Nitrogen deposition impacts in natural ecosystems

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.