Monocultures of ryegrass and large amounts of nitrogen fertiliser are not going to build soil carbon. This was the message from Dr Christine Jones, an Australian scientist who was speaking at the BioFarm online conference last week.

Dr Jones was speaking on the subject of carbon sequestration in farming. Carbon sequestration is the transfer of carbon in the atmosphere (greenhouse gas) to the soil where it can be safely stored and has many benefits. She repeated claims made recently that plantation forestry is not a net sequester of carbon because the lifespan of the harvested wood products is shorter than the time taken to grow the trees, plus the planting and harvesting of trees emits carbon. She said: “I’m not saying that trees don’t have positive benefits, I’m saying that short-rotation forestry is not a solution to emission reduction.”

An analysis of 2,700 soil profiles from around the world found that in grassland soils 42% of the carbon was in the 0 to 20cm increment

Dr Jones went on to say: “If we want to mitigate climate change we will need carbon capture and storage options which increase biodiversity, which is one thing plantation forestry does not do, it has to support cultural values and we need long-term removal and storage of carbon from the atmosphere. In order to do all these things we have to have an alternative and forestry is not it.”

She said soils are the world’s largest terrestrial carbon sink and that topsoil holds three times as much carbon as the vegetation growing on them.

“An analysis of 2,700 soil profiles from around the world found that in grassland soils 42% of the carbon was in the 0 to 20cm increment and 58% in the 20 to 100cm increment. This is important because a lot of measurements being done on soil carbon are being done at 0 to 10cm or 0 to 15cm or maximum 0 to 20cm and really we need to be looking at the whole of the top metre when we’re looking to see what the sequestration potential of the soil is.”

She said carbon stored below 30cm is the sequestration of significance as it is more stable than topsoil carbon and it also has many benefits for the land, such as mitigation for floods and droughts and also for farm productivity.

“It also improves the mineral density in plants, animals and people because roots are down there actively foraging for the trace elements that are so missing in today’s forages and food production systems.”

Creating the conditions for high amounts of carbon to be sequestered into soil involves a holistic approach that enhances biodiversity and puts soil microbes front and centre. According to Jones this involves growing multi-species swards with up to 16 different plant species and cutting out synthetic fertiliser. She said that grass and clover swards are insufficient to build up enough soil carbon.

When you have one species such as ryegrass or maybe two species such as ryegrass and white clover there will be a response to nitrogen

“There has been many studies, including ones in Ireland to show that diversity replaces nitrogen. Once we get over eight species for example, we will find that pastures will be more productive at zero nitrogen than they will at 200kg N/ha/year.

“When you have one species such as ryegrass or maybe two species such as ryegrass and white clover there will be a response to nitrogen. The reason that nitrogen is used in those low-diversity situations is that it is in fact replacing soil function. It is used as a surrogate for soil function. Once we get over eight species we no longer need fertiliser. In fact they are a detriment to the system,” Jones said.

She said that studies have shown that high-diversity plots, such as those with more than eight species accumulated almost 22% more carbon than low diversity plots. Jones said that in March 2019 Australian farmer Niels Olsen was the first farmer in the world to be paid for sequestering carbon in a regulated government-run carbon credit scheme. The amount of carbon in 1m deep soil cores was measured using special coring equipment. Two depth increments are used, 0 to 30cm and 30 to 100cm, with the largest increase in carbon observed on the Olsen farm in the 30 to 100cm soil depth range.

In 2019 Olsen was paid for sequestering 11.2t CO2e/ha and in 2020 he was paid for sequestering 13.7t CO2e/ha, both over 12-month periods. Jones disputed claims made by some experts that soil carbon becomes saturated over time and can no longer continue to sequester carbon. She said that it was Olsen’s experience that carbon sequestration increases as the amount of carbon in the soil increases because soils become better aggregated and deeper as carbon content increases.

The current price of a carbon credit in Australia is around €10/t, so sequestering 13.7t/ha would generate an extra €137/ha in income on top of income generated from farming. According to Jones, farm profit should increase and not decrease while growing diverse swards as pasture growth increases while inputs decrease.

In Australia, a baseline carbon content is measured and annual measurements thereafter are used to assess if carbon is increasing or not

Carbon credits in Europe were trading at around €25/t prior to COVID-19 so if Olsen was farming in Europe he could claim €342/ha in extra income. However, the EU emissions trading scheme does not yet recognise carbon that is sequestered in soils as no mechanism exists as yet to quantify and trade the carbon sequestered in soils.

In Australia, a baseline carbon content is measured and annual measurements thereafter are used to assess if carbon is increasing or not. Jones said that the measurement procedure is heavily regulated and conducted through grid area measurements. She said that the minimum amount of carbon that needs to be sequestered to pay for the costs involved in sampling is 1t CO2e/ha/year.

Comment

Dr Jones is clearly not a fan of plantation forestry or perennial ryegrass monocultures or even ryegrass and clover pastures because she says they are not sequestering enough carbon.

Whether you fully agree with her views or not is secondary to the fact that there is an obvious opportunity for farmers to benefit from sequestering carbon.

This is happening albeit it on a very small scale in Australia. The notion that farmers could get paid to sequester carbon has been around for a while, but this is the first time that I have learned of it actually happening in a government-regulated scheme.

Getting paid to sequester carbon is an incentive to increase sequestration and ultimately to take carbon out of the atmosphere and reduce global warming. Only at that point will farmers adopt practices to maximise sequestration, such as growing multi-species or re-wetting peatland.

Research in Ireland on soil as a carbon sink is still very much in its infancy and progress is slow because carbon content changes slowly. We have no real data on how much carbon is currently in Irish soils and how much carbon we are capturing or losing each year under different farming practices. In time, this information could be as important as soil fertility indexes for phosphorus and potassium are now. It is of course a complicated area and research is playing catch-up as the world continues to heat up. To move away from ideologies we need a standardised approach to measuring soil carbon, something the new Teagasc Signpost Programme needs to deliver on.