Scottish grasslands and soil carbon

'The UK Committee on Climate Change (2019)1 estimates that more than 50% of agricultural greenhouse gas (GHG) emissions could be removed through the introduction of mitigation approaches that target methane and nitrous oxide emissions.

But what about the other 50%? One common assertion is that carbon (C) is sequestered in soils, particularly in permanent pastures and is not taken account of in GHG inventories.

There is some incentive to maintain permanent pastures through the Greening element of the Common Agricultural Policy, but is it true that grasslands in Scotland can help bridge the gap between actual and net zero GHG emissions?

Soil C sequestration is the process whereby C, in the form of organic matter (e.g. from plant roots, manures and plant litter), is incorporated into the soil. The organic matter can be stabilised through binding to soil minerals, preventing it from being further decomposed by soil microbes and releasing CO2 back to the atmosphere, but this process is incredibly slow.

Soil C can be lost rapidly due to disturbances, including ploughing pasture to reseed. It can take many years for organic matter to become stabilised in soil and up to 100 years for soil to recover to an equilibrium C stock (the C stock is the total mass of C sequestered in the soil).

Sequestration rates are rapid to begin with but get progressively slower over time. Where permanent grasslands are observed to be sequestering C, it is highly likely that they are recovering from some previous land management practice.

So, the potential for soil C sequestration to mitigate GHG emissions in grassland is not insignificant, but it is limited and easily reversible.

It is currently assumed in the UK Inventory of GHG Emissions that grasslands that have not been recently associated with a land-use change maintain a constant C stock. The justification for this assumption is provided by national soil survey data and detailed analysis of C fluxes from individual sites.

Although this assumption will not allow for individual circumstances where management activities have been used to increase C stocks, it is likely to be broadly true of the C balance in most cases.

What is arguably more important is protecting current soil C stocks from being lost and to value them for more than just their ability to sequester C. Soils also provide vital ecosystem services such as flood prevention, maintaining fertility, encouraging biodiversity, water quality and nutrient cycling, which are often supported by higher levels of soil organic matter. These wider benefits will be covered in greater detail in the next blog.

• I would like to thank Professor Bob Rees (SRUC), Professor Pete Smith (University of Aberdeen), and Dr Alasdair Sykes (SRUC) for advice and comments they provided during the writing of this blog and the accompanying factsheet “What is the potential of managed grasslands in Scotland to sequester carbon?”.

Fact Sheet

Carbon (C) sequestration, the permanent removal of C from the atmosphere, will be needed to meet Net-Zero emissions in agriculture. On a global scale, it is believed that grassland soils have a huge potential to sequester C, but is this true for Scotland?

Managed grasslands in Scotland hold approximately 172 million tonnes of carbon (MtC) and available data suggest that there has been little change in soil C stocks in recent decades (the C stock being the C already sequestered in the soil).

There is a lack of evidence on the potential for Scottish managed grasslands to sequester C, and the UK Greenhouse Gas Inventory does not currently take account of sequestration that may be occurring on areas where land use has not changed (e.g. long-term grasslands). However, one detailed seven year study of a grazed, fertilised and occasionally mowed permanent grassland in SE Scotland found that soil C stocks were stable (e.g. not sequestering or losing C). Soil C is important for soil health, fertility, water and nutrient availability and increases resilience to extreme weather. Therefore, it is extremely important to maintain existing soil C stocks.

What can farmers do to maintain grassland carbon stocks?

Grazing management: There is a lack of evidence related to grazing management and carbon stocks in managed grasslands in Scotland. However, it is widely accepted that over-grazing reduces soil C stocks.

Increasing grassland productivity leads to increased C inputs through plant roots. This can be achieved through fertilisation (inorganic or organic i.e. manure) and seeding with high-yielding grass species and legumes. Liming increases the pH of acidic soils which makes them more productive, but it also increases CO2 emissions from soils. The net effect on soil C stocks is uncertain.

Challenges

Limited capacity: Soils have a limited capacity to sequester C, the capacity depends on the soil type and management. After a change in management, rapid C sequestration slows (~20 years) and typically reaches a new equilibrium (within ~100 years).

Permanence: Disturbing soil leads to rapid C loss. To maintain C stocks, the same management needs to be continued indefinitely. Soil C can be lost much more easily than it can be sequestered.

Trade-offs: The nitrogen in fertiliser helps increase grass productivity, but if too much is applied it creates an excess of nitrogen in the soil. This excess nitrogen can be used by soil microbes to produce nitrous oxide (N2O), and so N2O emissions increase with fertiliser application. One tonne N2O (half-life 114 years) is equivalent to 298 tonnes CO2 (half-life 50-200 years).

Measurement: Differences in methods used to measure soil C can lead to wide variation in estimates of soil C stocks, with values differing by up to 31% for Scottish soils. This uncertainty has implications for detecting if soil C stocks are increasing or decreasing.

(This material and references was first published on www.nfus.org.uk)