How does liming and nutrient use impact carbon stocks in soil?

Experiments with liming and nutrient use on grassland and soil indicate there are “no effects” to carbon stocks and this is a very “positive development”.

Liming, it seems, even if used in the short term, can have a positive impact on soil carbon stocks.

This is according to Dr. Dario Fornara, principal scientific officer, Agri-Food and Biosciences Institute (AFBI) who was speaking about soil carbon sequestration under grassland intensification during the Irish Creamery Milk Suppliers’ Association (ICMSA) think-tank on ‘Climate Change and Dairy Farming in Ireland’, which was held in Dublin on Wednesday, September 4 last.

The event was designed to facilitate an exchange of views, and other speakers on the day included: Dr. Ken Byrne, UL; Colm McCarthy, economist, UCD; Dr. David Styles, NUIG; and Philip O’Brien of the Environmental Protection Agency (EPA).

‘Improving and maintaining carbon stocks’

Meanwhile, Fornara highlighted how there were “ways of improving or maintaining carbon stocks” in grasslands.

We have discovered that as a result of liming there is a positive impact on soil carbon stocks.

He continued: “Liming – when applied even in the short term – can increase CO2 and carbon loses from the soil.

“Long term it has very positive effects on carbon stocks with much of the carbon in a more ‘resistant’ state.”

Experimenting on grassland

The expert went on to say that other grassland experiments carried out indicate that liming “certainly doesn’t have a negative effect on carbon stocks”.

He says there are also “benefits” to soil PH and nutrient use efficiency – as well as to other aspects.

We have a long-term experiment south of Belfast that was established in 1970.

Fornara added: “It started out initially as an experiment examining the potential effects of animals – including pigs and cattle – on nutrient use efficiency.

“Now, the experiment has allowed us to look at how carbon has been changing in the soils over the last 50 years.

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“We have different plots – some received no nutrients; some received 200kg of nitrogen (N); then we have pig slurries and cattle slurries of different concentration levels.

“We want to establish how these nutrient treatments affect carbon; what we are determining – so far – is that there is an increase in carbon since 1970 – even within the controlled plots which haven’t been receiving any nutrient fertilisation.”

Accumulating carbon

Fornara went on to point out that the cattle slurry plots have been accumulating more carbon than the other plots and one of the key reasons for this, he believes, is because of the type of slurry that is being applied.

“We estimate that about 10% to 15% of the carbon that has been added every year because of cattle slurry is in this soil.”

He continued: “The pig slurry, meanwhile, does not appear to have the same positive effect on carbon accumulation through time.”

Meanwhile, Fornara pointed out that the experiment is ongoing and the rationale behind it is that another common practice is soil tillage – plough and reseeding every few years.

The common knowledge is that carbon will be lost through soil tilling – at least in the short term – and this is actually true.

He added: “We wanted to find out then if there is any long-term effect of soil tillage on carbon stocks. So we selected 11 farms across Northern Ireland with different fields chosen on each farm.

“The fields were divided based on the knowledge of the farmer and on written records about how many times each field was actually reseeded over the previous 50 years.

“Based on all that we are finding now that the carbon stocks decreased from the top soil layers to deeper soil layers – different carbon stocks with the highest ones in the top 20cm.

“So there are no effects of soil tillage on the carbon stocks of these grasslands.

“The message is that if you reseed every four or five years – under certain climatic conditions and add slurries and N – then you might not have a negative effect on carbon stocks because of the large stocks that are already there.”

‘Work in progress’

Fornara does warn though – that all of this is a “work in progress”.

We are going to select more grasslands so we can confirm these results.

He concluded: “My experience with working on relatively long-term grassland experiments is that biodiversity – adding different species and having grasses and herbs – results in more carbon sequestration in grasslands which do not receive full nutrient applications.”