Scottish Nitrogen Balance Sheet 2022

Overview

The Climate Change (Nitrogen Balance Sheet) (Scotland) Regulations 2022, requires the SNBS to provide the basis of a whole-economy calculation of Nitrogen Use Efficiency (NUE).

The section “Methodology for estimating Nitrogen Use Efficiency (NUE) metrics” sets out key elements of the methods used for this calculation. 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 the previous section.

In order to contextualise this analysis, which 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.

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, and ii) international calculations of NUE at this level are widely undertaken.

Table 1 Crop production NUE for Scotland based on the SNBS data is estimated at 70%. This reflects 60.1 kt / yr of useful outputs produced, relative to 85.8 kt / yr of inputs.

Contributors to NUE in crop production

Inputs to arable land	kt N
mineral fertiliser (to arable/crops)	57.8
slurry/manure (to arable/crops)	16.1
atmospheric N deposition (to arable)	4
digestate (non-crop/crop waste feedstocks only) (data not available as split between arable/grass - using 50%)	2.1
seeds (sowing/planting)	1.7
biological N fixation (BNF) by arable crops	2
sewage sludge (data not split into arable/grass - using 50%)	1.2
compost (assumed to go to arable/horticulture)	0.89
Total	85.8

 

Useful outputs	kt N
harvest (as food, includes human-edible crops that end up as livestock feed, seed materials or biomass)	58.5
harvest (planted as fodder crops)	1.6
Total	60.1

Note: Recycling terms are not included in either inputs or outputs for the purpose of this NUE calculation: digestate from crops, crop residues.

NUE = 60.1 ÷ 85.8 = 70%

The 70% figure compares well with international data published for 124 countries (up to 2009) , 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, weather), 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.

Even in the absence of management changes, some variability in NUE due to natural effects can be expected. 2022 had good weather for farming, and this resulted in the highest cereal yields for 20 years. The result of this for the efficiency calculations is much higher outputs for very similar inputs and so an increase in efficiency. Years with more typical weather would have more typical outputs and so efficiencies closer to those of the years previous to 2022.

Livestock Feed Conversion Nitrogen Use Efficiency

Table 2. Looking only at livestock’s ability to turn feed (including grazing) into produce yields a NUE of around 10%, reflecting 19.3 kt N /yr of produce from 197 kt N /yr of feed.

Contributors to NUE in Livestock Feed Conversion

Inputs to arable land	kt N
feed (includes grass, fodder crops, concentrates)	197
Total	197

 

Useful outputs	kt N
livestock produce (milk, eggs, meat, wool)	19.3
Total	19.3

Note: Recycling terms (not included in either inputs or outputs for the purpose of this NUE calculation): manures, materials used to grow the feed which is entangled with crop farming

NUE = 19.3 ÷ 197 = 10%

Whole-agriculture Nitrogen Use Efficiency

For mixed crop / livestock production systems, the output side of the NUE equation includes both livestock and crop produce. 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.

Table 3 Whole-agriculture NUE for Scotland is estimated at 33%. This reflects 53.6 kt N / yr of useful outputs produced, relative to 162.8 kt N / yr of inputs.

Contributors to whole-agriculture NUE

Inputs to whole agriculture	kt N
mineral fertiliser (to arable/crops and grass)	104.7
livestock feed (not grown on Scottish farms, e.g. soy)	21.5
biological N fixation (BNF) by arable crops and grass	18.4
atmospheric N deposition (to arable and grass)	12.1
sewage sludge to agricultural land	2.5
digestate (non-agricultural feedstocks only)	2.6
compost to agricultural land	0.89
seeds (sowing/planting - net import)	0.12
Total	162.8

 

Useful outputs	kt N
harvest (as food, excludes human-edible crops that are used as livestock feed or for seed)	34.3
livestock produce (milk, eggs, meat, wool)	19.3
Total	53.6

Note: Recycling terms (not included in either inputs or outputs for the purpose of this NUE calculation): manures/slurries, digestate of agricultural origin, straw (comes back to fields via manures), fodder crops grown on farm

NUE = 53.6 ÷ 162.8 = 33%

The whole-agriculture NUE figure of 33% being so much lower than the figure for crop production alone ( 70%) 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.

As such, any country with an agriculture sector that contains a relatively large proportion of livestock 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. 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.

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.

Table 4. The estimated NUE value for aquaculture is 32%. This reflects useful output products of 6.3 kt N / yr, relative to inputs of 19.9 kt N / yr.

Contributors to aquaculture NUE

Inputs to aquaculture	kt N
aquaculture feed	19.9
Total	19.9

 

Useful outputs	kt N
harvest (finfish coastal waters)	5.8
harvest (shellfish)	0.26
harvest (finfish freshwater)	0.25
Total	6.3

Note: Recycling terms (not included in either inputs or outputs for the purpose of this NUE calculation): assuming no harvested outputs from aquaculture produce are recycled into aquaculture feed.

NUE = 6.3 ÷ 19.9 = 32%

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 ( 32%) than it is for agricultural livestock ( 10%), as fish are cold-blooded and a larger proportion of their feed is converted into protein.

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.

Table 5. The estimated all-food NUE value is 33%. This reflects total useful outputs of 59.9 kt N / yr, relative to total inputs of 182.6 kt N / yr (a full breakdown is provided in the above tables).

Contributors to all-food-production NUE

Inputs to all food production	kt N
mineral fertiliser (to arable/crops and grass)	104.7
livestock feed (not grown on Scottish farms, e.g. soy)	21.5
aquaculture feed	19.9
biological N fixation (BNF) by arable crops and grass	18.4
atmospheric N deposition (to arable and grass)	12.1
sewage sludge to agricultural land	2.5
digestate (non-agricultural feedstocks only)	2.6
compost to agricultural land	0.89
seeds (sowing/planting - net import)	0.12
Total	182.6

 

Useful outputs	kt N
harvest (as food, excludes human-edible crops that are used as livestock feed or for seed)	34.3
livestock produce (milk, eggs, meat, wool)	19.3
harvest (finfish coastal waters)	5.8
harvest (shellfish)	0.26
harvest (finfish freshwater)	0.25
Total	59.9

Note: Recycling terms (not included in either inputs or outputs for the purpose of this NUE calculation): manures/slurries, digestate of agricultural origin, straw (comes back to fields via manures), fodder crops grown on farm, assuming no harvested outputs are recycled into aquaculture feed (however modern fish feed contains plant ingredients, including some homegrown).

NUE = 59.9 ÷ 182.6 = 33%

The all-food-production NUE figure is dominated by the much larger overall magnitude contribution from agriculture (with an estimated NUE of 33%), with aquaculture (with an estimated NUE of 32 %) having much less influence. 

Forestry NUE

Forestry is the only non-food related sector producing a significant amount of useful outputs from which a NUE can be calculated. Inputs to forestry are primarily from the trees and the woodland they are in more generally absorbing nitrogen from the atmosphere. Outputs are the wood harvested for material use, fuel and export.

Table 6. The estimated NUE for forestry is 27% from inputs of 25.8 kt N /yr and outputs of 7 kt N /yr.

Contributors to Forestry NUE

Inputs to forestry	kt N
atmospheric N deposition to woodland	17.1
biological N fixation (BNF) by forests	10
mineral fertiliser to woodland planting	0.013
seedlings (planting)	0.0099
Total	27.2

 

Useful outputs	kt N
Forestry harvest - material use	4.6
Forestry harvest - wood fuel	2
Forestry harvest - export	0.43
Total	7

NUE = 7 ÷ 27.2 = 26%

N.B. Forestry NUE is not all that useful, as much of forest N is locked up in stocks/standing forests. NUE could be increased by depleting stocks (i.e. increased felling), which is not a useful concept. That the anthropogenic inputs (fertiliser and seedlings) represent such a small proportion of the inputs to forestry further limits the utility of this measure.

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.

Table 7. The estimated value of the figure for economy-wide NUE is 27%. This reflects total useful outputs of 66.9 kt N / yr, relative to total inputs of 251.9 kt N / yr.

Contributors to Whole Economy NUE

Inputs at whole economy level	kt N
mineral fertiliser (all land)	104.7
biological N fixation (all land)	44.4
atmospheric N deposition (imported NOx + NHx only)	38.3
NOx emissions from fuel burnt in Scotland (to account for transport, energy & industry)	22.9
livestock feed (not grown in Scotland)	21.5
aquaculture feed	19.9
import of wood (roundwood)	0.074
seeds (sowing/planting - net import)	0.12
Total	251.9

 

Useful outputs	kt N
harvest (as food, excludes human-edible crops that are used as livestock feed or for seed)	34.3
livestock produce (milk, eggs, meat, wool)	19.3
Forestry harvest - all uses (inc. export)	7
aquaculture produce (finfish freshwater, coastal))	6.3
exported materials (that aren’t already accounted for above) - e.g. industrial output	0
Total	66.9

Note: Recycling terms (not included in either inputs or outputs for the purpose of this NUE calculation): manures/slurries, other recycled materials (composts, sewage, digestate), straw (comes back to fields via manures), assuming no harvested outputs are recycled into aquaculture feed (however modern fish feed contains plant ingredients, including some homegrown), N deposition originating from Scottish ammonia and NOx emissions, all livestock feed grown in Scotland (grass and fodder), seeds/planting materials grown in Scotland

NUE = 66.9 ÷ 251.9 = 27%

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.

The economy-wide NUE figure for Scotland of 27% is dominated by the NUE value associated with food production (of 33%).

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 whole-economy NUE metric; and have been done so on the basis proposed by the OECD .
• 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 the methodology subsection “Methodology for estimating Nitrogen Use Efficiency (NUE) metrics”.

Comparison to Previous Years

Table 8. Agriculture and related NUEs show a higher efficiency in the most recent year.

Total inputs and useful outputs, and the resultant NUEs, for years from 2019 to 2022.

NUE Groups	Input 2019	Input 2020	Input 2021	Input 2022
Arable	91.7	88.9	86.7	85.8
Livestock	198.7	197.3	197.2	197.0
Whole Agriculture	197.7	184.9	186.9	162.8
Aquaculture	21.2	22.9	21.2	19.9
All Food	218.9	207.8	208.1	182.6
Forestry	28.1	28.1	27.2	27.2
Whole Economy	295.3	280.0	279.0	251.9
NUE Groups	Output 2019	Output 2020	Output 2021	Output 2022
Arable	58.0	58.2	56.1	60.1
Livestock	20.1	20.0	19.7	19.3
Whole Agriculture	55.0	55.4	51.5	53.6
Aquaculture	7.3	6.9	7.4	6.3
All Food	62.3	62.3	59.0	59.9
Forestry	7.5	7.7	8.2	7.0
Whole Economy	69.8	70.0	67.2	66.9
NUE Groups	NUE 2019	NUE 2020	NUE 2021	NUE 2022
Arable	63%	65%	65%	70%
Livestock	10%	10%	10%	10%
Whole Agriculture	28%	30%	28%	33%
Aquaculture	34%	30%	35%	32%
All Food	28%	30%	28%	33%
Forestry	27%	27%	30%	26%
Whole Economy	24%	25%	24%	27%

 

A year-to-year change in efficiency was seen in Arable Agriculture, which saw an increase in efficiency from 65% in 2021 to 70% in 2022. This is mostly due to an increase in useful outputs due to an increased crop harvest in 2022. The long periods of hot and dry conditions resulted in the highest total cereal yield of the last 20 years. Given that this increase in efficiency was due to the unusually good weather in 2022 rather than human action, we should expect that future years with more typical weather patters would show efficiencies closer to those of previous years.

This increase in cereal yield also formed part of the reason for the increase in the Whole Agriculture efficiency from 28% in 2021 to 33% in 2022. However, the increase in efficiency at the Whole Agriculture level was not just driven by an increase in outputs due to the cereal yields, but also a decrease in inputs. The bulk of this decrease in inputs as a Whole Agriculture level was driven by a reduction in the use of mineral fertilisers on grasslands, likely due to less being required as a result of the unusually good weather. As with Arable Agriculture, we should expect future years to resemble previous years rather than 2022.

As food production is the largest part of Nitrogen in the economy, this agricultural increase in efficiency in 2022 resulted in an increase in efficiency at the whole economy level, from 24% in 2021 to 27% in 2022.

There is a reduction in efficiency in Forestry (from 30% in 2021 to 26% in 2022) however as mentioned above in the NUE calculations for Forestry, the Forestry NUE is not as useful a measure as the other NUEs due to so much Nitrogen being locked up in stocks of existing trees. Increases or decreases in efficiency may represent forest stocks being depleted or augmented rather than changes in practice.

Caution should be taken when comparing NUE values from year to year. The SNBS is a relatively new publication and there are still further improvements to be made with data sources. The filling in of any gaps in either the measured inputs or measured outputs would result in either a decrease or increase in the NUE. Methodology related changes in the NUE may also be seen when switching sources to sources more suitable for yearly updates. See sections Revisions and Next Steps for changes to the data for this release and planned changes in future releases.