Summary of technical approach
The technical approach builds from work set out in a previous technical study undertaken by the UK Centre for Ecology & Hydrology for SEPA in 2019. That study made a limited, first attempt at a national Nitrogen Balance Sheet for Scotland, but with several substantial data gaps. The current SNBS builds from this, in particular by seeking to fill data gaps, broaden scope and adapt the methods to be compatible with international guidelines. However, key features are summarised below.
Nitrogen Use Efficiency (NUE) is an important summary indicator metric that can be calculated from the comprehensive dataset on nitrogen flows assembled in the SNBS. In line with the statutory requirements for the SNBS, NUE is the ratio (expressed as a percentage) of useful nitrogen-containing outputs to all nitrogen inputs. This can be expressed as shown below:
NUE = Useful N Outputs ÷ N inputs × 100%
Calculations of NUE can be undertaken at a range of scopes and scales. Whilst sector-specific calculations (especially for crop production) are commonly used in existing international analyses, the statutory requirement for the SNBS is for a whole-economy metric, which remains a relatively novel concept. The whole-economy NUE calculations undertaken for the SNBS and summarised in the Nitrogen Use Efficiency section of this report have been undertaken in line with a 2013 OECD paper and other relevant international guidance on methodology (e.g. as available from the UN Economic Commission for Europe and the EU Nitrogen Expert Panel).
The SNBS draws on the latest published Official Statistics and other data sources that were available as of publication. This means that the majority of the data relates to nitrogen flows for the calendar year 2020, but some of the data relates to other years in the broader period 2010-2020. This is because many of the data used in the SNBS are from derived datasets dependent on the collation and processing of complex data and modelling (such as national atmospheric emission inventories), with the resulting time delay in reporting. Therefore, the baseline figures for NUE presented in this report should be understood as reflecting the most up-to-date available overall estimate for the national position as of publication, rather than any single specific year.
It should also be noted that the inherent uncertainties in the underlying data means that the SNBS may need to be revised in the future for purely technical reasons, as scientific knowledge improves over time or newer data become available. Such revisions have the potential to affect all historic time periods referenced, thereby potentially affecting the baseline analysis set out in previous versions of the SNBS.
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), apart from the information on industrial discharges and aquaculture which originate from SEPA’s Scottish Pollutant Release Inventory and Scotland’s Aquaculture website
The data underpinning the agricultural 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 (which is based on a wide range of Official Scottish statistics for 2019), hydrological modelling commissioned by SG from UKCEH (2010 data), other Official Statistics (including the Scottish Agricultural Census) and expert advice from researchers at e.g. SRUC and UKCEH.
The methodology for estimating the aquaculture nitrogen flows matches used by SEPA for regulatory purposes 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.
The transport emission figures are taken directly from the UK’s air pollutant and GHG emission inventories. The road fuel consumption figures are based on Scottish Transport Statistics, the rail fuel consumption figures are based on BEIS figures.
The industry and energy emissions data 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 on the nitrogen flows associated with fossil fuels extracted on Scottish territory, and the related imports, exports.
All atmospheric emissions data for the humans and settlements 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. Protein intake figures are based on National Records of Scotland population estimates combined with average protein intakes per individual.
Methodology for estimating the nitrogen flows which comprise the SNBS
Over recent years, an international agreement under the Gothenburg Protocol to the UN Economic Commission for Europe (ECE) Convention on Long-Range Transboundary Air Pollution (CLRTAP) has established an international reporting scheme for some key aspects of nitrogen flows. A guidance document on national nitrogen budgets was developed by the UN ECE Task Force on Reactive Nitrogen’s Expert Panel on Nitrogen Budgets and contains detailed draft annexes outlining the recommended methodology. This guidance builds on existing national data collections wherever possible, including the international greenhouse gas and air quality pollutant emission inventory reporting mechanisms and the OECD/EUROSTAT methodology for Gross Nutrient Budgets (GNB, formerly known as Gross Nitrogen Balances). The SNBS has been developed using the UN ECE guidance documents where possible. So far, to our knowledge, only Germany has published a national nitrogen budget that is largely, but not entirely, based on this draft guidance.
The SNBS uses existing published Official Statistics, such as the GHG and air quality emission inventories and SEPA pollutant datasets, where available. Additional information has been gathered from key expert institutions such as Scotland’s Rural College (SRUC), SEPA, Forest Research, the UK Centre for Ecology & Hydrology (UKCEH), and Rothamsted Research. All of these data sources are documented in detail in the SNBS spreadsheet. For some less well understood nitrogen flows, default values have been applied to relevant activity data. This approach ensures consistency and compatibility with existing long-term statistics data series wherever possible, and will aid the development of a time series where trends in nitrogen flows can be observed over time.
The UK’s atmospheric emission inventory reports air quality pollutants in their full chemical composition - for example, ammonia is reported as amounts of NH3 rather than as amounts of N alone. Furthermore, emissions of non-CO2 greenhouse gases (including N2O) are reported within the national greenhouse gas inventory as CO2 equivalents through use of GWP conversion factors. To convert these published emissions data into the common unit for the SNBS (of kt N / yr), which relates to flows of nitrogen (only), a conversion is undertaken based on the respective molecular weights (where Nitrogen (N) = 14, Hydrogen (H) = 1 and Oxygen (O) = 16). For example, ammonia (NH3) consists of one molecule of Nitrogen and 3 molecules of Hydrogen, therefore the total molecular weight of NH3 = 14 + (3 x 1) = 17. To convert the amounts of such emissions as reported in the national inventories into flows of nitrogen alone, these values need to be divided by the total molecular weight (17) and multiplied by that of the nitrogen present (14).
Methodology for estimating Nitrogen Use Efficiency (NUE) metrics
As set out in the Nitrogen Use Efficiency section of this report, NUE calculations can be derived for some individual sectors of the economy, and also at the whole-economy level. The former is much more commonly used in existing international analysis, with widespread application in particular for crop production systems.
A key question to consider when calculating NUE for any system is the definition of the system boundaries. This determines which parameters should be used on the input and output sides of the NUE equation, and which become internal to the calculations, as “recycling terms” within the system.
Using the example of crop production systems, the useful outputs are defined as the nitrogen contained in harvested crops removed from the land. Inputs include purchased (mineral) fertilizers, livestock manures and slurries, composts or other organic materials that aren’t recycled within the system, planting materials, but also atmospheric deposition and biological nitrogen fixation by legumes.
Alternatively, if the scope of the NUE calculation is then expanded to cover a mixed crop/livestock agriculture system, the useful outputs also include animal produce, such as milk, meat, eggs or wool. However, in this case manures become a “recycling term” within the systems boundaries rather than an input (as was the case for the crop production example).
Forestry can be considered as a further example, illustrating an instance where the “recycling terms” become relatively very important for an NUE calculation. In this case, both the useful outputs (mainly wood) and inputs (mainly from atmospheric deposition and biological nitrogen fixation) hide a wide range of other processes. In other words, large amounts of nitrogen are locked up in woodland as stocks, and much internal recycling of e.g. leaf litter, or brash etc. left behind after felling operations.
The main elements of the definitions of system boundaries and recycling terms used in the calculation of whole-economy NUE that forms the main output from the SNBS are explained in the section on Nitrogen Use Efficiencies, alongside the calculations. However, further more technical points are:
- To avoid any double counting at this scale of calculation, the Nitrogen deposition caused by local emissions (i.e. re-deposition of Scottish emissions on the Scottish territory, derived from modelling) was removed from the input side of the NUE equation.
- Similarly, ammonia deposition originating from emissions from within Scotland becomes a recycling term in the whole-economy approach, and therefore only deposition imported to Scotland from across the border is counted as inputs.
- On the other hand, Nitrogen fixation (on the input side) is estimated for the whole territory, rather than agricultural and forestry land only.
- Fish landings are included on both the input and output sides of the economy-wide NUE calculation, i.e. they are landed at Scottish ports and therefore are an input to the Scottish economy, and also a useful output (as food). Thereby this term effectively cancels out within the calculation.
- As noted in the subsection below, the import and export of goods and materials across the Scottish border represents an uncertainty in the current calculation (as insufficient data currently exist).
As well as the choices of scale (both in terms of sectors vs whole-economy and spatial scale), NUE metrics will also be influenced by the length of the time period under consideration. This is especially important given the potential for natural variations in the nitrogen locked-up temporarily in “stock” (for example livestock on farms, or trees in the context of forestry activities). However, as the NUE outputs from the current SNBS are using a national, whole-economy scale, an annual (or indeed multi-annual) summary NUE value for Scotland should provide a valid methodology for establishing trends over time.
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