data in the calibration and validation of biodiversity and ecosystem services models. Addressing these challenges would allow the development of more integrated global projections of biodiversity and ecosystem services, thereby improving their policy relevance in supporting the interlinked international conservation and sustainable development agendas.

Aims In human-dominated ecosystems, the presence of a given species is the result of both the ecological suitability of the site and human impacts such as land-use choices. The influence of land-use choices on the predictions of species distribution models (SDMs) has, however, been often neglected. Here, we provide a theoretical analysis of the land-use selection bias affecting classical SDMs in the case of either presence-only or presence–absence datasets. Land-use selection bias in the predictions of SDMs is then quantified for four widespread European tree species. Location Continental France. Methods We describe a bivariate selection model (BSM) that estimates simultaneously the economics of land-use choices and species responses to bioclimatic variables. The land-use equation, based on an econometric model of landowner choices, is joined to an equation of species responses to bioclimatic variables. Results We found a significant land-use selection bias in all the species studied. The sign and the magnitude of the bias varied among species and were strongly related to the type of dataset used in the SDM calibration (presence-only or presence–absence). In addition, the BSM estimates the spatial covariance between the probability of presence and the presence of compatible land use. We found that, depending on the species, sites with high ecological suitability could present a high probability of compatible land use (positive covariance) or a low probability (negative covariance). Main conclusion We showed that the use of classical SDMs in human-dominated areas can lead to strong miss-estimations of actual species distributions and could therefore prevents sound projections of the effects of climate change. The proposed BSM represents a crucial step to account for the economic forces shaping species distribution in anthropized areas and paves the way for a direct assessment of trade-offs and opportunities that may arise in a context of global change.

A major challenge today and into the future is to maintain or enhance beneficial contributions of nature to a good quality of life for all people. This is among the key motivations of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), a joint global effort by governments, academia, and civil society to assess and promote knowledge of Earth's biodiversity and ecosystems and their contribution to human societies in order to inform policy formulation. One of the more recent key elements of the IPBES conceptual framework (1) is the notion of nature's contributions to people (NCP), which builds on the ecosystem service concept popularized by the Millennium Ecosystem Assessment (MA) (2). But as we detail below, NCP as defined and put into practice in IPBES differs from earlier work in several important ways. First, the NCP approach recognizes the central and pervasive role that culture plays in defining all links between people and nature. Second, use of NCP elevates, emphasizes, and operationalizes the role of indigenous and local knowledge in understanding nature's contribution to people.

Global biodiversity targets have far-reaching implications for nature conservation worldwide. Scenarios and models hold unfulfilled promise for ensuring such targets are well founded and implemented; here, we review how they can and should inform the Aichi Targets of the Strategic Plan for Biodiversity and their reformulation. They offer two clear benefits: providing a scientific basis for the wording and quantitative elements of targets; and identifying synergies and trade-offs by accounting for interactions between targets and the actions needed to achieve them. The capacity of scenarios and models to address complexity makes them invaluable for developing meaningful targets and policy, and improving conservation outcomes.

Aim To analyse global patterns of climate during the mid-Holocene and conduct comparisons with pre-industrial and projected future climates. In particular, to assess the exposure of terrestrial biomes and ecoregions to climate-related risks during the Holocene–Anthropocene transition starting at the pre-industrial period. Location Terrestrial ecosystems of the Earth. Methods We calculated long-term climate differences (anomalies) between the mid-Holocene (6 ka cal bp, mH), pre-industrial conditions and projections for 2100 (middle-strength A1B scenario) using six global circulation models available for all periods. Climate differences were synthesized with multivariate statistics and average principal component loadings of temperature and precipitation differences (an estimate of climate-related risks) were calculated on 14 biomes and 766 ecoregions. Results Our results suggest that most of the Earth's biomes will probably undergo changes beyond the mH recorded levels of community turnover and range shifts because the magnitude of climate anomalies expected in the future are greater than observed during the mH. A few biomes, like the remnants of North American and Euro-Asian prairies, may experience only slightly greater degrees of climate change in the future as compared with the mH. In addition to recent studies that have identified equatorial regions as the most sensitive to future climate change, we find that boreal forest, tundra and vegetation of the Equatorial Andes could be at greatest risk, since these regions will be exposed to future climates that are well outside natural climate variation during the Holocene. Conclusions The Holocene–Anthropocene climate transition, even for a middle-strength future climate change scenario, appears to be of greater magnitude and different from that between the mH and the pre-industrial period. As a consequence, community- and biome-level changes due to of expected climate change may be different in the future from those observed during the mH.

International audience; Climate change is a threat to biodiversity, and adaptation measures should be considered in biodiversity conservation planning. Protected areas (PA) are expected to be impacted by climate change and improving their connectivity with biological corridors (BC) has been proposed as a potential adaptation measure, although assessing its effectiveness remains a challenge. In Mesoamerica, efforts to preserve the biodiversity have led to the creation of a regional network of PA and, more recently, BC. This study evaluates the role of BC for facilitating plant dispersal between PA under climate change in Mesoamerica. A spatially explicit dynamic model (cellular automaton) was developed to simulate species dispersal under different climate and conservation policy scenarios. Plant functional types (PFT) were defined based on a range of dispersal rates and vegetation types to represent the diversity of species in the region. The impacts of climate change on PA and the role of BC for dispersal were assessed spatially. Results show that most impacted PA are those with low altitudinal range in hot, dry, or high latitude areas. PA with low altitudinal range in high cool areas benefit the most from corridors. The most important corridors cover larger areas and have high altitude gradients. Only the fastest PFT can keep up with the expected change in climate and benefit from corridors for dispersal. We conclude that the spatial assessment of the vulnerability of PA and the role of corridors in facilitating dispersal can help conservation planning under a changing climate.

Improving understanding of how biodiversity and ecosystems respond to environmental change is necessary to guide policy and management. To this end, the bioDISCOVERY project of the international programme on global change, Future Earth, initiates and supports international networks of scientists to advance research on monitoring and observations, scenarios and models, and assessments of biodiversity and ecosystems. bioDISCOVERY activities seek collective solutions to key research challenges, and provide support for the international science community by participating in the development of global databases. This global working-group approach is essential for directing cutting-edge science toward supporting international policies, addressing urgent environmental issues, and closing research gaps through transdisciplinary integration and mobilisation of the scientific community.

Scenarios are powerful tools toenvision how nature might respondto different pathways of futurehuman development and policy choices1.Most scenarios developed for globalenvironmental assessments have exploredimpacts of society on nature, such asbiodiversity loss, but have not includednature as a component of socioeconomicdevelopment2. They ignore policyobjectives related to nature protectionand neglect nature?s role in underpinningdevelopment and human well-being.This approach is becoming untenablebecause targets for human developmentare increasingly connected with targetsfor nature, such as in the United Nations?Sustainable Development Goals. The nextgeneration of scenarios should explorealternative pathways to reach theseintertwined targets, including potentialsynergies and trade-offs between natureconservation and other developmentgoals, as well as address feedbacks betweennature, nature?s contributions to people,and human well-being. The developmentof these scenarios would benefit from theuse of participatory approaches, integratingstakeholders from multiple sectors (forexample, fisheries, agriculture, forestry)and should address decision-makersfrom the local to the global scale3, therebysupporting assessments being undertakenby the Intergovernmental Platformon Biodiversity and EcosystemServices (IPBES). Fil: Rosa, Isabel M. D.. German Centre For Integrative Biodiversity Research; Alemania. Martin-universitat Halle-wittenberg; Alemania Fil: Pereira, Henrique M.. German Centre For Integrative Biodiversity Research; Alemania. Martin-universitat Halle-wittenberg; Alemania. Universidad de Porto; Portugal Fil: Ferrier, Simon. Csiro Land And Water; Australia Fil: Alkemade, Rob. Pbl Netherlands Environmental Assessment Agency; Países Bajos Fil: Acosta, Lilibeth A.. University Of The Philippines Los Banos; Filipinas Fil: Akcakaya, H. Resit. Stony Brook University; Estados Unidos Fil: Den Belder, Eefje. Pbl Netherlands Environmental Assessment Agency; Países Bajos. Wageningen University And Research Centre; Países Bajos Fil: Fazel, Asghar M.. Eco Institute Of Environmental Science And Technology; Irán. University Of Environment; Irán Fil: Fujimori, Shinichiro. International Institute For Applied Systems Analysis; Austria. National Institute For Environmental Studies Of Japan; Japón Fil: Harfoot, Mike. United Nations Environment Programme World Conservation Monitoring Centre; Reino Unido Fil: Harhash, Khaled A.. Egyptian Environmental Affairs Agency; Egipto Fil: Harrison, Paula A.. Lancaster University; Reino Unido Fil: Hauck, Jennifer. Coknow Consulting; Alemania. Helmholtz Centre for Environmental Research; Alemania Fil: Hendriks, Rob J. J.. Ministry Of Economic Affairs; Países Bajos Fil: Hernández, Gladys. Centre For World Economy Studies; Cuba Fil: Jetz, Walter. Imperial College London; Reino Unido. University of Yale; Estados Unidos Fil: Karlsson Vinkhuyzen, Sylvia I.. Wageningen University And Research Centre; Países Bajos Fil: Kim, Hyejin. National Institute Of Ecology; Corea del Sur Fil: King, Nicholas. North-west University; Sudáfrica Fil: Kok, Marcel T. J.. Pbl Netherlands Environmental Assessment Agency; Países Bajos Fil: Kolomytsev, Grygoriy O.. Schmalhausen Institute of Zoology of National Academy of Sciences of Ukrain; Ucrania Fil: Lazarova, Tanya. Pbl Netherlands Environmental Assessment Agency; Países Bajos Fil: Leadley, Paul. Université Paris Sud; Francia Fil: Lundquist, Carolyn J.. National Institute Of Water And Atmospheric Research; Nueva Zelanda. The University of Auckland; Nueva Zelanda Fil: García Márquez, Jaime. Humboldt-Universität zu Berlin; Alemania Fil: Meyer, Carsten. German Centre For Integrative Biodiversity Research; Alemania. University of Yale; Estados Unidos Fil: Navarro, Laetitia M.. German Centre For Integrative Biodiversity Research; Alemania. Martin-universitat Halle-wittenberg; Alemania Fil: Nesshöver, Carsten. Helmholtz Centre for Environmental Research; Alemania. German Centre for Integrative Biodiversity Research; Alemania Fil: Ngo, Hien T.. Ipbes Secretariat; Alemania Fil: Ninan, Karachepone N.. Centre For Economics, Environment And Society; India Fil: Palomo, Maria Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; Argentina Fil: Pereira, Laura M.. Stockholms Universitet; Suecia Fil: Peterson, Garry D.. Stockholms Universitet; Suecia Fil: Pichs, Ramon. Centre For World Economy Studies; Cuba Fil: Popp, Alexander. Potsdam Institute for Climate Impact Research; Alemania Fil: Purvis, Andy. Natural History Museum; Reino Unido Fil: Ravera, Federica. Consejo Superior de Investigaciones Científicas. Centre de Recerca Ecológica I Aplicacions Forestals; España. University Of Évora; Portugal. Universitat Central De Catalunya; España Fil: Rondinini, Carlo. Università degli studi di Roma "La Sapienza"; Italia Fil: Sathyapalan, Jyothis. Centre For Economic And Social Studies; India Fil: Schipper, Aafke M.. Pbl Netherlands Environmental Assessment Agency; Países Bajos Fil: Seppelt, Ralf. Coknow Consulting; Alemania. Martin Luther University Halle-Wittenberg; Alemania Fil: Settele, Josef. German Centre for Integrative Biodiversity Research; Alemania. Helmholtz Centre for Environmental Research; Alemania Fil: Sitas, Nadia. The Council For Scientific And Industrial Research; Sudáfrica Fil: Van Vuuren, Detlef. Pbl Netherlands Environmental Assessment Agency; Países Bajos

Model-based projections of shifts in tree species range due to climate change are becoming an important decision support tool for forest management. However, poorly evaluated sources of uncertainty require more scrutiny before relying heavily on models for decision-making. We evaluated uncertainty arising from differences in model formulations of tree response to climate change based on a rigorous intercomparison of projections of tree distributions in France. We compared eight models ranging from niche-based to processbased models. On average, models project large range contractions of temperate tree species in lowlands due to climate change. There was substantial disagreement between models for temperate broadleaf deciduous tree species, but differences in the capacity of models to account for rising CO2 impacts explained much of the disagreement. There was good quantitative agreement among models concerning the range contractions for Scots pine. For the dominant Mediterranean tree species, Holm oak, all models foresee substantial range expansion.

Plant traits – the morphological, anatomical, physiological, biochemical and phenological characteristics of plants and their organs – determine how primary producers respond to environmental factors, affect other trophic levels, influence ecosystem processes and services and provide a link from species richness to ecosystem functional diversity. Trait data thus represent the raw material for a wide range of research from evolutionary biology, community and functional ecology to biogeography. Here we present the global database initiative named TRY, which has united a wide range of the plant trait research community worldwide and gained an unprecedented buy-in of trait data: so far 93 trait databases have been contributed. The data repository currently contains almost three million trait entries for 69 000 out of the world's 300 000 plant species, with a focus on 52 groups of traits characterizing the vegetative and regeneration stages of the plant life cycle, including growth, dispersal, establishment and persistence. A first data analysis shows that most plant traits are approximately log-normally distributed, with widely differing ranges of variation across traits. Most trait variation is between species (interspecific), but significant intraspecific variation is also documented, up to 40% of the overall variation. Plant functional types (PFTs), as commonly used in vegetation models, capture a substantial fraction of the observed variation – but for several traits most variation occurs within PFTs, up to 75% of the overall variation. In the context of vegetation models these traits would better be represented by state variables rather than fixed parameter values. The improved availability of plant trait data in the unified global database is expected to support a paradigm shift from species to trait-based ecology, offer new opportunities for synthetic plant trait research and enable a more realistic and empirically grounded representation of terrestrial vegetation in Earth system models.

Biological monitoring has a long history in freshwaters, where much of the pioneering work in this field was developed over a 100 years ago—but few of the traditional monitoring tools provide the global perspective on biodiversity loss and its consequences for ecosystem functioning that are now needed. Rather than forcing existing monitoring paradigms to respond to questions they were never originally designed to address, we need to take a step back and assess the prospects for novel approaches that could be developed and adopted in the future. To resolve some of the issues with indicators currently used to inform policymakers, we highlight new biological monitoring tools that are being used, or could be developed in the near future, which (1) consider less-studied taxonomic groups, (2) are standardised across regions to allow global comparisons, and (3) measure change over multiple time points. The new tools we suggest make use of some of the key technological and logistical advances seen in recent years—including remote sensing, molecular tools, and local-to-global citizen science networks. We recommend that these new indicators should be considered in future assessments of freshwater ecosystem health and contribute to the evidence base for global to regional (and national) assessments of biodiversity and ecosystem services: for example, within the emerging framework of the Intergovernmental Platform on Biodiversity and Ecosystem Services.

The objective of this study was to determine what patterns, if any, existed in the response of nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), soil microbial N and soil inorganic N to elevated CO2 across a broad range of grassland environments. We studied the response of these N pools and microbial activities in four CO2-enrichment sites of the MEGARICH project (Managing European Grasslands as a Sustainable Resource in a Changing Climate). CO2 treatment was studied in factorial combination with a cutting frequency treatment at two sites and with a temperature treatment at one site. Our study showed that microbial biomass N, NEA, DEA and extractable soil [NH4 1 ] and [NO3 ] were generally not affected by elevated CO2 in these grassland ecosystems after several years of treatment, nor by cutting frequency or temperature at the sites that included these treatments. Exceptions to this were that DEA and soil [NO3 ] decreased by 22% and 45%, respectively, at the French site at elevated CO2. We discuss the possible explanations for this lack of response.

The LPJ-GUESS dynamic vegetation model uniquely combines an individual- and patch-based representation of vegetation dynamics with ecosystem biogeochemical cycling from regional to global scales. We present an updated version that includes plant and soil N dynamics, analysing the implications of accounting for C–N interactions on predictions and performance of the model. Stand structural dynamics and allometric scaling of tree growth suggested by global databases of forest stand structure and development were well reproduced by the model in comparison to an earlier multi-model study. Accounting for N cycle dynamics improved the goodness of fit for broadleaved forests. N limitation associated with low N-mineralisation rates reduces productivity of cold-climate and dry-climate ecosystems relative to mesic temperate and tropical ecosystems. In a model experiment emulating free-air CO2 enrichment (FACE) treatment for forests globally, N limitation associated with low N-mineralisation rates of colder soils reduces CO2 enhancement of net primary production (NPP) for boreal forests, while some temperate and tropical forests exhibit increased NPP enhancement. Under a business-as-usual future climate and emissions scenario, ecosystem C storage globally was projected to increase by ca. 10%; additional N requirements to match this increasing ecosystem C were within the high N supply limit estimated on stoichiometric grounds in an earlier study. Our results highlight the importance of accounting for C–N interactions in studies of global terrestrial N cycling, and as a basis for understanding mechanisms on local scales and in different regional contexts.

DIVERSITAS, the international programme on biodiversity science, is releasing a strategic vision presenting scientific challenges for the next decade of research on biodiversity and ecosystem services: “Biodiversity and Ecosystem Services Science for a Sustainable Planet”. This new vision is a response of the biodiversity and ecosystem services scientific community to the accelerating loss of the components of biodiversity, as well as to changes in the biodiversity science-policy landscape (establishment of a Biodiversity Observing Network — GEO BON, of an Intergovernmental science-policy Platform on Biodiversity and Ecosystem Services — IPBES, of the new Future Earth initiative; and release of the Strategic Plan for Biodiversity 2011–2020). This article presents the vision and its core scientific challenges. Fil: Larigauderie, Anne. DIVERSITAS. Muséum National d’Histoire Naturelle; Francia Fil: Prieur Richard, Anne Helene. DIVERSITAS. Muséum National d’Histoire Naturelle; Francia Fil: Mace, Georgina. Imperial College London. Center for Population Biology; Reino Unido Fil: Londsdale, Mark. CSIRO Ecosystem Sciences; Australia Fil: Mooney, Harold A.. Stanford University. Department of Biological Sciences; Estados Unidos Fil: Brussaard, Lijbert. Wageningen University, Soil Quality Department; Países Bajos Fil: Cooper, David. Secretariat of the Convention on Biological Diversity; Canadá Fil: Wolfgang, Cramer. Institut Méditerranéen de Biodiversité et d’Ecologie marine et continentale; Francia Fil: Daszak, Peter. EcoHealth Alliance. Wildlife Trust; Estados Unidos Fil: Diaz, Sandra Myrna. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; Argentina Fil: Duraiappah, Anantha. International Human Dimensions Programme; Alemania Fil: Elmqvist, Thomas. University of Stockholm. Department of Systems Ecology and Stockholm Resilience Center; Suecia Fil: Faith, Daniel. The Australian Museum; Australia Fil: Jackson, Louise. University of California; Estados Unidos Fil: Krug, Cornelia. DIVERSITAS. Muséum National d’Histoire Naturelle; Francia Fil: Leadley, Paul. Université Paris. Laboratoire Ecologie Systématique Evolution, Ecologie des Populations et Communautés; Francia Fil: Le Prestre, Philippe. Laval University; Canadá Fil: Matsuda, Hiroyuki. Yokohama National University; Japón Fil: Palmer, Margaret. University of Maryland; Estados Unidos Fil: Perrings, Charles. Arizona State University; Estados Unidos Fil: Pulleman, Mirjam. Wageningen University; Países Bajos Fil: Reyers, Belinda. Natural Resources and Environment; Sudáfrica Fil: Rosa, Eugene A.. Washington State University; Estados Unidos Fil: Scholes, Robert J.. Natural Resources and Environment; Sudáfrica Fil: Spehn, Eva. Universidad de Basilea; Suiza Fil: Turner II, B. L.. Arizona State University; Estados Unidos Fil: Yahara, Tetsukazu. Kyushu University; Japón

International audience; Global environmental changes are expected to alter ecosystem carbon and nitrogen cycling, but the interactive effects of multiple simultaneous environmental changes are poorly understood. Effects of these changes on the production of nitrous oxide (N2O), an important greenhouse gas, could accelerate climate change. We assessed the responses of soil N2O fluxes to elevated CO2, heat, altered precipitation, and enhanced nitrogen deposition, as well as their interactions, in an annual grassland at the Jasper Ridge Global Change Experiment (CA, USA). Measurements were conducted after 6, 7 and 8 years of treatments. Elevated precipitation increased N2O efflux, especially in combination with added nitrogen and heat. Path analysis supported the idea that increased denitrification due to increased soil water content and higher labile carbon availability best explained increased N2O efflux, with a smaller, indirect contribution from nitrification. In our data and across the literature, single-factor responses tended to overestimate interactive responses, except when global change was combined with disturbance by fire, in which case interactive effects were large. Thus, for chronic global environmental changes, higher order interactions dampened responses of N2O efflux to multiple global environmental changes, but interactions were strongly positive when global change was combined with disturbance. Testing whether these responses are general should be a high priority for future research.

Plant growth and development is dependent on factors internal and external to the plant. The effect of nutrient on root morphology has been widely studied in artificial soils or solution cultures. However, the effect of this experimental procedure has never been assessed by comparing plant response in artificial and natural soils. In a laboratory experiment, plants were submitted to three levels of nitrogen supply, in one artificial substratum made of pure sand and one natural soil. Deep differences in plant homeostasis, growth and development were observed between natural and artificial soils: (i) the chemical composition of plant tissues (%C and %N) was changing according to the nitrogen gradient in the artificial substratum, whereas it remained constant in the natural soil; (ii) the same amount of added nitrogen was responsible for a 17% increase in total plant production in the natural soil as compared with the artificial substratum; (iii) plant development differed deeply in the two substrata: in the artificial substratum, the shoot to root ratio was stable whereas the thick to fine roots ratio was increasing with increasing N supply, suggesting an exploration strategy to found an absent limiting nutrient. Conversely, in the natural soil, the thick to fine roots ratio was stable whereas the shoot to root ratio was decreasing with increasing N supply, suggesting an exploitation strategy dedicated to extract a limiting nutrient present in soil components. Even if mechanisms responsible for changes in homeostasis, growth and development were not elucidated, these results stress the importance to consider substrata with a minimal level of complexity if we expect to understand plant growth and development in natural conditions thanks to laboratory experiments.

Background: Little is known about the combined impacts of global environmental changes and ecological disturbances on ecosystem functioning, even though such combined impacts might play critical roles in shaping ecosystem processes that can in turn feed back to climate change, such as soil emissions of greenhouse gases.[br/] Methodology/Principal Findings: We took advantage of an accidental, low-severity wildfire that burned part of a long-term global change experiment to investigate the interactive effects of a fire disturbance and increases in CO(2) concentration, precipitation and nitrogen supply on soil nitrous oxide (N(2)O) emissions in a grassland ecosystem. We examined the responses of soil N(2)O emissions, as well as the responses of the two main microbial processes contributing to soil N(2)O production - nitrification and denitrification - and of their main drivers. We show that the fire disturbance greatly increased soil N(2)O emissions over a three-year period, and that elevated CO(2) and enhanced nitrogen supply amplified fire effects on soil N(2)O emissions: emissions increased by a factor of two with fire alone and by a factor of six under the combined influence of fire, elevated CO(2) and nitrogen. We also provide evidence that this response was caused by increased microbial denitrification, resulting from increased soil moisture and soil carbon and nitrogen availability in the burned and fertilized plots. [br/] Conclusions/Significance: Our results indicate that the combined effects of fire and global environmental changes can exceed their effects in isolation, thereby creating unexpected feedbacks to soil greenhouse gas emissions. These findings highlight the need to further explore the impacts of ecological disturbances on ecosystem functioning in the context of global change if we wish to be able to model future soil greenhouse gas emissions with greater confidence.

International audience; Responses of soil nitrogen (N) cycling to simultaneous and potentially interacting global environmental changes are uncertain. Here, we investigated the combined effects of elevated CO2, warming, increased precipitation and enhanced N supply on soil N cycling in an annual grassland ecosystem as part of the Jasper Ridge Global Change Experiment (CA, USA). This field experiment included four treatments—CO2, temperature, precipitation, nitrogen—with two levels per treatment (ambient and elevated), and all their factorial combinations replicated six times. We collected soil samples after 7 and 8 years of treatments, and measured gross rates of N mineralization, N immobilization and nitrification, along with potential rates of ammonia oxidation, nitrite oxidation and denitrification. We also determined the main drivers of these microbial activities (soil ammonium and nitrate concentrations, soil moisture, soil temperature, soil pH, and soil CO2 efflux, as an indicator of soil heterotrophic activity). We found that gross N mineralization responded to the interactive effects of the CO2, precipitation and N treatments: N addition increased gross N mineralization when CO2 and precipitation were either both at ambient or both at elevated levels. However, we found limited evidence for interactions among elevated CO2, warming, increased precipitation, and enhanced N supply on the other N cycling processes examined: statistically significant interactions, when found, tended not to persist across multiple dates. Soil N cycling responded mainly to single-factor effects: long-term N addition increased gross N immobilization, potential ammonia oxidation and potential denitrification, while increased precipitation depressed potential nitrite oxidation and increased potential ammonia oxidation and potential denitrification. In contrast, elevated CO2 and modest warming did not significantly affect any of these microbial N transformations. These findings suggest that global change effects on soil N cycling are primarily additive, and therefore generally predictable from single factor studies.

Assessing Biodiversity Declines Understanding human impact on biodiversity depends on sound quantitative projection. Pereira et al. (p. 1496 , published online 26 October) review quantitative scenarios that have been developed for four main areas of concern: species extinctions, species abundances and community structure, habitat loss and degradation, and shifts in the distribution of species and biomes. Declines in biodiversity are projected for the whole of the 21st century in all scenarios, but with a wide range of variation. Hoffmann et al. (p. 1503 , published online 26 October) draw on the results of five decades' worth of data collection, managed by the International Union for Conservation of Nature Species Survival Commission. A comprehensive synthesis of the conservation status of the world's vertebrates, based on an analysis of 25,780 species (approximately half of total vertebrate diversity), is presented: Approximately 20% of all vertebrate species are at risk of extinction in the wild, and 11% of threatened birds and 17% of threatened mammals have moved closer to extinction over time. Despite these trends, overall declines would have been significantly worse in the absence of conservation actions.

International audience; Future climate scenarios predict simultaneous changes in environmental conditions, but the impacts of multiple climate change drivers on ecosystem structure and function remain unclear. We used a novel experimental approach to examine the responses of an upland grassland ecosystem to the 2080 climate scenario predicted for the study area (3.5A degrees C temperature increase, 20% reduction in summer precipitation, atmospheric CO2 levels of 600 ppm) over three growing seasons. We also assessed whether patterns of grassland response to a combination of climate change treatments could be forecast by ecosystem responses to single climate change drivers. Effects of climate change on aboveground production showed considerable seasonal and interannual variation; April biomass increased in response to both warming and the simultaneous application of warming, summer drought, and CO2 enrichment, whereas October biomass responses were either non-significant or negative depending on the year. Negative impacts of summer drought on production were only observed in combination with a below-average rainfall regime, and showed lagged effects on spring biomass. Elevated CO2 had no significant effect on aboveground biomass during this study. Both warming and the 2080 climate change scenario were associated with a significant advance in flowering time for the dominant grass species studied. However, flowering phenology showed no significant response to either summer drought or elevated CO2. Species diversity and equitability showed no response to climate change treatments throughout this study. Overall, our data suggest that single-factor warming experiments may provide valuable information for projections of future ecosystem changes in cool temperate grasslands.

Despite increasing interest in the effects of climate change on soil processes, the response of nitrification to elevated CO2 remains unclear. Responses may depend on soil nitrogen (N) status, and inferences may vary depending on the methodological approach used. We investigated the interactive effects of elevated CO2 and inorganic N supply on gross nitrification (using 15N pool dilution) and potential nitrification (using nitrifying enzyme activity assays) in Dactylis glomerata mesocosms. We measured the responses of putative drivers of nitrification (NH 4 + production, NH 4 + consumption, and soil environmental conditions) and of potential denitrification, a process functionally linked to nitrification. Gross nitrification was insensitive to all treatments, whereas potential nitrification was higher in the high N treatment and was further stimulated by elevated CO2 in the high N treatment. Gross mineralization and NH 4 + consumption rates were also significantly increased in response to elevated CO2 in the high N treatment, while potential denitrification showed a significant increase in response to N addition. The discrepancy between the responses of gross and potential nitrification to elevated CO2 and inorganic N supply suggest that these measurements provide different information, and should be used as complementary approaches to understand nitrification response to global change.

International audience; Plant-plant and plant-soil interactions play a key role in determining plant community structure and ecosystem function. However, the effects of global change on the interplay between co-occurring plants and soil microbes in successional communities are poorly understood. In this study, we investigated competition for nitrogen (N) between soil microorganisms, grass plants and establishing tree seedlings under factorial carbon dioxide (CO2) and N treatments. Fruxinus excelsior seedlings were germinated in the presence or absence of grass competition (Dacrylis glomerata) at low (380 mu mol mol(-1)) or high (645 mu mol mol(-1)) CO2 and at two levels of N nutrition in a mesocosm experiment. Pulse N-15 labelling was used to examine N partitioning among plant and soil compartments. Dactylis exerted a strong negative effect on Fraxinus biomass, N capture and N-15 recovery irrespective of N and CO2 treatment. In contrast, the presence of Dactylis had a positive effect on the microbial N pool. Plant and soil responses to N treatment were of a greater magnitude compared with responses to elevated CO2, but the pattern of Fraxinus- and microbial-N pool response to N and CO2 varied depending on grass competition treatment. Within the Dactylis competition treatment, decreases in Fraxinus biomass in response to N were not mirrored by decreases in tree seedling N content, suggesting a shift from below- to above-ground competition. In the Dactylis-sown pots, N-15 recovery could be ranked Dactylis > microbial pool > Fraxinus in all N and CO2 treatment combinations. Inequalities between Fraxinus and soil microorganisms in terms of N-15 recovery were exacerbated by N addition. Contrary to expectations, elevated CO2 did not increase plant-microbe competition. Nevertheless, microbial N-15 recovery showed a small positive increase in the high CO2 treatment. Overall, elevated CO2 and N supply did not interact on plant/soil N partitioning. Our data suggest that the competitive balance between establishing tree seedlings and grass plants in an undisturbed sward is relatively insensitive to CO2 or N-induced modifications in N competition between plant and soil compartments.

International audience; Plants modify nutrient availability by releasing chemicals in the rhizosphere. This change in availability induced by roots (bioavailability) is known to improve nutrient uptake by individual plants releasing such compounds. Can this bioavailability alter plant competition for nutrients and under what conditions? To address these questions, we have developed a model of nutrient competition between plant species based on mechanistic descriptions of nutrient diffusion, plant exudation, and plant uptake. The model was parameterized using data of the effects of root citrate exudation on phosphorus availability. We performed a sensitivity analysis for key parameters to test the generality of these effects. Our simulations suggest the following. ( 1) Nutrient uptake depends on the number of roots when nutrients and exudates diffuse little, because individual roots are nearly independent in terms of nutrient supply. In this case, bioavailability profits only species with exudates. ( 2) Competition for nutrients depends on the spatial arrangement of roots when nutrients diffuse little but exudates diffuse widely. ( 3) Competition for nutrients depends on the nutrient uptake capacity of roots when nutrients and exudates diffuse widely. In this case, bioavailability profits all species. Mechanisms controlling competition for bioavailable nutrients appear to be diverse and strongly depend on soil, nutrient, and plant properties.

International audience; The competitive interactions between woody seedlings and herbaceous vegetation have received increasing interest in recent years. However, little is known about the relative contributions and underlying mechanisms of above- and below-ground competition between species. We used a novel experimental approach to assess the responses of Fraxinus excelsior seedlings to different combinations of root and shoot competition imposed by the grass Dactylis glomerata under greenhouse conditions. Seedling growth was significantly reduced by competition for soil resources, but neither biomass nor height were significantly affected by shoot competition for light. Competitive response indices based on biomass confirmed that below-ground competition was more important than above-ground competition, and indicated that root and shoot competition did not interact to influence plant growth. Fraxinus biomass allocation and seedling traits were almost all significantly affected by root competition; these responses varied depending on the trait examined. In contrast, morphological responses to shoot competition were limited. In the absence of root competition, seedlings showed a significant increase in the biomass allocated to leaves and a greater leaf area ratio in response to shoot competition. Our findings suggest that morphological modifications help to mitigate the negative effects of competition, but the expression of plasticity may be suboptimal due to resource constraints. The present study also highlights the importance of appropriate experimental controls and analysis to avoid confounding effects of experimental design and ontogeny on the interpretation of competitive responses.

We have developed a spatially explicit model of plant root and soil bacteria interactions in the rhizosphere in order to formalise and study the microbial loop hypothesis that postulates that plants can stimulate the release of mineral N from the soil organic matter by providing low molecular weight C molecules to C-limited microorganisms able to liberate into the soil enzymes that degrade the organic matter. The model is based on a mechanistic description of diffusion of solutes in the soil, nutrient uptake by plants, bacterial activity and bacterial predation. Modelled soil bacterial populations grow, mediate transformations among several forms of nitrogen (mineral and organic) and compete for nitrogen with plants. Our objectives were to see if we could simulate the stimulation of turnover of the microbial loop by exudates and to study the effects of diffusion of C and N in the rhizosphere on these different processes. The model qualitatively mimics most of the characteristics of the microbial loop hypothesis. In particular, (1) plant exudates increase the growth of bacteria in the soil and (2) increase the degradation of soil organic matter and N mineralisation. (3) The increased bacterial biomass induces an increase in predator biomass and, as a result, (4) plant mineral N uptake is increased threefold compared with scenarios without exudation. However, the temporal dynamics simulated by the model are much slower than observed dynamics (the increase in uptake appears after a few months). Taking into consideration the diffusion of C and N containing molecules in soil has large effects on the spatial structure of the bacterial and predator biomass. However, the average biomass of bacteria and predators, N mineralisation and plant N uptake were not affected by these properties. The model provides a quantitative and mechanistic explanation of how plants could benefit from liberating low molecular organic matter and the subsequent stimulation of the microbial loop and increases N mineralisation.

International audience; The effects of global change on below-ground processes of the nitrogen (N) cycle have repercussions for plant communities, productivity and trace gas effluxes. However, the interacting effects of different components of global change on nitrification or denitrification have rarely been studied in situ. 2. We measured responses of nitrifying enzyme activity (NEA) and denitrifying enzyme activity (DEA) to over 4 years of exposure to several components of global change and their interaction (increased atmospheric CO2 concentration, temperature, precipitation and N addition) at peak biomass period in an annual grassland ecosystem. In order to provide insight into the mechanisms controlling the response of NEA and DEA to global change, we examined the relationships between these activities and soil moisture, microbial biomass C and N, and soil extractable N. 3. Across all treatment combinations, NEA was decreased by elevated CO2 and increased by N addition. While elevated CO2 had no effect on NEA when not combined with other treatments, it suppressed the positive effect of N addition on NEA in all the treatments that included N addition. We found a significant CO2-N interaction for DEA, with a positive effect of elevated CO2 on DEA only in the treatments that included N addition, suggesting that N limitation of denitrifiers may have occurred in our system. Soil water content, extractable N concentrations and their interaction explained 74% of the variation in DEA. 4. Our results show that the potentially large and interacting effects of different components of global change should be considered in predicting below-ground N responses of Mediterranean grasslands to future climate changes.

We studied the symmetry of belowground competition at the level of species using a model of plant competition at the scale of individual roots. This model predicts the nutrient uptake of plants with varying root densities and uptake kinetics, competing for the same resource, at equilibrium between resource supply and plant uptake. A symmetry parameter is derived from this model. The analysis of the model behaviour suggests that the symmetry of belowground competition depends on the diffusive supply of the nutrient (buffer capacity × effective diffusion coefficient) for which competition occurs. As the diffusive supply is determined to a greater extent by soil water content and soil type rather than by nutrient type, this means that the symmetry of belowground competition should vary with soil water content and soil type. In particular, our model suggests that competition for nutrients that are primarily supplied by diffusion through the soil is (1) symmetrical at high diffusive supply when root uptake kinetics are identical or at low diffusive supply when roots are randomly distributed in the soil whatever their uptake kinetics are, (2) size-asymmetrical at high diffusive supply when root uptake kinetics are different, (3) partially symmetric or size-asymmetric at low diffusive supply when root are not randomly distributed in the soil.

Soil and nutrient properties, via their influence on nutrient diffusion rates in the soil, may play a key role in determining the outcome of plant competition for nutrients. We used two models to explore the potential contributions of nutrient uptake kinetics, root density, soil properties, and nutrient type to interspecific plant competition for soil nutrients. The first model uses well-known nutrient diffusion and absorption relationships to generate soil nutrient concentration maps and nutrient uptake at the scale of individual roots (PARIS- M). A second model (PARIS-E) was developed based on a fit of the Hill equation to the output of PARIS-M. The PARIS-E model provides an accurate and simple means of de- termining the relative contributions of sink strength (root surface area X uptake kinetics) vs. space occupation (number of roots per unit area) to competition at equilibrium as modeled by PARIS-M. An analysis based on these two models suggests the following: (1) Diffusive supply (soil nutrient buffer capacity x effective diffusion coefficient) determines the relative im- portance of space occupation vs. sink strength for nutrient competition. (2) At the low range of reported values of diffusive supply, competition depends on space occupation and, therefore, the species with the most roots per unit area is the most competitive. Sink strength gains in importance as diffusive supply increases and dominates competitive interactions at the high end of the range of reported diffusive supplies. (3) The relative importance of space occupation vs. sink strength depends primarily on soil water content and soil texture, because diffusive supply is sensitive to these factors. Diffusive supply is relatively insen- sitive to nutrient type. This analysis suggests that nutrient competition models should include the effects of soil properties as a determinant of the relative contributions of sink strength vs. space occupation.

Summary • The objective of this study was to better identify the mechanisms by which elevated CO2 (c. 665 µmol mol−1) alters soil nitrifying and denitrifying enzyme activity (NEA and DEA), and the dynamics of plant and microbial N pools. • We measured the effects of elevated CO2 on plant biomass and N, soil microbial biomass N, soil ammonium and nitrate concentrations, NEA and DEA in monospecific grassland mesocosms (Holcus lanatus and Festuca rubra) grown for 15 months in reconstituted grassland soil. • NEA strongly decreased at elevated CO2 in the Holcus mesocosms, but was not affected in Festuca systems. DEA was less sensitive to elevated CO2 than NEA. In Holcus mesocosms, microbial N showed an initial growth phase that appeared to limit plant N acquisition, but was only transient. • CO2-induced changes in NEA and DEA were best explained by factors that could reduce soil [O2] (increased soil water content and organic C). Elevated CO2 may increase microbial immobilisation in highly disturbed systems, this effect weakening as the system gets closer to equilibrium.