Calcium oxalate in soils, its origins and fate – a review

Calcium oxalate, a mineral of low solubility, is found widely in plants, but its fate in soils has been neglected until recently. This review considers the likely forms and reactions in soils that may be significant in the recycling of both calcium (Ca) and carbon (C). In soils, calcium oxalate is both formed by fungi and utilised by saprotrophic microbes and by some mesofauna as a source of energy and C. In acidic soils the oxidation releases soluble products, the Ca as Ca2+ and the C as bicarbonate, whereas in alkaline soils some Ca is released as Ca2+ but some C and Ca2+ form calcium carbonate, which may contribute to the formation of calcrete, usually in soils found in arid regions. This latter reaction has been considered as a possible major pathway, which may be utilised in the sequestration of carbon dioxide.