Exploring inorganic carbon dynamics in soil via the Oxalate-carbonate pathway: A methodological approach for monitoring carbon dioxide removal

Carbon dioxide (CO₂) assimilation into organic carbon through photosynthesis is widespread, but the biogenic conversion of CO₂ into inorganic carbon compounds is often overlooked. One biogenic pathway, facilitated by oxalogenic plants, fungi, and oxalotrophic bacteria, is known as "The Oxalate Carbonate Pathway" (OCP) (Rowley et al., 2017). The process entails the plant uptake of soil calcium, the transformation to calcium oxalate (CaOx) crystals within plant tissues, and their return to the soil via tissues decomposition or as exudes, where CaOx is subsequently catabolised and stored as calcium carbonate in the soil (CaCO₃). 

Afforestation/reforestation with plants that perform OCP holds significant global CDR implications. For example, Oxisols that are free of carbonates (the Amazon basin and other ecoregions featuring tropical weathered acidic soils) cover >750m ha, with the potential for gigaton scale carbon removal (i.e. 1-2 tCDR/ha/year as CaCO₃, could yield 0.75-1.5 Gt/year). Also, OCP in alkaline karst environments may contribute to delaying the return of CO2 to the atmosphere. 

During the OCP, bicarbonate (HCO3^-) is also produced, and is the dominant carbon/mole species between soil pH 6-10, with carbonate precipitation from pH 8.3, and above. The carbon removal efficiency and fate of the CDR product thus depends on the environmental conditions.

Notably, OCP-based biomineralization has not yet been covered by existing MRV methods. Existing enhanced weathering frameworks, have paved foundations for quantifying carbon removal through bicarbonate ion flushing into the ocean (Mills et al., 2024), and the Microbial Carbon Mineralization methodology quantifies carbonate minerals stored in soil (Andes and Ecoengineers, 2023). The OCP pathway represents a potential carbon removal approach that combines carbonate mineral storage in soils and bicarbonate ion flushing to the ocean that will require a combination of quantification methods and a new methodology.

This mesocosm study aims to elucidate the mechanisms governing CaOx decomposition and its impact on soil pH, soil oxalates, and the relative contribution of CaCO₃ precipitation and HCO₃^- flushing on net carbon removal in the system. Furthermore, it seeks to assess impact on soil organic carbon and potential SOC destabilization risks.

We present early results from a benchtop trial, comparing two soil types from the dry tropics of Yucatàn (slightly basic-rendzic Leptosols vs alkaline soils), and litter (CaC₂O₄.H₂0-treatment vs KBr-control) on inorganic carbon dynamics. The soil mesocosms received distilled water and were run under ambient conditions, for 20 days, to track pH, carbonate content, bicarbonate flushing, and soil organic carbon during decomposition. Using titrations, we estimated CO₂ removal as soil-based carbonates, and as flushed HCO₃^- in effluent. Potential implications for leveraging the OCP for carbon removal will be discussed.