How grassland can help combat global warming

Research studies suggest that various grassland management practices can build carbon stores in soils over a long period of time, members of the NI Institute of Agricultural Science (NIIAS) were told on Monday evening.

Speaking at a conference in Templepatrick, Dr Dario Fornara from the Agri Food and Biosciences Institute (AFBI) said that the process, known as soil carbon sequestration, allows the greenhouse gas carbon dioxide to be removed from the atmosphere.

However, he said that the long-term effects of grassland management practices on soil carbon stocks are still not fully understood, and research is ongoing.

“I know everyone wants an answer now, but carbon sequestration takes time.”

That said, scientists have been able to re-analyse soil samples that were taken as part of other studies into grassland, and then compare how each management practice affected soil carbon stores over time.

Rothamsted

The longest-running grassland experiment in the world is in Rothamsted, England, where a set of grassland plots has been managed and sampled since 1856.

As part of his research, Fornara re-analysed the soil samples and found that plots which have been receiving lime had more carbon stored.

“You lose some carbon initially, but in the long term soil liming has had a positive impact on stocks,” he said.

Fornara explained that limed soils had higher grass yields, which led to more carbon dioxide being removed from the atmosphere in a process known as photosynthesis.

Some of this carbon then stays in the plant and is eventually added to the soil after either being consumed by livestock or decomposed when the plant dies.

Liming also increases the activity of microbes in the soil and helps stabilise soil organic matter, both of which have a positive effect on soil carbon levels.

“Even after 100 years, some soils are still taking up a bit of carbon,” Fornara said.

Slurry

A similar study on nutrient applications has been running at AFBI Hillsborough since 1970.

Analysis of historic soil samples from these plots found that soil carbon stocks have continued to grow in plots that have been receiving cattle slurry for 50 years.

“Our study found approximately 15% of carbon in cattle slurry is retained each year,” Fornara said.

Plots that received pig slurry and compound fertilisers also had a rise in carbon levels, but to a much lesser extent than cattle slurry.

Fornara also discussed a study on commercial farms across NI which examined the impact that ploughing and reseeding grassland has on carbon stores. Soil samples were taken from 126 plots and farmers gave information about reseeding in the fields over the past 50 years.

Researchers were expecting reseeded soils to have less carbon as ploughing can allow it to be released to the atmosphere.

However, the results showed that ploughing and reseeding every few years did not have a negative effect on soil carbon stores. A larger study spanning over 500 plots is now planned to investigate this further.

Diversity

Fornara said that swards made up of different species of grass can lead to higher rates of soil carbon sequestration. This was found in a study in the US, although the AFBI researcher added that the soils in the trial were different to NI farms and local research on the topic is needed.

With research into carbon sequestration ongoing, NIIAS members were told that scientists are working to establish an agreed system for measuring and monitoring soil carbon stocks over time.

“Ideally, we will measure soil carbon every five years, for the next 20 years,” Fornara said.

He suggested that carbon sequestration studies should also go beyond grassland plots and include hedgerows and trees.

Livestock

Some of the management practices discussed by Fornara, such as lime and slurry applications, would lead to increased grass yields on most farms. However, he suggested that increasing livestock numbers to utilise extra grass could prove counterproductive for offsetting global warming as ruminants produce the greenhouse gas methane.

“The benefits that you have from carbon sequestration might not be enough to offset more greenhouse gas emissions from a higher number of animals,” he said.

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