Foliar gypsum formation and litter production in the desert shrub, Acacia bivenosa, influences sulfur and calcium biogeochemical cycling in arid habitats

Aims: Some sulfur-accumulating species from arid habitats facilitate a little-understood foliar gypsum (CaSO₄•2H₂O) biomineralisation. This study seeks to increase our understanding of the ecophysiological and nutritional function of phytogenic gypsum, including how gypsum-formers influence soil S and Ca (S/Ca) cycling mineralogy and distribution. Methods: We studied leaf composition and mineralogy (ICP-OES, SEM-EDXS) through leaf senescence and litter degradation in Acacia bivenosa DC, together with detailed soil profile analysis (composition, S chemistry & stable isotopes). Results: Acacia bivenosa accumulated foliar gypsum even when growing in surface soils without high S/Ca concentrations, accreting tissue-encapsulated gypsum, which was relatively recalcitrant to degradation, within the litter beneath the crown. Though A. bivenosa regulated limiting or potentially harmful elements during leaf senescence, it did not remobilise S/Ca or preferentially accrete gypsum in senescing foliage to enhance S/Ca excretion with litter. Instead, A. bivenosa maintained high S concentrations through reabsorption from phytogenic accretion zones supplemented by alternative sources, most likely in the deeper regolith. Conclusions: Acacia bivenosa influences S/Ca cycling, mineralogy and spatial distribution with the soil environment by readily accumulating S/Ca, which it concentrates within the topsoil as phytogenic gypsum. These phytogenic accretion zones can provide a sink for S/Ca salts and other potential phytotoxins, which could assist with revegetating sulfate-saline substrates.