What are soil carbon stocks and how are they measured?

THE British Society of Soil Science has released a guide to soil carbon monitoring, reporting and verification for land managers. The briefing, Soil Carbon: What are carbon stocks and how can they be measured?, is the first of a new series, Land Use for Net Zero.

The British Society of Soil Science (BSSS) says that soil has a critical role to play in climate change. It can reduce and remove atmospheric greenhouse gases (GHGs), and provide a location for clean energy production to take place. Their briefing series aims to help land managers understand where they should focus their efforts and the trade-offs involved in each of decision.

“Around 30% of the Earth’s surface is land. The way we use and interact with land will have an impact on climate change. Land and soil can contribute towards climate change mitigation for example, by removing carbon dioxide from the atmosphere and providing locations for clean energy production.”

The briefing addresses soil carbon finance schemes, storage techniques, soil sampling and analysis, monitoring, reporting and verification, challenges in using soils as carbon stores and scope three emissions.

The BSSS says that soils are the dominant natural land-based carbon sink, holding more carbon than all terrestrial habitats combined. There is a significant opportunity to increase soil carbon stocks and reduce GHG emissions from many agricultural soils by changing management practices and how land is used.

They say that government-funded incentive schemes can help finance soil management practices that deliver improvements in soil health along with wider environmental benefits such as improved water quality. However, they point out that UK public finance does not at present, specifically incentivise soil carbon increases, while the majority of private carbon finance schemes combine soil carbon stock increases and soil derived GHG emission reductions to calculate a net gain in soil carbon. Carbon finance schemes, they note, only consider the amount of organic carbon in soils as tonnes per hectare (t/ha) to a defined soil depth (e.g. 0-30cm, 30-60cm, 60cm and deeper) converted into a CO2 equivalent.

Continuing scientific debate
Interestingly, the briefing says that soil carbon stocks will eventually reach a plateau (i.e. equilibrium) where no further carbon will be store. However, as 8.9ha has previously reported, this is widely contested, and many believe that the peer-review processes will eventually catch up with the observed on-the-ground experiences of farmers and other stakeholders.

The briefing also warns that soil carbon stocks are easily reversible and will need continual management to permanently maintain them. While this in itself is not in dispute, these concerns should be understood alongside the fact that grassland sequestration has great potential, and that other natural carbon stores, such as forests, are also highly vulnerable, for example from fire and disease, which can quickly decimate years of sequestration.

There is however no dispute that the rate of sequestration will depend on a range of factors including soil texture, new and past management, weather conditions and soil nutrient status. This means that field verification is essential.

The briefing ends in a series of recommendations:

  • Before signing up to any scheme, it is important to consider that soil carbon credits can only be sold once through the Voluntary Soil Carbon Market
  • Soil carbon removals remain with the land and can be transferred across the supply chain e.g. for Scope 3 reporting
  • There may be financial and non-financial consequences, such as taxes and potential reduced income from other contracts, of signing up to any scheme and these should be considered when selecting a scheme
  • For reliable measurement of soil carbon stocks, field sampling should be carried out in conjunction with an appropriate sampling design, approved analytical methods and reliable modelling, where relevant
  • Field sampling must go to depth, ideally below the management layer, to ensure that soil carbon stock changes are captured
  • Measurement of soil carbon stocks requires analyses of soil carbon content and fine dry bulk density, along with soil dry mass for change assessments
  • Carbon removals from soils (i.e. increased soil carbon stocks) should be considered alongside carbon reductions from soils (i.e. reduced greenhouse gas emissions)
  • Permanence in soil carbon stocks requires a long-term commitment to changing how soil is managed
  • Expert advice from an independent advisor will support you in making the right decision for you and your land.

Read the full briefing


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