Erthygl Carbon ar gyfer ‘Y Tir’

The production of greenhouse gases (GHG) is known to accelerate the process of climate change. The agriculture sector was responsible for 12% of Welsh GHG emissions in 2016 (Welsh Government, 2016). Wales is committed to a target of 95% reduction (of 1990 levels) in GHG emissions by 2050 (Welsh Government, 2019). Due to this, agriculture, along with all other sectors, is under considerable pressure to reduce its emissions, as well as its broader environmental impacts. To help meet this challenge, approaches that farmers can take include 1) reducing GHG emissions produced on-farm, and 2) sequestering (fixing) carbon by removing carbon dioxide from the atmosphere; therefore ‘offsetting’ some of the emissions generated on-farm. A farm carbon footprint provides estimates of both the GHG produced on-farm as well carbon sequestered on the farm. It can be measured using various carbon footprint calculators.

GHGs are gases within the atmosphere that trap heat. The three main greenhouse gases in agriculture are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). These three are converted to their ‘carbon dioxide equivalents’ (CO2e) in carbon footprint calculators, based on their Global Warming Potential (GWP) values used internationally by the Intergovernmental Panel on Climate Change (IPCC). These GHGs vary in both their strength and lifespan; the GHGs with the greatest impact from agriculture being methane and nitrous oxide as both gases are very potent in terms of their GWP.

Reported figures suggest that 56% of UK agricultural GHGs is methane, 31% is nitrous oxide and 13% is carbon dioxide (AHDB, 2024).

Each GHG derives from different sources on-farm. Here are some examples of their sources -

  • Carbon dioxide - burning of fossil fuels, fuel and energy usage, fertiliser, land cultivation

  • Methane - ruminant livestock via enteric fermentation, manure handling and spreading, slurry stores

  • Nitrous oxide - fertiliser deposition, manure spreading on pastures, crop residues

Measures to lower a farm’s carbon footprint can be specific to an individual farm/enterprise. Many are not new practices and/or will not cost the farming business, and are often techniques that are already being implemented on-farm. Measures to reduce a farm’s carbon footprint also result in improved efficiency in many cases, generating cost savings. These measures include those relating to livestock, land and energy.

It is not possible to avoid all GHG emissions produced but there may be opportunity to offset them on-farm. Agriculture is one of few sectors that is able to ‘sequester’ and store carbon. Carbon sequestration is the natural process by which carbon dioxide is captured and removed from the atmosphere during photosynthesis. A portion of this is then transferred to the soil through the roots and stored below-ground in soil, as well as in trees and hedges. Carbon stock refers to the existing quantity of carbon stored on a farm within trees, hedges, and soils. 

Soil carbon stocks are impacted by various factors, including soil type, rainfall and management of the land. Estimates suggest that UK soil carbon stocks exceed 4,000 megatonnes of carbon (Bradley et al., 2005). Current research suggests that existing soil carbon stocks are constant or increasing under permanent grasslands (Emmett et al., 2023). Soils will reach a saturation point in terms of carbon storage, whereby the potential to store additional carbon in them will become limited (Smith, 2014). Some soils will already be at this saturation point, and due to this, further carbon sequestration gains in soil carbon stocks are variable. Furthermore, degraded soils may currently have a low carbon stock though therefore may have greater potential for further carbon sequestration. There has been no increase in topsoil carbon reported in the UK over the last 30 years (Emmet et al., 2010).

Farming Connect’s Welsh Soil Project is a Pan-Wales initiative that aims to estimate the soil carbon stock of various fields to a soil depth of 50 cm on a number of different farms. A total of 56 farms from Farming Connect’s ‘Our Farms’ Network have been involved in the project to-date, and the detailed data collected can be used to provide an insight into how soil carbon stocks vary between fields of varying management and between farming systems (depending on factors such as land use, topography, climate). The findings have indicated great variation in soil organic carbon content and estimated organic carbon stock between farms. A snapshot of the results for a group of 15 farms sampled in Autumn 2023 are provided below -

 

Figure 1: Average Soil Organic Matter content (%) of each sampling depth within each field type for all farms.

Table 1: Average Soil Carbon Stock (t/ha) of each sampling depth within each field type.

Carbon levels are more stable at depth, and a minimum sampling depth of 30 cm is recommended to account for differences within the soil profile according to best practice (IPCC, 2006). Research has shown that significant changes in soil carbon stocks generally occur in the top 20 cm of the soil (Fornara et al., 2020), and therefore, carbon losses are reduced at greater depth due to less soil disturbance. Carbon is lost from soils much quicker than it is gained, and so, maintaining current carbon stocks is important (Smith, 2014). Measures to maintain current soil carbon stocks, and potentially increase carbon sequestration include minimising soil disturbance and using minimum tillage techniques such as direct drilling and scarification of seeds where possible/suitable to do so. Permanent pastures generally have deeper-rooting systems than annual crops as plant species grow a larger rooting network over time, thus will accumulate deeper layers of carbon fixing in soils (Thorup-Kristensen et al., 2020). Many of these measures may also lead to additional benefits, such as increased pasture productivity, improvement in soil health, structure, fertility and water retention capability.

Trees (and hedges), as with grass, absorb carbon dioxide during photosynthesis and store it as biomass. They therefore have good carbon sequestration potential, acting as both an above and below-ground carbon storage. Trees may not start sequestering much carbon instantly after planting, but generally, soil carbon stock will increase with greater tree growth. Potential sequestration rates by new and existing woodlands depend on a number of factors. In terms of woodland creation, the species planted will impact sequestration rates (i.e., fast growing species such as conifers may result in higher sequestration rates). Factors that limit tree growth, such as lack of management or diseases, will limit carbon sequestration potential, and in some cases, result in the opposite process (release of carbon to the atmosphere). As with soils, trees and hedges will also reach a plateau in terms of their carbon stock capabilities.

In summary, there has never been a greater need to improve our understanding of both GHG emissions and carbon sequestration in agriculture. This may become even more of a priority in the future as further research is carried out. Careful consideration should be taken when interpreting and broadly discussing carbon (whether that is emissions produced or sequestration) as there are many ways of expressing it, as well as some variability in methodology for calculating it. A key point when repeatedly measuring and assessing a farm carbon footprint over time is that the same method (and calculator) is used each time. Carbon should not be considered in isolation as many farming practices that benefit a farm’s carbon footprint can also offer economic and environmental gains.

References

AHDB (2024). Greenhouse gas emissions: agriculture. Available Here Accessed November 11th, 2024.

Bradley, R.I., Milne, R., Bell, J., Lilly, A., Jordan, C., Higgins, A. (2005). A soil carbon and land use database for the United Kingdom. Soil Use and Management 21(4), 363-369.

Emmett, B., Evans, C., Matthews, R., Smith, P., Thompson, A. (2023). Environment and

Rural Affairs Monitoring & Modelling Programme (ERAMMP). ERAMMP Report-101: The opportunities and limitations of carbon capture in soil and peatlands. Report to Welsh Government (Contract C208/2021/2022)(UK Centre for Ecology & Hydrology Project 08435).

Emmett, B.A., Reynolds, B., Chamberlain, P.M., Rowe, E., Spurgeon, D., Brittain, S.A., Frogbrook, Z., Hughes, S., Lawlor, A.J., Poskitt, J., Potter, E., Robinson, D.A., Scott, A., Wood, C., Woods, C. (2010). Countryside Survey: Soils Report from 2007. Technical Report No. 9/07 NERC/Centre for Ecology & Hydrology 192pp. (CEH Project Number: C03259). 

Fornara, D., Olave, R., Higgins, A. (2020). Evidence of low response of soil carbon stocks to grassland intensification. Agriculture, Ecosystems and Environment 287, 106705.

IPCC (2006). Chapter 2. Generic methodologies applicable to multiple land-use categories. In: 2006 IPCC Guidelines for National Greenhouse Gas Inventories. Volume 4. Agriculture, forestry and other land use. Prepared by the National Greenhouse Gas Inventories Programme (eds Eggleston HS, Buendia L, Miwa K, Ngara T, TanabeK), 2.1–2.59. IGES, Japan.

Smith, P. (2014). Do grasslands act as a perpetual sink for carbon?. Global Change Biology 20(9), 2708-2711.

Thorup-Kristensen, K., Halberg, N., Nicolaisen, M., Olesen, J.E., Crews, T.E., Hinsinger, P., Kirkegaard, J., Pierret, A., Dresbøll, D.B. (2020). Digging deeper for agricultural resources, the value of deep rooting. Trends in Plant Science 25(4), 406-417.

Welsh Government (2019). Wales accepts Committee on Climate Change 95% emissions reduction target. Available Here Accessed November 11th, 2024.

Welsh Government (2016). Agriculture: Sector Emission Pathway. Available Here Accessed November 11th, 2024.