Establishing a Scottish Nitrogen Balance Sheet

Annex A – Additional technical information

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^[33] was developed by the UN ECE Task Force on Reactive Nitrogen’s Expert Panel on Nitrogen Budgets^[34] and contains detailed draft annexes outlining the recommended methodology^[35]. 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^[36] 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our knowledge, only Germany has published a national nitrogen budget that is largely, but not entirely, based on this draft guidance^[37].

The SNBS re-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, in the same way as for Tier 1 emission inventory methodologies (i.e. the use of simple emission factors). 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 (see Chapter 2). 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)^[38]. 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 Chapter 2 and Chapter 4 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 Chapter 4. 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.

Summary of current data gaps for the SNBS

The main remaining data gaps in the SNBS (i.e. the grey arrows in Figure 1) can be summarised as follows:

Detailed import/export statistics for volumes of goods and materials can generally only be obtained at the UK level, and not enough detail is currently available to extract data for Scotland from the few data sources that do exist (e.g. summary HMRC statistics, Input-Output tables). Partial import/export data are available or can be reasonably inferred, but only for some specific sub-sectors of the economy (e.g. data exists on roundwood exported from Scottish forestry, or any use of soya as animal feed in Scotland can be assumed to be imported). The Scottish Material Flow Accounts^[39] also attempt to derive some flows of materials and produce from the available statistics, but there remain significant data gaps.

In addition to these trade flows, there are some natural processes in terrestrial (and aquatic) ecosystems where nitrogen is taken up and recycled, such as through the senescing and decomposition of vegetation, and these are also not quantified here. These processes, which occur across all types of vegetation, woodlands, heathlands, etc. are distinct from those described for forestry operations and the other specific cases covered in Chapter 3, where data does exist. These processes are acknowledged as difficult to quantify in the UN ECE guidance and the scientific knowledge the guidance draws on, and data are not required for any of the NUE calculations set out in this report.

There is also currently very limited knowledge on the extent of clover on Scotland’s pastures, and no specific data exist to our knowledge. If this could be improved, it would assist with the quantification of the biological nitrogen fixation for agriculture in the SNBS.

In terms of scope for improving the secondary calculation methods (rather than the underpinning data or main economy-wide NUE calculation methods), there are several potential areas for future improvement:

• Agriculture livestock feed conversion calculations: The simple calculations applied in Chapter 4 are based on work by SRUC^[40] and its representation in Scotland’s Material Flow Accounts (MFA)^[41]. It would be preferable to develop these into more comprehensive approaches, should further relevant data (e.g. around the nitrogen content of the wide range of grassland types in Scotland and how these feed into the different livestock sectors) become available in the future.
• Aquaculture livestock feed conversion calculations: The estimates of feed conversion and losses in Chapter 4 are based on the current modelling used by SEPA for regulatory purposes. However, SEPA recently started a review of their evidence base, and any outcomes should be incorporated into the relevant SNBS calculations once the review has been completed.