Official Statistics

Annex

Published 27 October 2022

Section 1: Drivers of emissions

1.1 Drivers of recorded sector emissions

The methodology used to report agricultural emissions has been predominantly based on the number of livestock animals and the amount of nitrogen-based fertiliser applied to land. A variety of important factors influence emissions which are not captured by this methodology (see “Other drivers of emissions” below for details); research has been undertaken to better reflect the position. The results of this research have been incorporated into an upgraded greenhouse gas (GHG) inventory for agriculture, implemented since 2017.

1.2 Other drivers of emissions

There are other factors which are not captured in estimated emissions, but which are likely to affect the true level of emissions. For example, some areas of farming practice will have an impact, e.g. timing of fertiliser application, efficiency of fertiliser use, feed conversion ratios, genetic improvements. Some of these relate to efficiency: there have been productivity gains in the sector, through more efficient use of inputs over the last twenty years and some of these gains will have had a positive impact, though some may have had a negative impact on emissions. Soil moisture and pH are also highly important to soil emissions. On a national basis these drivers are expected to have a subtle, but significant impact, rather than a dramatic impact on the true level of emissions over the period. On a regional basis, the drivers of soil emissions are likely to have a more dramatic impact for some land use types.

1.3 Nitrous oxide emissions

Direct emissions of nitrous oxide from agricultural soils are estimated for the following: use of inorganic fertiliser, biological fixation of nitrogen by crops, ploughing in crop residues, cultivation of histosols (organic soils), spreading animal manures on land and manures dropped by animals grazing in the field. In addition to these, the following indirect emission sources are estimated: emission of nitrous oxide from atmospheric deposition of agricultural nitric oxide and ammonia and the emission of nitrous oxide from leaching of agricultural nitrate and runoff. Also, nitrous oxide emissions from manures during storage are calculated for a number of animal waste management systems.

1.4 Methane emissions

Methane is produced as a by-product of enteric fermentation and from the decomposition of manure under anaerobic conditions. Enteric fermentation is a digestive process whereby feed constituents are broken down by micro-organisms into simple molecules. Both ruminant animals (e.g. cattle and sheep), and non-ruminant animals (e.g. pigs and horses) produce methane, although ruminants are the largest source per unit of feed intake. When manure is stored or treated as a liquid in a lagoon, pond or tank it tends to decompose anaerobically and produce a significant quantity of methane. When manure is handled as a solid or when it is deposited on pastures, it tends to decompose aerobically and little or no methane is produced. Hence the system of manure management used affects emission rates.

1.5 Uncertainty in emissions estimates

There are relatively large uncertainties in estimating agricultural emissions as they are generated by heterogeneous natural systems for which we do not have precise measures.

Uncertainties around nitrous oxide emissions are particularly large; they incorporate spatial and temporal variation in emissions factors (e.g. soil texture variations etc), and more structural uncertainties relating to the way the farming industry and biological processes are represented in the current model. Some of these uncertainties are already understood to some extent, whilst others have undergone further research as part of the recent inventory improvement programme.

The table below shows typical uncertainties (for 2020 estimates) in the current methodology and reflects recent improvements in the analysis^{[footnote 1]} although, it will not be possible to remove all uncertainty.

Table 1. Emissions uncertainty

IPCC Category	Gas	2020 emissions (Gg CO2e)	Combined activity and emission factor uncertainty (%)
3A Enteric fermentation	Methane	20,937.60	13.7%
3B Manure management	Methane	3,812.01	10.5 %
3B Manure management	Nitrous oxide	2,813.54	17.3 %
3D Agricultural soils	Nitrous oxide	11,648.42	21.7 %

Source: UK National Inventory Report Annex 2

Section 2: Emission intensity and background

2.1 Why agriculture produces GHGs

GHG emissions in agriculture are dominated by non-carbon dioxide emissions that occur from three main sources:

• Emissions of methane from ruminant livestock burps: Ruminant livestock produce methane during their digestive processes. Micro-organisms in the rumen degrade carbon from feeds in the absence of oxygen, producing methane gas. This gas is subsequently emitted to air by eructation (burps). Emissions are affected by diet, health and livestock management. In addition it may be possible to use treatments that regulate or destroy methane producing micro-organisms in the rumen.

• Emissions of nitrous oxide from fertilisers applied to land: Nitrogen in fertilisers and manures is transformed to nitrous oxide during the biological processes of nitrification and denitrification. These processes are driven by soil bacteria and are mediated by soil type, soil management and weather. Emissions can be controlled to some extent by nutrient planning, fertiliser application methods and management of soil condition. In addition urea based fertilisers produce carbon dioxide when applied to soils during the process of hydrolysis. Liming of soils results in emissions of carbon dioxide from the breakdown of lime but can enhance carbon sequestration on acidic grasslands. All nitrogen containing fertiliser media applied to soils can release ammonia gas (an important air pollutant) when applied to soils in a process called volatilisation. This is particularly true of anaerobic digestates, slurries and urea based fertilisers. Although not a GHG in itself, deposition of ammonia to land results in indirect emissions of nitrous oxide.

• Emissions of methane and nitrous oxide from livestock manures during storage and application to land: Livestock excreta contain carbon and nitrogen that is subject to chemical transformation via biological processes. Depending on how excreta is managed and applied to land varying amounts of methane and nitrous oxide are produced.

Further information can be found on the Defra Livestock numbers webpage.

Further information can be found on the Soil Nutrient Balances, Fertiliser Usage and Farm Practices Survey webpages.

In addition there are two carbon dioxide based sources of emissions from agriculture:

• Emissions of carbon dioxide from machinery use, heating etc: These only account for 9% of agricultural emissions. The 2016 inventory estimate (which covers the period 1990 to 2014) was 4.7 million metric tonnes of carbon dioxide equivalent (MtCO₂e). These emissions can be controlled by low energy technologies, changes in management practice (e.g. minimum tillage) and implementation of renewable energy generation

• Emissions of carbon dioxide from land use change and land management: Soils store carbon at equilibrium concentrations that reflect the balance of organic material inputs versus biological turnover of carbon (bugs eat organic materials and respire carbon dioxide). Croplands have lower soil carbon content than grasslands due to lower organic inputs and greater disturbance from ploughing etc. Lands converted to cropland produced about 12 MtCO₂e in 2014. About 10% of the cropland emissions result from historic drainage of lowland peat for agriculture. This is offset to some extent by land converted to pasture, which sequestered about 9 MtCO₂e in 2014. Agriculture was therefore a net source of land use emissions (3 MtCO₂e) in 2014. Overall the Land Use, Land Use Change and Forestry (LULUCF) sector was a net sink 9 MtCO₂e in 2014 due to the large forest sink.

Agriculture has a complex set of biological processes that control emissions of three major GHGs via a diverse range of pathways. In addition soils and biomass possess the ability to absorb (sequester) carbon to some extent. As such agricultural emissions can be difficult to model and monitor. However, the range of gasses and emissions pathways also means that there is a diverse set of entry points for mitigation of emissions from the sector.

It is always important to ensure mitigation of GHGs does not result in detrimental impacts to other environmental outcomes (for example air and water quality) and this is particularly true in agriculture, where there are real risks of unintended consequences if mitigation is not planned carefully. Fortunately, many agricultural mitigation options (in particular those that reduce the losses of nitrogen to the environment) provide co-benefits for air and water quality as well as reducing soil acidification and protecting biodiversity. Since nitrogen in fertiliser and feed is procured at a cost to the farmer, such options are often economically beneficial as well.

Enquires to: farmingscience@defra.gov.uk

Section 3: Greenhouse gas emissions

Uptake of on-farm mitigation measures

Details of uptake rates for a wide range of on-farm mitigation measures can be found in the results of the February 2021 Farm Practices Survey.

The survey focused on practices relating to greenhouse gas mitigation with topics including nutrient and manure management, manure and slurry storage, farm health plans, cattle and sheep breeding and feeding regimes and anaerobic digestion.

1. The 95% confidence interval given in the “Analysis of uncertainties in the estimates of nitrous oxide and methane emissions in the UK’s greenhouse gas inventory for agriculture” for the estimate of total nitrous oxide emissions from soils in 2010 is (−56%, +143%). This reduced uncertainty reflects improved analysis and is substantially different to that given by Brown et al. (2012). Their confidence interval, based on expert opinion, was (−93%, +253%). However (−56%, +143%) is still much larger than that derived by Monni et al. (2007) who quote a 95% confidence interval of (−52%, +70%). Their analysis was based on more conservative estimates for the uncertainty in emissions factors (from IPCC 1997) whereas the 95% confidence interval of (−56%, +143%) was derived using more recent IPCC guidelines (Eggleston et al., 2006). ↩