Calcium oxalate and calcium cycling in forest ecosystems

An understorey shrub is an extreme CaOx accumulator that plays a disproportionately large role in Ca cycling; however, transformations of CaOx and other Ca forms in forest soils demand detailed attention. Calcium oxalate (CaOx) plays an important but neglected role in Ca cycling in terrestrial ecosystems. CaOx crystals are present in leaves and roots (less commonly in bark and wood) of many trees and vary greatly in size, shape, and crystallinity. We extend an Australian study of an Eucalyptus regnans forest ecosystem, showing that the shrub Pomaderris aspera plays a significant role in Ca cycling. X-ray fluorescence microscopy (XFM) shows that crystals of CaOx in the leaves of both species contain up to 15% Ca; the density of crystals being much greater in P. aspera than in E. regnans. The concentration of oxalate in P. aspera is about 6% dry weight, ranking P. aspera as an extreme oxalate accumulator, in common with a number of other members of the family Rhamnaceae. The fate of CaOx in nutrient cycling is poorly known and often misunderstood. Upon committal of CaOx to the soil, transformations are brought about by fire or saprophytic heterotrophic bacteria and fungi; the latter derive carbon and energy from the oxidation of oxalate. The primary reaction is one of the oxidations which produces CaCO3 and CO2, such that half of the C is sequestered in the CaCO3 which will remain stable under alkaline conditions and is therefore a potential sink for atmospheric CO2.