Occurrence of calcium oxalate in karri (Eucalyptus diversicolor F. Muell.) forest ecosystems of south western Australia.

The amount of calcium in Eucalyptus diversicolor (karri) ecosystems is high in comparison with other eucalypt forest ecosystems in Australia. A large proportion of this calcium is present as crystalline Ca oxalate. Whewellite (CaC2O4·H2O) accumulates in leaf tissue of the over- and understorey species. Up to 70% of the calcium in the leaves of karri is stored as whewellite, the highest concentrations occurring in the oldest leaves. Synthesis of Ca oxalate by karri may be related in part to the high levels of exchangeable soil calcium. Oxalate of plant origin is rapidly metabolized on the forest floor during decomposition of leaf litter. About 70% of the whewellite in fresh karri leaf litter is degraded during the first wet season following leaf fall. However, additional oxalate is formed in the soil and litter layer from microbial production of oxalic acid. These crystals are of the dihydrate calcium salt, weddelite (CaC2CO4· 2H2O). Microbial production of oxalate, particularly by ectomycorrhizal fungi, may be an important mechanism for mobilizing of phosphate from the strongly phosphateadsorbing soils on which these forests grow.