Reactive nitrogen (N) deposition from intensive agricultural and industrial activity has been identified as the third greatest threat to global terrestrial biodiversity, after land-use and climate change. While the impacts of N deposition are widely acknowledged, their magnitude is poorly quantified. We combine N deposition models, empirical response functions, and vegetation mapping to simulate the effects of N deposition on plant species richness from 1900 to 2030, using the island of Great Britain as a case study. We find that current species richness values -when averaged across five widespread habitat types -are approximately one-third less than without N deposition. Our results suggest that currently expected reductions in emissions will achieve no more than modest increases in species richness by 2030, and that emissions cuts based on habitat-specific "critical loads" may be an inefficient approach to managing N deposition for the protection of plant biodiversity. The effects of N deposition on biodiversity are severe and are unlikely to be quickly reversed.

The selection of climate policies should be an exercise in risk management reflecting the many relevant sources of uncertainty. Studies of climate change and its impacts rarely yield consensus on the distribution of exposure, vulnerability, or possible outcomes. Hence policy analysis cannot effectively evaluate alternatives using standard approaches such as expected utility theory and benefit-cost analysis. This Perspective highlights the value of robust decision-making tools designed for situations, such as evaluating climate policies, where generally agreed-upon probability distributions are not available and stakeholders differ in their degree of risk tolerance. This broader risk management approach enables one to examine a range of possible outcomes and the uncertainty surrounding their likelihoods.

Findings from nitrogen (N) manipulation studies have provided strong evidence of the detrimental impacts of elevated N deposition on the structure and functioning of heathland ecosystems. Few studies, however, have sought to establish whether experimentally observed responses are also apparent under natural, field conditions. This paper presents the findings of a nationwide field-scale evaluation of British heathlands, across broad geographical, climatic and pollution gradients. Fifty two heathlands were selected across an N deposition gradient of 5.9 to 32.4 kg ha−1 yr−1. The diversity and abundance of higher and lower plants and a suite of biogeochemical measures were evaluated in relation to climate and N deposition indices. Plant species richness declined with increasing temperature and N deposition, and the abundance of nitrophilous species increased with increasing N. Relationships were broadly similar between upland and lowland sites, with the biggest reductions in species number associated with increasing N inputs at the low end of the deposition range. Both oxidised and reduced forms of N were associated with species declines, although reduced N appears to be a stronger driver of species loss at the functional group level. Plant and soil biochemical indices were related to temperature, rainfall and N deposition. Litter C:N ratios and enzyme (phenol-oxidase and phosphomonoesterase) activities had the strongest relationships with site N inputs and appear to represent reliable field indicators of N deposition. This study provides strong, field-scale evidence of links between N deposition - in both oxidised and reduced forms - and widespread changes in the composition, diversity and functioning of British heathlands. The similarity of relationships between upland and lowland environments, across broad spatial and climatic gradients, highlights the ubiquity of relationships with N, and suggests that N deposition is contributing to biodiversity loss and changes in ecosystem functioning across European heathlands.

Urban green spaces (UGS) provide health benefits to city dwellers, which may be even more important during times of crisis such as the COVID-19 pandemic. However, lack of access to UGS or important UGS features, in addition to concerns about UGS safety or maintenance, could prevent people from receiving these benefits. We designed an online survey to understand how people were using and perceiving UGS during the COVID-19 pandemic in New York City during the spring of 2020. The survey included questions about how people’s visits to UGS and perceptions of the importance of UGS for health had changed since the start of the pandemic, as well as the concerns people had and features of UGS they considered important. Of the 1372 people who took the survey, most respondents were concerned about a lack of social distancing and crowded UGS, and respondents with these concerns were less likely to visit UGS and had visited UGS less often during than before the pandemic. In addition, generalized linear models showed differences in some concerns and important features of UGS across gender, race and ethnicity, demonstrating the importance of considering community needs in UGS design and management. Although concerns about lack of access were not common in our study population, these also appeared to prevent people from using UGS, and were more common in certain areas of the city that were also hard-hit by COVID-19 in the beginning of the pandemic. To ensure that people can get health benefits from UGS during times of crisis, cities must eliminate barriers by providing equitable access to UGS, considering what amenities communities need from UGS, and provide consistent communication about public health policies.

In this study, we compare annual fluxes of methane (CH₄), nitrous oxide (N₂O) and soil respiratory carbon dioxide (CO₂) measured at nine European peatlands (<i>n</i> = 4) and shrublands (<i>n</i> = 5). The sites range from northern Sweden to Spain, covering a span in mean annual air temperature from 0 to 16 °C, and in annual precipitation from 300 to 1300 mm yr⁻¹. The effects of climate change, including temperature increase and prolonged drought, were tested at five shrubland sites. At one peatland site, the long-term (> 30 yr) effect of drainage was assessed, while increased nitrogen deposition was investigated at three peatland sites. <br><br> The shrublands were generally sinks for atmospheric CH₄, whereas the peatlands were CH₄ sources, with fluxes ranging from −519 to +6890 mg CH₄-C m⁻² yr⁻¹ across the studied ecosystems. At the peatland sites, annual CH₄ emission increased with mean annual air temperature, while a negative relationship was found between net CH₄ uptake and the soil carbon stock at the shrubland sites. Annual N₂O fluxes were generally small ranging from −14 to 42 mg N₂O-N m⁻² yr⁻¹. Highest N₂O emission occurred at the sites that had highest nitrate (NO₃⁻) concentration in the soil water. Furthermore, experimentally increased NO₃⁻ deposition led to increased N₂O efflux, whereas prolonged drought and long-term drainage reduced the N₂O efflux. Soil CO₂ emissions in control plots ranged from 310 to 732 g CO₂-C m⁻² yr⁻¹. Drought and long-term drainage generally reduced the soil CO₂ efflux, except at a hydric shrubland where drought tended to increase soil respiration. <br><br> In terms of fractional importance of each greenhouse gas to the total numerical global warming response, the change in CO₂ efflux dominated the response in all treatments (ranging 71–96%), except for NO₃⁻ addition where 89% was due to change in CH₄ emissions. Thus, in European peatlands and shrublands the effect on global warming induced by the investigated anthropogenic disturbances will be dominated by variations in soil CO₂ fluxes