The potential for permanent carbon storage in ecosystems that use the oxalate-carbonate pathway (OCP) could be greater than previously estimated, according to a new study.

Most carbon accounting standards don’t take this pathway into account, yet it could be a game-changer, according to the paper ‘Oxalate and oxalotrophy: an environmental perspective’, one of the first to be published in Sustainable Microbiology, an Applied Microbiology International publication.


Source: Cathy Clarke

Calcium carbonate in old termite mounds, thought to be the result of the Oxalate Carbonate Pathway process.

The paper is a review from an interdisciplinary perspective concerning oxalate, its use by microorganisms and its potential role in removing carbon dioxide from the Earth’s atmosphere, according to corresponding author Professor Don A Cowan of the University of Pretoria.

Atmospheric carbon sequestration

“Oxalotrophy (broadly, the metabolism of plant- and fungi-derived oxalic acid by soil microorganisms) is environmentally relevant due to the Oxalate Carbonate Process (OCP) which, under the right circumstances, can lead to atmospheric carbon sequestration,” he explained.

“Oxalate is a common metabolic product in many plants, and is a product of plant photosynthesis and atmospheric CO2 assimilation.  In addition, many fungi known to grow on decaying plant material produce exogenous oxalate. 

“Oxalate-rich plant tissue (fallen leaves etc) is processed by a wide diversity of soil microorganisms, where much of the oxalate carbon is re-released by microbial respiration as CO2. However, in high pH calcium-rich soils, this CO2 is fixed as mineral calcium carbonate. Insoluble carbonate layers in soil profiles can survive for very long periods.”

Soil remedy

Oxalotrophy can do two things - improve soil fertility by reducing acidity and making calcium available, and sequester atmospheric carbon.

Recent research has looked at the potential of using oxalotrophy to ameliorate acid soils. Soil acidity is a major problem in growing crops, and affects 40% of arable land. Applying crop residues/plant waste that contains oxalate, and managing the environment around this waste to support oxalotrophy has the potential to make the soil less acidic. 

Oxalotrophy also plays an important role in making calcium available. Calcium in soil improves soil fertility and improves soil physical structure, allowing the soil to remain porous and not as compacted during tilling/working the soil. 

Game changing

“Learning how to manipulate the oxalotrophic system to benefit agriculture is game changing because it would improve soil condition while improving biodiversity. Oxalotrophy happens best in an intact, healthy and functioning ecosystem with soil fauna and intact pore spaces needed for the processes of calcium bioaccumulation to take place,” Professor Cowan said. “Earthworms, termites, mites and springtails are needed to break down (eat and excrete) the calcium oxalate bearing leaf litter in the soil first, to maximise the potential for oxalotrophy in this pre-worked organic matter by bacteria. 

“Using oxalotrophy in degraded lands as a soil amendment in the future would need careful land management to ensure this process can function, and this needs a healthy soil, meaning a double benefit - more soil health, and problem of acidity fixed.”

Permanent carbon storage

Meanwhile, oxalate also offers potential for permanent carbon storage in ecosystems where the oxalate-carbonate pathway (OCP) takes place. 

Most organic carbon is usually stored in soil for a relatively short time because it is consumed by microbes and the carbon stored in the organic matter is released again to the atmosphere by the respiration of these microbes. 

However, the OCP is a microbially-driven transformation of organic carbon to inorganic carbon in the form of bicarbonate. This inorganic carbon is stored over a much longer time-frame than organic carbon. 

Carbon accounting standards

“This means that nature-based solutions, with a value multiplier such as organic-inorganic carbon transformation, offer increased value compared to a more transient organic carbon storage,” co-author Dr Michele Francis of the University of Stellenbosch said. 

“This has not been recognized yet in the main carbon accounting standards, mainly because the OCP has not been widely publicised and there have only been a few studies that actually quantify the carbon storage. 

“What is game changing is that given the extent of oxalotrophy and its importance for soil fertility and carbon sequestration, this would make forests, for example, “more expensive” to cut down, or more worth preserving, if the carbon or biodiversity credits issued for them are worth more.”

Surprisingly widespread

What was surprising for the authors was how widespread oxalotrophy is, both in terms of the variety of bacteria with the genetic capacity to utilise oxalate vs what has been characterised; and the number and diversity of ecosystems that are able to support this by producing oxalate.

“The true extent of the OCP and its real-world role in atmospheric carbon sequestration is yet to be determined. Given that oxalate is such a common and abundant plant product across the world, it is possible that OCP makes a much larger contribution to carbon sequestration than previously thought,” Professor Cowan said.

The authors said more funding is needed to detect the OCP in more ecosystems, and more publicity for the general public and the funders to recognize that this is an important issue.

Global south

“Many of the potential ecosystems are in the ‘global south’, in countries that are under-funded in terms of research,” Professor Cowan said. 

“These are ‘developing country Parties’ under the Paris Agreement, meaning that research can be funded under the Paris agreement. Article 9 of the Paris Agreement requires developed country Parties to provide financial resources, technology development and transfer, and capacity-building to developing country Parties, to assist developing country Parties with respect to both mitigation and adaptation. 

“The extent of these ecosystems in developed countries also needs to be investigated, for example in Europe under EU’s biodiversity strategy for 2030, which is a plan to protect nature and reverse the degradation of ecosystems in Europe.”

This review of oxalotrophy grew out of a collaborative research project, jointly supported by the South African National Research Foundation and the US National Science Foundation and led by Cathy Clarke (University of Stellenbosch, South Africa).  The project involves researchers from three other South African Universities and two US institutions.

‘Oxalate and oxalotrophy: an environmental perspective’ is published in Sustainable Microbiology, an Applied Microbiology International publication.