A new method to simultaneously quantify oxalate and carbonate phases in Arenosols using Rock-Eval® thermal analysis

The oxalate-carbonate pathway (OCP) is a metabolic process that converts atmospheric carbon dioxide into biogenic carbonates, representing a potential long-term sink of carbon in the form of inorganic carbon in soils, particularly in drylands. This pathway involves oxalates, organic crystals ubiquitous in plants. To date, quantifying soil oxalates in low carbon content soils remained challenging owing to the absence of suitable methods: existing approaches require extensive sample pretreatment, have prohibitively high quantification limits, and have not been adapted for soil matrices. This study presents a novel approach for oxalate quantification in low carbon soils using Rock-Eval® thermal analysis, which simultaneously characterises and quantifies soil organic and inorganic carbon. Bare Arenosol samples from a hyperarid ecosystem (Sharaan National Park, Saudi Arabia) were amended with pure oxalate across a gradient from 0 to 7.14 mg g −1 . Thermal analysis revealed a distinctive four-peak of CO and CO 2 emissions associated with oxalate content. However, the thermal decomposition of oxalate into carbonate hinders direct oxalate quantification and its differentiation from background soil carbonates. We address this by developing oxalate and carbonates quantification formulas based on the CO emitted during pyrolysis between 400 and 550 °C as a reliable linear proxy for oxalate content. Validation through application to Arenosols samples from Sharaan yields results consistent with active OCP expression. These findings advance our understanding of the role of oxalates and the OCP in the carbon cycle, particularly for assessing the carbon sequestration capacity of hyperarid ecosystems and monitoring ecological restoration in such ecosystems.