Establishing a Scottish Nitrogen Balance Sheet

2. Summary of technical approach

The technical approach to the initial version of the SNBS builds from work set out in a previous technical study undertaken by the UK Centre for Ecology & Hydrology for SEPA in 2019^[3]. That study made a limited, first attempt at a national Nitrogen Balance Sheet for Scotland, but with several substantial data gaps. The present SNBS builds from this, in particular by seeking to fill data gaps, broaden scope and adapt the methods to be compatible with international guidelines. Further information on the technical design of the SNBS, including the international guidelines followed, is provided in Annex A. 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 = 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 Chapter 4 of this report have been undertaken in line with a 2013 OECD paper^[4] and other relevant international guidance on methodology (e.g. as available from the UN Economic Commission for Europe and the EU Nitrogen Expert Panel)^[5].

This first version of the SNBS draws on the latest published Official Statistics and other data sources that were available as of November 2021. This means that the majority of the data relates to nitrogen flows for the calendar year 2019, 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 November 2021, 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 this first version of the SNBS.