Soil Sequestration Might Not Be The ‘Win’ Hoped For

The exact climate action potential of soil carbon sequestration had always been strongly debated.

Eminent scientists have warned there is little justification for excitement around soil carbon sequestration for climate action and for food security.

Increasing the carbon content of agricultural soils is promoted by the United Nations, the EU, the Intergovernmental Panel on Climate Change, and others as a win-win for climate change mitigation and food security.

But researchers at Wageningen University & Research in the Netherlands and a colleague from the Dutch Environmental Assessment Agency have published their findings that such a win-win is the exception, not the rule.

A benefit for climate change action often leads to a loss for food security or vice versa, they said.

Wageningen University is rated one of the world’s best for agriculture and forestry teaching.

“But three of their crop and soil biology experts have contributed to the conclusion that the existing knowledge base does not justify current global agendas that focus first and foremost on soil carbon sequestration”.

Sequestration occurs when carbon dioxide is absorbed from the atmosphere by plants, assimilated into its organic form, and stored in the soil or in woody biomass.

Globally, soils and forests store vast amounts of carbon, particularly in grasslands, peatlands and woodlands.

In Ireland, farm grasslands have significant potential to sequester carbon in root biomass and in the soil, and enhancing this is seen as vital for climate action and for greenhouse gas reduction.

Teagasc is now quantifying soil carbon stocks and carbon sequestration rates on Irish farms. Their findings could help farmers to implement emission-reducing farming systems.

But the latest Dutch research findings are that the climate mitigation potential of soil organic carbon (SOC) is modest, at best, on a global scale, and context-specific. They said it is highly uncertain whether any causal link exists between SOC sequestration and crop yield.

Up to now, SOC sequestration was claimed to be a win-win with numerous co-benefits and few trade-offs. It has long been claimed to enhance soil fertility and productivity, increase soil biodiversity, improve water retention and purification, and reduce erosion, compaction, runoff and water pollution, while leading to larger and more stable yields globally.

Limiting global warming to the crucial 1.5°C or 2°C is thought impossible without removing carbon dioxide, the main greenhouse gas, from the atmosphere, and soil sequestration has been seen as a vital component for carbon dioxide removal.

The exact climate action potential of SOC sequestration had always been strongly debated, but now the Dutch study says that even where it can go hand-in-hand with improved soil fertility and crop yields, strategies to promote one may not be the best suited to promote the other.

The soil type is crucial in this conflict. Clay soils may best store SOC, especially if depleted, but the effect on crop yield may be largest in sandy soils.

“Ensuring global food security in a stabilised climate is a tremendously complex endeavour,” said the Dutch scientists.

“For example, while it is doubtful that increasing food production in Europe or North America would yield any benefit for global food security, the link between food production and food security is absolutely critical for smallholder African farmers, with an estimated 7% reduction in poverty for every 10% increase in yield”.

They did not question that soil organic matter, with the carbon it contains, is fundamental to maintaining soil fertility and supporting plant production. But their review demonstrated that increasing SOC stocks does not necessarily lead to an increase in yield.

“We conclude that, if we are to ensure food security in a changing climate, let alone other sustainable development goals, SOC sequestration is simply not up to the challenge.

“SOC sequestration is indeed one of many approaches, one small piece of a very large puzzle, and it should be treated as such. We argue that including SOC sequestration in the narrative of global climate change mitigation is ill-suited and we call for a soil-smart approach, which will not always be ‘climate-smart’.”

One of the authors of the new report, Gabriel Moinet, called for “synergies for land sustainability to be maximised, and agronomic requirements for food security to be fulfilled. Soil carbon sequestration may occur along this pathway, and contribute to climate change mitigation, and should be regarded as a co-benefit.”

They said existing scientifically estimated contributions of soil carbon sequestration mostly do not include sequestration decreasing exponentially when soil carbon saturation approaches.

And many large social, economic and political barriers threaten the implementation of SOC sequestration, casting yet more doubt on its climate mitigation potential.

Their findings put a damper on recent ever more optimistic predictions, for example, claims that SOC sequestration would be key to supporting up to 12 of the United Nations’ Sustainable Development Goals The four parts in 1000 (of carbon in soil) initiative launched during the COP21 in 2015, and the COP23 and FAO calls for rapid upscaling and implementation of SOC sequestration are also challenged.

Soils are the largest reservoir of carbon in the terrestrial biosphere, containing four times as much as global vegetation, and twice as much as the atmosphere.

However, the emission of other greenhouse gases, such as nitrous oxide, can negate or exceed the benefit of sequestering SOC in some conditions.

Also, SOC remains vulnerable to losses and is sensitive to a range of environmental factors, said the Dutch scientists.

(Source – Irish Examiner – Farming – Stephen Cadogan – 01/02/2023)