Jury still out on carbon sequestration in soil
While the potential for the farming industry to sequester carbon in soils under the country’s grasslands as a means of offsetting greenhouse gas emissions from ruminants and other sources is “not insignificant”, it is, however, also limited – and easily reversible.
That was the finding of a researcher working in a unique fellowship which partnered the practical minds of NFU Scotland with the country’s scientific community.
Climate research scientist Dr Gemma Miller who has been working with the union on climate issues has produced a factsheet on Scottish grasslands and soil carbon, the first in a series which investigates some of the science behind both GHG emissions and carbon sequestration in the farming sector.
Managed grasslands in Scotland hold approximately 172 million tonnes of carbon said Miller who said available data suggested that there had been little change in carbon stock sequestered in the soil in recent decades.
“It is believed that grassland soils have a huge potential to sequester C, but is this true for Scotland?” asked Miller.
She said that soil carbon sequestration occurred where organic matter from plant roots, manures and plant litter was 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.”
She said that while there was a lack of evidence to support the belief that any major increase in soil carbon was currently occurring, one detailed study of permanent grassland in SE Scotland found that soil C stocks were stable, neither sequestering or losing carbon.
Stating that soil carbon could be lost much more easily than it could be sequestered she warned that it was extremely important to maintain existing soil C stocks, and pointed out that soil carbon could 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).”
Miller said that while sequestration rates could be rapid to begin with, they became progressively slower over time.
“So, the potential for soil C sequestration to mitigate GHG emissions in grassland is not insignificant, but it is limited and easily reversible”.
She said that the jury was still out on practices which encouraged better use be made of grasslands by increasing productivity levels:
“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.”
She said that the use of fertilisers and seeding with high-yielding grass species and legumes could increase the amount of carbon taken up by plants but added that while liming increased the ph of acidic soils, making them more productive, it also increased CO2 emissions from soils.
“The net effect on soil C stocks is uncertain,” she concluded.