Establishing a Scottish Nitrogen Balance Sheet

3. Results from the initial version of the Scottish Nitrogen Balance Sheet

3.1 Overview

Scotland’s main national nitrogen flows can be summarised in the simplified flow diagram set out below as Figure 1. The full SNBS dataset^[6] contains hundreds of individual flows of nitrogen, which have necessarily been aggregated to provide such an overview.

The colour coding of the flow arrows on Figure 1 reflects:

Useful (i.e. desired) nitrogen-containing outputs are shown as dark blue arrows. These include foodstuffs (cereals, fruit, vegetables, meat, dairy, eggs, fish/seafood), as well as natural fibres such as wool and wood.

Losses of nitrogen into the environment are shown as orange arrows. These include both emissions to the atmosphere (as the greenhouse gas nitrous oxide, N₂O, or as the air quality pollutants ammonia (NH₃) and nitrogen dioxide, NO₂) and flows into freshwater and coastal hydrological systems (as nitrate, NO₃, dissolved organic and inorganic nitrogen, particulates, etc).

Inputs of nitrogen are shown as light blue arrows. These mainly represent deliberate anthropogenic uses of nitrogen (e.g. fertiliser use towards food production), although there are also some inputs from natural processes (e.g. biological fixation of nitrogen from the atmosphere by terrestrial ecosystems). Also included, for the purpose of this diagram, are nitrogen flows that are recycled within the wider system before forming either a useful output or a loss to the environment (e.g. as organic materials from waste, or emissions of air pollutants that are then deposited back to soils from the atmosphere).

Grey arrows on the diagram represent instances where available data quality remains insufficient to quantify the flows. These instances mainly relate to import/export flows of nitrogen across Scotland’s borders (see Annex A for more detail). Addressing these data gaps, where possible and useful to do so, will be a priority for the ongoing further development of the SNBS.

These arrows represent flows of nitrogen in a wide range of different chemical forms (e.g. nitrous oxide, ammonia, nitrates, etc.). For the purpose of the SNBS, all flows have been converted to units of kilotonnes of nitrogen per annum (kt N / yr) as a common currency which can then be added up and compared (see Annex A for technical detail of these conversion methods). The width of the arrows on Figure 1 reflect, in a general sense, the relative size of the flows.

It is clear from Figure 1 that nitrogen flows across the Scottish economy and environment are highly complex, even in such a relatively simplified form. In particular, much of the nitrogen which enters into the system (e.g. from fertiliser use) is recycled within it – often in complex ways – before forming either a useful output (e.g. via food products) or a loss to the environment (e.g. via emissions to air).

Nonetheless, this summary also makes clear that by far the largest overall “engine” of nitrogen use in Scotland is associated with food production. Overall, out of the 78 kt N / yr of total useful nitrogen-containing outputs produced in Scotland, almost 90% of these are associated with food production (namely 55 kt N / yr of foodstuffs from agriculture^[7], with the remainder from aquaculture and landings from sea fisheries).

The nitrogen cycle for food production is also then closely linked with the waste management aspects of the wider summary diagram, through the consumption of food for human nutrition and subsequent excretion.

There are also several sets of nitrogen flows on the summary diagram that are largely independent of the food production system, including those associated with the combustion of fossil fuels (via transport, industry and wider energy use), although these are generally much smaller in magnitude and much simpler in structure (i.e. direct emissions to the air).

Finally, it should be noted that the forestry nitrogen cycle is not set out explicitly within the format of this summary diagram, with only the useful outputs (i.e. wood products) being shown explicitly and the inputs and losses included within the wider flow arrows associated with nitrogen recycling in soils and terrestrial ecosystems (the reasons for this approach are discussed in more detail below).

The remaining sub-sections of this chapter provide further detail on the nitrogen flows for the following key sectors of Scotland’s economy and environment:

• Summary of nitrogen flows to/from the atmosphere
• Summary of nitrogen flows to/from the hydrosphere and aquatic ecosystems
• Food production: agriculture
• Food production: aquaculture
• Transport
• Industry and energy
• Humans and settlements (including waste management)
• Forests, woodlands and terrestrial semi-natural ecosystems

All of these results are at the national scale. Annex B sets out additional regional-scale results for some of the key hydrological nitrogen flows, where the SNBS dataset allows for such analysis.

3.2 Summary of nitrogen flows to/from the atmosphere

The atmosphere acts as both a sink for nitrogen-containing emissions from both anthropogenic activities and natural processes, and as a source of the nitrogen deposited to soils and habitats. In addition, the atmosphere acts as a transport medium, bringing imports of air pollutants to Scotland and carrying away some of the pollutants beyond Scotland’s borders (this is referred to as transboundary air pollution). These flows are summarised in Figure 2 below.

Total emissions of nitrogen to air in Scotland amount to around 59 kt N / yr.

Nitrous oxide (N₂O), which is a greenhouse gas, amounts to around 8 kt N / yr of these total emissions. The majority of nitrous oxide emissions come from agriculture (4.8 kt N / yr) with other contributions from land use and land use change (2.5 kt N / yr) and more minor ones from industry, transport and waste processing. Nitrous oxide is a potent greenhouse gas, with a global warming potential (i.e. conversion factor to carbon dioxide equivalent) of almost 300. Scottish greenhouse gas emissions statistics for 2019 show that the 8 kt of N in the nitrous oxide emissions amounted to 3.8 Mt of CO₂ equivalent (see Annex A for further information on the conversion between these figures), which represents 8% of Scotland’s total greenhouse gas emissions for that year. This makes it the third most significant greenhouse gas, after CO₂ itself (which represents 70% of the total) and methane (which represents 19% of the total).

The remaining emissions of nitrogen to the atmosphere are as other (i.e. non-greenhouse gas) air quality pollutants. These total 52 kt N / yr, split broadly evenly between nitrogen dioxide (NO₂, 26 kt N / yr), mainly from combustion in transport, industry and wider energy use) and ammonia (NH₃, 26 kt N / yr), mainly from agricultural sources. Compared with these total air quality pollutant emissions from Scotland’s territory, it is also estimated that 68 kt N / yr of nitrogen is deposited from the atmosphere, making Scotland a net importer of air pollution. The imported atmospheric nitrogen arrives in Scotland through regional and long-range transport and dispersion of air quality pollutant emissions, from both the rest of the British Isles (UK and Republic of Ireland), international shipping and the European mainland^[9]. The nitrogen deposition figures in the SNBS are estimated through modelling (in combination with measurements from the UK’s national monitoring networks), and data are derived annually for Official Statistics on behalf of Defra^[10].

Additional nitrogen inputs to semi-natural habitats from atmospheric deposition can cause substantial biodiversity effects, damaging sensitive vegetation and the wildlife depending on it for feeding and/or breeding. Atmospheric nitrogen pollutants can also cause damage to both vegetation and human health in areas of high concentrations near the emission sources.

The spatial distribution of air pollutant emissions and atmospheric concentrations therefore matters in terms of where impacts occur. For nitrogen deposition, there are two main pathways, dry deposition (which occurs closer to the sources) and wet deposition (which can travel long distances, often many 100s of kilometres). Impacts on remote areas are typically therefore due to wet deposition, especially in high rainfall areas. By contrast, greenhouse emissions are not re-deposited, but remain in the atmosphere, contributing to climate warming on a global scale.

A further pathway of nitrogen to the atmosphere (which is not shown on Figure 1 and Figure 2) is through denitrification as di-nitrogen (N₂), which is neither a greenhouse gas nor an air pollutant. Denitrification is a microbial process that converts nitrogen from soils (especially wetlands) and water bodies through a series of reactions. Denitrification also plays an important role in sewage treatment, helping to remove nitrogen from water and thereby cleaning it. Denitrification is difficult to estimate as it involves a complex series of processes with high degrees of uncertainty. However, rough estimates can be made using existing scientific literature and expert knowledge. Overall, about 50 kt N / yr are estimated to be emitted as N₂ in Scotland, with 36 kt N / yr from terrestrial soils (including wetlands) and the remainder from hydrological systems.

3.3 Summary of nitrogen flows to/from the hydrosphere and aquatic ecosystems

Scotland’s hydrosphere and aquatic (freshwater and coastal) ecosystems act as both recipients and transport media for nitrogen losses, mainly originating from anthropogenic activities but also in some cases from natural nitrogen cycling processes. The main direction of these flows is through river catchments and groundwater bodies into coastal waters.

The main hydrological flows of nitrogen within Scotland are due to run-off and leaching from soils, with the majority of these losses linked to grasslands (estimated at 58 kt N / yr), arable (46 kt N / yr) and semi-natural habitats (18 kt N / yr from a combination of woodlands, heaths, grasslands, montane, etc.). Discharges from sewage processing and industrial sources contribute a further 12 kt N / yr.

These sources, combined with legacy nitrates in groundwater (mainly in some aquifers in eastern Scotland and estimated at 27 kt N / yr), together contribute to a total estimated discharge of nitrogen into Scottish coastal waters of around 140 kt N / yr. A further 15 kt N / yr is exported to England via the River Tweed, which crosses the border shortly before reaching the coast at Berwick-upon Tweed.

Within coastal waters themselves, excreta from aquaculture are a further source of nitrogen input, estimated at around 13 kt N / yr.

All of the hydrological flow data described here are derived from new modelling carried out for the SNBS by UKCEH (this uses a base year of 2010)^[11], apart from the information on industrial discharges and aquaculture which originate from SEPA’s Scottish Pollutant Release Inventory^[12] and Scotland’s Aquaculture website^[13].

Flows of nitrogen into the hydrosphere can lead to eutrophication, with subsequent harmful impacts of marine ecosystems. Eutrophication occurs when the enrichment of waters by nutrients (nitrogen and phosphorus) causes excessive growth of phytoplankton resulting in an undesirable disturbance of the marine ecosystem. Eutrophication is a problem if there is evidence of nutrient enrichment from human activities resulting in concentrations in seawater exceeding background concentrations. Nitrogen entering the sea from rivers, and from direct sources, undergoes complex chemical and biological processes which can lead to enrichment in some cases. This enrichment can cause direct effects such as elevated chlorophyll concentrations and increased blooms and indirect effects such as oxygen deficiency and biomass changes.

Monitoring to assess eutrophication status in the marine environment is required to develop measures and actions needed to meet our vision for clean and safe Scottish seas. In addition, such monitoring is required in order to ensure Scotland fulfils national and international obligations under environmental legislation. The most current assessment of coastal eutrophication was published as part of the Scotland’s Marine Assessment 2020^[14].

For Scottish Marine Regions receiving the highest nutrient inputs (Clyde, Forth & Tay, North East and Moray), there were no significant increasing trends observed (2007-2017) and inputs were lower than in the period 1990-2007. The North Coast, Orkney Islands and Outer Hebrides did show statistically significant increasing trends although loads there were typically an order of magnitude lower than the high load sea areas.

3.4 Food production: agriculture

Agricultural activities in Scotland produce a diverse range of useful nitrogen-bearing outputs: crops (for human consumption, livestock feed, biomass and seed production) and also livestock produce (e.g. dairy, meat, eggs, wool), adding up to an estimated total of 55 kt N / yr.

The production of these outputs requires the input of nitrogen as a nutrient to soils, which happens principally through anthropogenic intervention via both mineral (artificial) fertilisers and organic manures and slurries. A total of 153 kt N / yr is estimated to be applied in this manner, which includes 10 kt of other recycled organic materials (composts, digestates, sewage sludge).

In addition to the anthropogenic inputs, 45 kt N / yr is gained from biological nitrogen fixation through legumes. This is mainly achieved through grass-clover systems, as there are only relatively small areas of peas and beans grown in Scotland. A further 12 kt N / yr is supplied by atmospheric deposition to arable and grasslands.

Finally, an estimated 21 kt N / yr of nitrogen is imported into Scotland as livestock feed (e.g. soy, maize, beet pulp, etc.) which, when combined with Scottish-grown crops and grass forage, provides a total of 199 kt N / yr of nitrogen fed to animals.

Nitrogen losses from agriculture to the environment follow two main pathways:

• emissions to the atmosphere, as the air quality pollutants ammonia (NH₃, emissions of 24 kt N / yr) and nitrogen dioxide (NO₂, emissions of 1.4 kt N / yr) and as the greenhouse gas nitrous oxide (N₂O, emissions of 4.8 kt N / yr).
• run-off and leaching from agricultural soils^[15] to catchments and groundwater (flows of 104 kt N / yr, mainly as nitrates, but also dissolved organic and inorganic and particulate forms).

The data underpinning the figures set out above are based on a combination of the UK’s National Atmospheric Emission Inventory (NAEI) and GHG Inventory and the underlying datasets^[16] (which is based on a wide range of Official Scottish statistics for 2019), hydrological modelling commissioned by SG from UKCEH^[17] (2010 data), other Official Statistics (including SEPA’s Materials to Land report^[18]) and expert advice from researchers at e.g. SRUC and UKCEH.

3.5 Food production: aquaculture

The dominant activity in terms of nitrogen flows for Scottish aquaculture is fin fish farming, mainly salmon in cages in coastal waters. The sector also includes the much smaller (in nitrogen terms) shellfish and freshwater aquaculture operations.

Useful outputs of nitrogen in harvested fish and shellfish produce are estimated at 7.3 kt N / yr, whereas the estimated anthropogenic input of feed into the system is at 21.3 kt N / yr. Losses of nitrogen into coastal waters are estimated at 13.4 kt N / yr, this being mostly in the form of nitrogen excreted by the fish, with only a small amount (estimated at 3%) of feed itself being lost.

The methodology for estimating these nitrogen flows matches that currently (Autumn 2021) used by SEPA for regulatory purposes^[19], and is thought to provide a conservative set of estimates. This method is under review, and the SNBS estimates can be revisited once this has been completed.

3.6 Transport

The key nitrogen flows for the transport sector are emissions to the atmosphere (13 kt N / yr), resulting from fuel combustion. The majority of these emissions are of air quality pollutants, mainly nitrogen dioxide (NO₂, emissions of 12.4 kt N / yr), and small amounts of ammonia (NH₃, emissions of 0.3 kt N / yr). Greenhouse gas emissions of nitrous oxide (N₂O) only amount to 0.3 kt N / yr, with the main climate effect from transport sources being due to emissions of CO₂.

These figures are all taken directly from the UK’s air pollutant and GHG emission inventories.

3.7 Industry and Energy

As with transport, the key nitrogen flows in the industry and energy sectors relate to fossil fuel combustion emissions to the atmosphere, totalling 12 kt N / yr. The vast majority of this is emitted as the air quality pollutant nitrogen dioxide (NO₂) with a minor contribution from ammonia (NH₃). Emissions of the greenhouse gas nitrous oxide (N₂O) only make up a small part of the overall emissions from these sectors (at 0.3 kt N / yr). This is because, again in the same way as for transport, the majority of the climate impact from fossil fuel combustion in these sectors occurs as CO₂.

The atmospheric emissions data set out above are taken directly from the UK’s air pollutant and GHG emission inventories. Additional data have also been compiled from the Scottish Energy Statistics Hub^[20] on the nitrogen flows associated with fossil fuels extracted on Scottish territory, and the related imports, exports.

3.8 Humans and settlements (including waste management)

Important flows of nitrogen related to human activity in Scotland include the food (i.e. protein) intake of the population and the related nitrogen excretion that is collected and processed/recycled or disposed of as sewage (around 23 kt N / yr). There are also minor emissions to the atmosphere of ammonia (NH₃, emissions of around 1 kt N / yr in total) arising directly from activities such as household solvent use, cigarette smoking, pets, babies’ nappies, fertiliser application to golf courses, etc.

There are then also a range of other nitrogen flows closely linked to the waste management system, comprising activities such as anaerobic digestion, composting and waste water/sewage processing. These processes allow for valuable nutrients, including nitrogen, to be recycled and utilised for growing crops and grass, and land reclamation. However, these activities as well as others within the waste sector (such as incineration and landfill) do also produce some emissions to the air.

Of a total nitrogen flow of around 30 kt N / yr from household and business/industrial waste, plus the 23 kt N / yr of human excreta generated in Scotland (see above), it is estimated that around 10 kt N / yr are applied back to Scottish soils. A further 12 kt N / yr are lost to water, of which around half is from human sewage and half from discharge by industry and food production. Atmospheric emissions from waste processing are relatively minor and estimated at less than 1 kt N / yr (mainly of ammonia and nitrous oxide). A further c. 1 kt N / yr of emission to the air, again mainly as ammonia, are associated with losses during the application of digestates and sewage sludge as fertilisers to land^[21].

Other nitrogen associated with the waste sector is re-used within the Scottish economy (estimated at 13 kt N / yr) or buried in landfill sites (estimated at 9 kt N / yr). These flows are not shown as arrows on Figure 1 either for simplicity (in the case of re-use within the economy) or because they represent long-term storage of nitrogen (in the case of landfill).

All atmospheric emissions data for this sector of the SNBS are taken directly from the UK’s emission inventories. Further data are taken from SEPA’s Scottish Pollutant Release Inventory (SPRI) and UKCEH sources.

3.9 Forests, woodlands and terrestrial semi-natural ecosystems

Scotland’s forest and woodland resource consists of different woodland types and tree species, ranging from traditional mixed Highland estates to the highly productive forests, such as the Tay Forest Park, and from urban forests in and around our cities to the more sensitive (in terms of nitrogen deposition) native woodlands, such as the Atlantic oakwoods in Argyll. These diverse and versatile forests and woodlands provide considerable economic and environmental benefits, as well as helping to improve people’s quality of life.

One of the key economic benefits provided by Scotland’s forests and woodlands is the production of timber and other wood fibre. It should be noted that such wood products are largely composed of carbon and contain relatively little nitrogen, compared with forest residues, including leaf litter and brash, which are generally retained in the forest where nutrients are naturally recycled. This explains why the wider forestry nitrogen cycle is not identified as a full separate set of flow arrows on Figure 1, but is instead mainly treated as part of a wider arrow related to the recycling of nitrogen within (all) terrestrial ecosystems.

Nonetheless, the full SNBS dataset does allow for estimates to be made of aspects of the forestry nitrogen cycle in isolation. In terms of economic production, the nitrogen in wood harvested in Scotland during 2019 was mostly destined for material (5 kt N / yr) and energetic (2 kt N / yr) uses, with a further 0.5 kt N / yr exported as roundwood (against a much smaller import of 0.1 kt N / yr).

For re-planting existing woodlands and for establishing new woodlands, nitrogen-containing fertiliser is only applied on a site specific basis where a nutrient deficiency has been identified or is highly likely, with an estimated use of only 0.02 kt N / yr. As such, the vast majority of nitrogen input to Scottish forests and woodlands occurs through either biological nitrogen fixation (11 kt N / yr) or nitrogen deposition from the atmosphere (17 kt N / yr).

High levels of nitrogen deposition can also pose challenges for sensitive near-natural woodlands, adversely affecting biodiversity - especially lichens that grow on trees and require clean air to thrive. Elevated ammonia concentrations from nearby emission sources can also have a similar effect. Official Statistics are released annually for the UK, with separate country data for Scotland, on the area of habitats and designated sites that exceed critical thresholds for atmospheric nitrogen input for ammonia concentrations (critical level exceedance) and total nitrogen deposition (critical loads exceedance)^[22].

Other semi-natural habitats in Scotland, such as bogs, heathlands and montane vegetation, are similarly threatened by nitrogen input from atmospheric deposition (the SNBS estimates the deposition flow to these habitats as 33 kt N / yr).

A proportion of the nitrogen deposited from the atmosphere onto forests, woodlands and other semi-natural habitats then makes its way into waterbodies, through leaching and run-off (the SNBS estimates these flows as 18 kt N / yr)^[23].