Influences of CO₂ and Fungus-Assisted Bioweathering on Apatite

Fluoridated apatite (FAP) is the dominant P source for ecosystems. However, the bioweathering of FAP is still not fully elucidated. In this study, phosphate-solubilising fungus (Aspergillus niger) was firstly incubated in soil to examine the weathering of FAP, on both cross and longitudinal sections. It showed that the fungus induced more pronounced erosion channels on the cross section in a P-deficient soil. The relative contributions of biological (phosphate-solubilising fungus) and abiotic factors (CO₂ and crystal face orientation) were contrasted. To further investigate the weathering contrasts on different sections of FAP, incubation was conducted in a culture medium. Fungal colonisation on the cross section of FAP resulted in local deeper P depletion zones and enriched secondary minerals (primarily calcium oxalate) than those on the longitudinal section. Additionally, elevated CO₂ increased FAP surface roughness (especially the cross section) and induced distinct surface microstructures, while the combined effects of elevated CO₂ and fungal colonisation further modified the surface morphology, secondary mineral formation and subsequent conceivable bioweathering for FAP. Synergistic interactions between fungal colonisation and elevated CO₂ promoted FAP surface alteration, providing new insights into microscale fungus-mineral interactions in soil microenvironments.