Establishing a Scottish Nitrogen Balance Sheet

4. Baseline metrics for Nitrogen Use Efficiency

4.1 Overview

The Climate Change (Scotland) Act 2009, as amended by the Climate Change (Emissions Reduction Targets) (Scotland) Act 2019, requires the SNBS to provide the basis of a whole-economy calculation of Nitrogen Use Efficiency (NUE).

Chapter 2 set out key elements of the methods used for this calculation and Annex A provides further technical detail. The present chapter sets out the results arising from these methods being applied to the initial version of the SNBS dataset. Whilst this chapter can be read in isolation, we would encourage readers to first familiarise themselves with the main flows of nitrogen themselves, as set out in Chapter 3.

In order to contextualise this analysis, which is pioneering and has only very limited international precedents at the economy-wide scale, the approach taken is to build up an understanding of NUE across key elements of the Scottish nitrogen system, starting from those where there is the greatest scope for international comparability.

4.2 Crop production Nitrogen Use Efficiency

This is the natural starting point from which to build up a wider NUE calculation, as i) crop production underpins much of wider food production, which in turn is the main engine of overall national nitrogen use in Scotland (see Chapter 3), and ii) international calculations of NUE at this level are widely undertaken.

Crop production NUE for Scotland based on the SNBS data is estimated at 65%. This reflects 58 kt / yr of useful outputs produced, relative to 90 kt / yr of inputs (a full breakdown is provided in Table 1 below).

Table 1. Crop production NUE for Scotland (all data 2019, except for N deposition which is 2018)

Recycling terms (not included in either inputs or outputs for the purpose of this NUE calculation):

digestate from crops, crop residues

NUE = 65%

The 65% figure compares well with international data published for 124 countries (up to 2009)^[24], where crop production NUE ranged from 40-77% for EU countries.

It is also important to note that NUE in arable production inherently varies depending on farm type/systems, management, environmental conditions (soils, climate), etc. While good management can reduce losses, in practice some losses are inevitable due to continuous nitrogen transformation processes in soils and leaching. As such, crop production NUE values between 50-90% can generally be considered desirable but there is no simple one size fits all “good value”.

4.3 Whole-agriculture Nitrogen Use Efficiency

For mixed crop / livestock production systems, the output side of the NUE equation expands to include animal produce, such as milk, meat, eggs or wool. On the other hand, there are now further input terms associated with the additional use of fertiliser to produce animal feed (where not already accounted for under the crop production outputs, mainly grass forage) and directly imported animal feed. Finally, it should be noted that some of the terms which were inputs to an arable-only system NUE calculation become recycling terms at this scale. For example, nitrogen in livestock manures and slurries were an external input for a crop system, whereas in a whole-agriculture system they become a recycling term.

Whole-agriculture NUE for Scotland is estimated at 27%. This reflects 55 kt N / yr of useful outputs produced^[25], relative to 200 kt N / yr of inputs (a full breakdown is provided in Table 2 below, as part of a wider all-food-production analysis).

The whole-agriculture NUE figure of 27% being so much lower than the figure for crop production alone (65%) reflects the fact that livestock farming has an inherently relatively low NUE. This is because only a small proportion of the ingested nitrogen in livestock farming ends up in useful nitrogen-containing produce and most is excreted. This excreted nitrogen (and phosphorus) still constitutes a very valuable resource of nutrients. When well-managed, a greater proportion of these nutrients can be recycled, thereby reducing both losses to the environment and waste of resources through the need for additional mineral fertiliser purchase.

For Scotland, a simple feed-conversion calculation would result in a rough value of c. 10% NUE for livestock activity if this were taken in isolation (i.e. nitrogen in livestock produce 20 kt N / yr of useful outputs, versus inputs of nitrogen in livestock feed of 199 kt / yr). Such a value is broadly consistent with international comparisons for livestock feed conversion efficiency.

As such, any country with an agriculture sector that contains a relatively large proportion of livestock, especially of ruminants (pigs & poultry have relatively better feed conversion ratios for nitrogen than cattle and sheep, but only ruminant livestock can utilise the very large area of Scottish land that is not suitable for growing crops), will always have a relatively low set of overall NUE values.

It should also be noted that, at any given point in time, there is a considerable amount of nitrogen present in living animals (as protein). This could be considered as “stocks”, functionally equivalent to the nitrogen bound up in living vegetation or soils (see Chapter 3). However, for the purpose of a long-term (e.g. annually averaged) calculation of NUE, such nitrogen is neither an input nor an output (nor a loss) term and therefore does not feature in the breakdown in Table 2.

4.4 Aquaculture Nitrogen Use Efficiency

A simple feed conversion NUE calculation can also be carried out for the aquaculture sector, which in Scotland is dominated by salmon farming in coastal waters. This results in an estimated NUE value for aquaculture of c. 34%. This reflects useful output products of 7 kt N / yr, relative to inputs of 21 kt N / yr (a breakdown is provided in Table 2 below, as part of a wider all-food-production analysis). Of these useful outputs, the vast majority are from finfish production in coastal waters, i.e. mainly salmon farming, with the remainder split between freshwater finfish (mainly trout) and shellfish.

The feed conversion NUE value is higher for aquaculture (c. 34%) than it is for agricultural livestock (c. 10%), as fish are cold-blooded and a larger proportion of their feed is converted into protein.

4.5 All-food-production Nitrogen Use Efficiency

By combining the SNBS data across the agriculture and aquaculture sectors, a value for all-food-production NUE can be estimated as 28%. This reflects total useful outputs of 62 kt N / yr, relative to total inputs of 221 kt N / yr (a full breakdown is provided in Table 2 below).

The all-food-production NUE figure is dominated by the much larger overall magnitude contribution from agriculture (with an estimated NUE of 27%), slightly increased by the contribution from aquaculture (with an estimated NUE of 34%).

Table 2. Food production NUE for Scotland (all data 2019, except for N deposition which is 2018)

NUE = 28%

Recycling terms (not included in either inputs or outputs for the NUE calculation):

Manure, slurry, digestate of agricultural origin, crop residues, fodder crops grown in Scotland

4.6 Whole-economy NUE

An economy-wide NUE figure can then be arrived at by taking the all-food-production analysis and adding in the remaining SNBS sectors of forestry, transport, industry, energy and waste management.

The estimated value of the baseline^[26] figure for economy-wide NUE is 25%. This reflects total useful outputs of 78 kt N / yr, relative to total inputs of 308 kt N / yr (a full breakdown is provided in Table 3 below).

In visualising such a whole-economy NUE calculation, it may also be helpful for readers to refer back to Figure 1. The useful output arrows on this diagram (those in dark blue) refer to the same terms as set out in Table 3 above for the whole-economy NUE calculation. However, it should be noted that both the inputs and recycling terms in the context of this calculation are shown as being the same colour for the purpose of Figure 1 (i.e. as the light blue arrows). This approach is due to the complexity of the system boundaries for this scale of NUE calculation which, if reflected in full on Figure 1, would result in many additional smaller arrows needing to be included and a corresponding loss of accessibility. For example, only the part of N deposition that originates from emissions outside of Scotland represents an input at the whole-economy scale, whereas emissions to the atmosphere in Scotland depositing back to Scottish soils constitute a recycling term.

Table 3. Whole-economy NUE for Scotland (as of baseline period)

Recycling terms (not included in either inputs or outputs for the NUE calculation):

Manure, slurry, digestate of agricultural origin, crop residues and fodder crops produced in Scotland, atmospheric N deposition (where the NOx and NH₃ originated from Scottish emissions)

NUE = 25%

The economy-wide NUE figure for Scotland of 25% is dominated by the NUE value associated with food production (of 28%), as this forms by far the largest overall part – and core engine – of the SNBS as a whole.

Nonetheless, the addition of the other sectors does slightly reduce the economy-wide figure relative to the food-production one. There are several factors behind this, all of which carry substantial technical complexities:

• For the transport, energy and industry sectors, the useful outputs from these combustion processes are heat, energy and mobility. As these contain no nitrogen, these sectors inherently have an effective NUE value of zero. Nonetheless, these sectors do contribute NOx emissions from fuel burnt in Scotland and are statutorily required to be taken into account for the purposes of the relatively novel whole-economy NUE metric and can be done so on the basis laid out in an OECD paper^[29].
• The forestry sector could be considered to have an effective NUE value of c. 27%, based on production of 8 kt N / yr as useful nitrogen-bearing outputs (for material use, wood fuel and export), from 28.2 kt / yr of inputs. However, it is noted that this is not a very useful indicator – as anthropogenic fertiliser input to tree planting is relatively very small and the vast majority of nitrogen inputs to forests and woodlands are from atmospheric deposition and biological N fixation (this is explored further in the section on forestry in Chapter 3).
• For the waste management sector, NUE is simply not a sensible indicator, as almost all of the flows represent recycling from other sectors as composts, digestates and sludges.
• Further information on technical issues associated with avoiding double counting in an economy-wide NUE calculation can be found in Annex A.

As of the time of publication of this report there are, to our knowledge, no existing international comparators for such a comprehensive and detailed economy-wide NUE calculation. However, efforts are understood to be underway over the next year, through the UN Economic Commission for Europe (ECE) Task Force on Reactive Nitrogen, to encourage and enable countries to use the UN ECE guidance for national nitrogen budgets (see Annex A) to report economy-wide NUE estimates. In this context, Scotland’s SNBS plays a pioneering role on the international stage.