Nitrogen deposition impacts in natural ecosystems
Project Lead
Challenges
Nitrogen is an essential element for life, but human creation of reactive nitrogen far exceeds that occurring by natural processes. A high proportion of anthropogenically created nitrogen compounds fail to remain with their intended target, instead being lost to the environment and impact on natural ecosystems. Pollution is one of the five main direct drivers of reductions in biodiversity and ecosystem services over the last 50 years. The perturbation of the nitrogen cycle has been identified as one of the main ways in which humanity is pushing the earth's system beyond its safe operating space.
Reactive nitrogen released into the environment reduces air quality and may affect organisms directly in gaseous form or be deposited onto vegetation and soils as particulates or in rainfall. Deposited nitrogen accumulates in soils and vegetation where enhanced nitrogen supply alters interactions between species and negatively impacts biodiversity. Deposition of nitrogen also impacts ecosystem functioning via changes in plant productivity and decomposition of organic materials in the soil altering both carbon and nitrogen cycling, with implications for carbon sequestration and emissions of greenhouse gases. When nitrogen inputs exceed an ecosystem’s capacity to retain them, nitrogen escapes into surface waters, impacting aquatic ecosystems and water quality.
While emissions of nitrogen from industry and transport have declined in recent years, emissions from agriculture remain high. In Scotland, nitrogen deposition is impacting widely on natural ecosystems, with 30% of sensitive habitats and 80% of the land within Special Areas of Conservation receiving excess nitrogen. These natural ecosystems are also subject to the impacts of climate change and other drivers. Climate change has the potential to exacerbate the negative effects of nitrogen deposition on biodiversity and ecosystem functioning, both through climate-enhanced nitrogen emissions from agriculture and via interactive effects of climatic stressors and nitrogen deposition on organisms and processes. In turn, nitrogen deposition has the potential to feedback on climate change through its effects on the carbon balance of natural ecosystems.
Scottish natural ecosystems supply essential goods and services upon which our society and economy depend. Effective safeguarding of natural capital and ecosystem service supply in Scotland in the face of continuing nitrogen deposition and increasing impacts of climate change requires a thorough understanding of how these two drivers impact biodiversity and ecosystem processes and their potential interactions, across a wide range of natural ecosystems.
Questions
Solutions
This project seeks to further understand the impacts of nitrogen deposition on sensitive natural ecosystems in Scotland in the context of a changing climate.
Nitrogen-climate interactions
Review of nitrogen and climate impacts on Scottish ecosystems
We are providing evidence on how biodiversity and natural ecosystems are changing, what is driving this change and how best to manage and protect them. To do this we are reviewing the evidence for nitrogen impacts on Scottish ecosystems in the context of climate change and nitrogen deposition and identifying the gaps in knowledge needed to model future change, anticipate problems and target restoration and mitigation effects.
Empirical studies of nitrogen-climate interactions in Scottish ecosystems
We are conducting a series of empirical studies of nitrogen-climate interactions in Scottish ecosystems, starting to fill some of the gaps in current knowledge concerning effects on Scottish ecosystems. Building on previous analyses, we are conducting a unified analysis across all habitats to examine the impacts of nitrogen deposition and climate change, along with sulphur deposition, grazing and land management, on vegetation community composition, species richness and functional metrics. We explore the relative sensitivity of different habitats to nitrogen and climate and identify thresholds for change. This analysis includes alpine, moorlands, peatlands, wetlands, grasslands, dunes, and native forests and compares the sensitivity of different habitats to nitrogen and climate impacts.
Alpine ecosystems
Further work uses cutting-edge eDNA techniques to expand our knowledge of the impacts of nitrogen deposition on below-ground biodiversity and ecosystem function in alpine and woodland ecosystems. The study investigates changes in both above and below-ground biodiversity in Scottish alpine habitats over the last 20 years and their spatial relationships to climate and nitrogen deposition. It also investigates the impacts of nitrogen deposition on ectomycorrhizal fungi in native woodland and whether certain host trees vary in sensitivity to climate and nitrogen deposition. Experimental manipulations will also be conducted to investigate the long-term consequences of carbon and nitrogen cycling in alpine heath and bryophyte-dominated habitats. The aim is to understand the mechanisms underpinning ecosystem responses and fill gaps in knowledge.
Woodland ectomycorrhizal (ECM) fungi communities
In native woodland, ectomycorrhizal (ECM) fungi are known indicators of N impacts. ECM fungi occupy a vital junction between soil environment and tree, strongly influencing tree nutrition and carbon and Nitrogen cycling within the woodland. Nitrogen deposition is a major factor structuring ECM communities across Europe, but knowledge of its impacts in Scotland and potential interacting effects with the strong climatic gradient is limited. We use species distribution data from the National Biodiversity Network to investigate patterns of ECM occurrence concerning climate and Nitrogen deposition. ECM fungi have strong associations with tree species, and we will investigate whether communities on particular hosts vary in sensitivity to climate and Nitrogen. This is complemented by an analysis of molecular data on ECM communities in Scotland using existing small-scale datasets supplemented with a new survey of ECM communities of birch, oak and pine. New samples are being analysed for total fungal, bacterial and eukaryote communities to determine whether other soil organisms also have the potential as indicators of nitrogen deposition and climate impacts.
Climate and nitrogen manipulation plots at the Culardoch experimental platform
Survey approaches to understanding nitrogen-climate change interactions are being complemented by experimental manipulations to enable a mechanistic understanding of ecosystem responses. Long-term climate and nitrogen manipulation plots at the Culardoch experimental platform are being used to investigate impacts on carbon and nitrogen cycling in alpine heath and long-term consequences for above and below-ground biodiversity, carbon and nutrient stocks. Additionally, new controlled environment studies with field-collected mesocosms are used to test how nitrogen deposition influences ecosystem responses to scenarios of altered summer and winter climates focusing on upland and bryophyte-dominated habitats. This work focuses on understanding the mechanisms underpinning ecosystem responses and identifying and filling gaps in knowledge.
Nitrogen deposition and climate impacts on natural ecosystems
Finally, we are modelling nitrogen deposition scenarios and climate impacts on natural ecosystems using new and existing modelling frameworks exploring and mapping risks to biodiversity and ecosystem function for a series of scenarios of future nitrogen deposition rates and climate change, based on socioeconomic, climate change and air quality policy scenarios.
Mitigation potential
Potential for nitrogen mitigation in Scottish natural habitats
We are then determining effective ways of reducing and mitigating the impacts of excess nitrogen deposition on sensitive natural ecosystems in Scotland and exploring metrics for measuring success. The evidence for the impacts of nitrogen deposition and thresholds for response is patchy across Scottish natural ecosystems, with evidence lacking for many important habitats and no agreed methodologies for nitrogen impact assessment. We are reviewing critical loads/thresholds for nitrogen impacts in Scottish ecosystems, responses to excess nitrogen potential for mitigation of nitrogen impacts through habitat management and evidence from these interventions Our review will provide evidence for end users on the potential for nitrogen mitigation in Scottish natural habitats and will also be used to guide suitable metrics for monitoring impact/recovery from nitrogen deposition across a range of habitats. The work synthesises previous research on both thresholds for nitrogen impacts and mitigation potential including that commissioned by SEPA and NatureScot and builds on this work by considering a wider range of ecosystems.
Management to mitigate nitrogen impacts
In this activity, we test the potential of management to mitigate nitrogen impacts with a new experimental study in a high-priority ecosystem for which the review identifies potential for mitigation. We examine the impacts of mitigation on biogeochemical status, habitat structure and biodiversity, and will develop and test a set of metrics to quantify impact and recovery, drawing on potential metrics identified by the review and accounting for variation in response times between ecosystem components.
Progress
2024 / 2025
The focus across the whole project has been on gathering of field survey and experimental datasets. The second year of fieldwork for the resurvey of alpine vegetation plots previously surveyed in the 1970s and 2000s was completed. An additional 97 plots were surveyed in 2024, focussing on Mull and the Southern Uplands, bringing the total number of plots re-surveyed so far to 199. DNA has been extracted from soil samples collected from each plot over the first two years of the survey and has now been sequenced. Soil and moss chemical analyses have also been completed. Preparations have been made for an additional 100 plots to be surveyed in summer 2025, bringing the total to 300 plots and completing the field survey. DNA sequence data from all years will be combined prior to bioinformatic analysis, followed by statistical analysis of the impacts of nitrogen deposition and climate change on above and below ground biodiversity.
One of the key aims of the project is studying ectomycorrhizal fungi and wider soil biodiversity associated with native forests in Scotland. Field sampling for this survey was completed in autumn 2024 and chemical and DNA analysis of the samples is also now complete. DNA sequence data for these samples has just been received and data analysis will take place during 2025-26. Fifty-seven native oak, birch and pine woodland sites have been sampled during the survey, covering the full geographic range of these forest types in Scotland. Data analysis will now focus on examining relationships between forest soil biodiversity and gradients of climate and nitrogen deposition. Additional funding was obtained for 2024-25 from Forestry and Land Scotland to conduct sampling of plantation forests (Scots pine and Sitka spruce) in parallel with this survey, and these data will be analysed at the same time to compare biodiversity in native and plantation forest. An experiment investigating the interactive effects of nitrogen deposition and climate change (warming) on competition between upland mosses has also been set up. This experiment is investigating the mechanisms driving the changes observed in the alpine vegetation resurvey, where upland mosses such as Hylocomium splendens and Rhytidiadelphus sp. are expanding at the expense of alpine species including Racomitrium lanuginosum.
The project team has continued the study of habitat management options to mitigate the impacts of nitrogen deposition. We sampled five pairs of peatland sites, where one of the pair had been subject to restoration, along a gradient of nitrogen deposition, including sites from the northwest of Scotland and from the central belt. The sampling methodology focusses on vegetation composition and plant and soil nutrient stocks. Samples collected during 2024 have all been processed and sent for chemical analysis. We are currently in discussions with Cumbria Wildlife Trust to sample sites there in 2025 which will extend the deposition gradient to some of the most heavily polluted sites in the UK, aiding interpretation of the data.
2023 / 2024
This project is helping us understand how pollution and climate change affect Scotland’s fragile upland and woodland ecosystems, and how management actions might lessen these impacts.
Alpine heath experiments: In Year 2 we completed the resampling of a unique 23-year experiment testing how nitrogen pollution, burning, and warming affect alpine heath. Nitrogen was added for 11 years, followed by 12 years of recovery; burning was applied once at the start; and warming treatments have been running for 18 years. The findings show that nitrogen effects persist long after inputs stop. Although added nitrogen was no longer visible in the plants, soil nutrient ratios were still altered, and extra nitrogen remained in surface soils. Warming also strongly influenced nutrient cycling in unexpected ways. By contrast, burning had only small long-term effects.
Woodland soils and fungi: Work continued on studying how nitrogen and climate affect woodland fungi, which play key roles in soil health. Soils from oak, birch, and pine forests across Scotland were sampled, and DNA analysis is underway to map biodiversity patterns.
Tracking long-term change: A major resurvey of historic alpine vegetation plots is now underway. By comparing data from the 1970s, 2000s, and 2020s, we will be able to track how vegetation and soil biodiversity have changed over 50 years of shifting pollution and climate.
Peatland restoration: Finally, new work has begun on whether peatland restoration and re-wetting can reduce the impacts of nitrogen pollution. Partnering with SEPA, NatureScot, and Peatland Action, trial surveys have been carried out and a wider programme of sampling will take place in the coming years.
Together, these studies will give us unprecedented insights into how Scotland’s ecosystems respond to multiple pressures and how management can help protect them for the future.
2022 / 2023
This project is investigating how nitrogen pollution and climate change interact to affect Scotland’s ecosystems, and what can be done to reduce the impacts.
What we’ve learned so far:
Two reviews were completed in the first year. The first looked at how nitrogen and climate change together influence biodiversity and ecosystem function. While there is good evidence on the effects of each separately, studies that examine both together are limited, especially in the UK. Most research focuses on forests, grasslands, and bogs, leaving major gaps in knowledge for other important Scottish habitats. This evidence will help shape experimental work in future years.
The second review explored whether habitat management can help ecosystems recover from nitrogen pollution. Research suggests some techniques may work in forests, heathlands, and grasslands, but most evidence comes from lowland European studies, which may not always apply to Scotland. These findings will guide a new experimental study on mitigation.
What’s underway:
Existing Scottish datasets are being compiled and analysed to test how nitrogen and climate together drive long-term vegetation change.
Work has begun on understanding how nitrogen and climate affect fungi (Ectomycorrhizal communities) in woodlands, using both biodiversity records and new DNA data collected from 20 field sites.
In alpine heath, long-term experimental plots are being studied to see how nitrogen and warming influence plants, soil biodiversity, and carbon and nutrient stocks.
What’s next:
In Year 2, the team will begin a major resurvey of historic alpine plots, providing insight into vegetation change over 50 years. For the first time, this will also include soil biodiversity, adding a new dimension to our understanding of these fragile habitats.
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