Scottish Nitrogen Balance Sheet, 2021

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 65%. This reflects 56.1 kt / yr of useful outputs produced, relative to 86.7 kt / yr of inputs.

Contributors to NUE in crop production

Inputs to arable land	kt N
mineral fertiliser (to arable/crops)	58.6
slurry/manure (to arable/crops)	16.2
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	86.7

 

Useful outputs	kt N
harvest (as food, includes human-edible crops that end up as livestock feed, seed materials or biomass)	54.6
harvest (planted as fodder crops)	1.6
Total	56.1

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

NUE = 56.1 ÷ 86.7 = 65%

The 65% 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), 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.

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 19.7 kt N /yr of produce from 197.2 kt N /yr of feed.

Contributors to NUE in Livestock Feed Conversion

Inputs to arable land	kt N
feed (includes grass, fodder crops, concentrates)	197.2
Total	197.2

 

Useful outputs	kt N
livestock produce (milk, eggs, meat, wool)	19.7
Total	19.7

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 = 19.7 ÷ 197.2 = 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 28%. This reflects 51.5 kt N / yr of useful outputs produced, relative to 183.8 kt N / yr of inputs.

Contributors to whole-agriculture NUE

Inputs to whole agriculture	kt N
mineral fertiliser (to arable/crops and grass)	125.8
livestock feed (not grown on Scottish farms, e.g. soy)	21.2
biological N fixation (BNF) by arable crops and grass	18.3
atmospheric N deposition (to arable and grass)	12.1
sewage sludge to agricultural land	2.5
digestate (non-agricultural feedstocks only)	2.8
compost to agricultural land	0.89
seeds (sowing/planting - net import)	0.19
Total	183.8

 

Useful outputs	kt N
harvest (as food, excludes human-edible crops that are used as livestock feed or for seed)	31.9
livestock produce (milk, eggs, meat, wool)	19.7
Total	51.5

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 = 51.5 ÷ 183.8 = 28%

The whole-agriculture NUE figure of 28% 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.

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 35%. This reflects useful output products of 7.4 kt N / yr, relative to inputs of 21.2 kt N / yr.

Contributors to aquaculture NUE

Inputs to aquaculture	kt N
aquaculture feed	21.2
Total	21.2

 

Useful outputs	kt N
harvest (finfish coastal waters)	7
harvest (shellfish)	0.25
harvest (finfish freshwater)	0.21
Total	7.4

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 = 7.4 ÷ 21.2 = 35%

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 ( 35%) 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 29%. This reflects total useful outputs of 59 kt N / yr, relative to total inputs of 205 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)	125.8
livestock feed (not grown on Scottish farms, e.g. soy)	21.2
aquaculture feed	21.2
biological N fixation (BNF) by arable crops and grass	18.3
atmospheric N deposition (to arable and grass)	12.1
sewage sludge to agricultural land	2.5
digestate (non-agricultural feedstocks only)	2.8
compost to agricultural land	0.89
seeds (sowing/planting - net import)	0.19
Total	205

 

Useful outputs	kt N
harvest (as food, excludes human-edible crops that are used as livestock feed or for seed)	31.9
livestock produce (milk, eggs, meat, wool)	19.7
harvest (finfish coastal waters)	7
harvest (shellfish)	0.25
harvest (finfish freshwater)	0.21
Total	59

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 ÷ 205 = 29%

The all-food-production NUE figure is dominated by the much larger overall magnitude contribution from agriculture (with an estimated NUE of 28%), slightly increased by the contribution from aquaculture (with an estimated NUE of 35%).

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 30.2 kt N /yr and outputs of 8.2 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
import of wood	0.09
mineral fertiliser to woodland planting	0.017
seedlings (planting)	0.013
Total	27.3

 

Useful outputs	kt N
Forestry harvest - material use	5.8
Forestry harvest - wood fuel	1.9
Forestry harvest - export	0.47
Total	8.2

NUE = 8.2 ÷ 27.3 = 30%

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 25%. This reflects total useful outputs of 75.1 kt N / yr, relative to total inputs of 296 kt N / yr.

Contributors to Whole Economy NUE

Inputs at whole economy level	kt N
mineral fertiliser (all land)	125.8
biological N fixation (all land)	44.3
atmospheric N deposition (imported NOx + NHx only)	38.3
NOx emissions from fuel burnt in Scotland (to account for transport, energy & industry)	25
livestock feed (not grown in Scotland)	21.2
aquaculture feed	21.2
import: fish landings	7.9
import of wood (roundwood)	0.09
seeds (sowing/planting - net import)	0.19
import: food	11.9
import: consumer goods, etc	0
Total	296

 

Useful outputs	kt N
harvest (as food, excludes human-edible crops that are used as livestock feed or for seed)	31.9
livestock produce (milk, eggs, meat, wool)	19.7
import: fish landings	7.9
Forestry harvest - all uses (inc. export)	8.2
aquaculture produce (finfish freshwater, coastal))	7.4
exported materials (that aren’t already accounted for above) - e.g. industrial output	0
Total	75.1

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 = 75.1 ÷ 296 = 25%

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 25% is dominated by the NUE value associated with food production (of 29%).

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”.