Biocrusts facilitate organic carbon preservation in tropical coral islands undergoing primary succession

• Biocrusts enhance SOC accumulation during coral island primary succession. • FT-ICR MS reveals ROC buildup and molecular transformation in biocrusts. • ROC storage peaks when biocrust Chl a reaches 30.8 mg/kg. • Fungi and bacteria co-regulate soil carbon preservation via decomposition pathways. Tropical coral islands, often regarded as ‘oceanic deserts’, undergo primary succession mediated by biological soil crusts (biocrusts), which facilitate soil formation and regulate biogeochemical cycles. Despite their critical role in enhancing substrates essential for the tropical coral island development, the influence of biocrust successional stages on soil organic carbon composition, stability, and preservation remains poorly understood. In this study, we examined biocrust enzyme activity, microbial communities, and their role in soil organic carbon preservation and stabilization across three- and five-years development stages. As biocrusts developed on coral islands, recalcitrant organic carbon (ROC) content increased significantly, with semi-labile and ROC fractions increasing by 4.6-fold and 6.3-fold, respectively, highlighting the strong carbon-fixation capacity of biocrusts in calcareous sand on tropical coral islands. Notably, enhanced polyphenol oxidase and cellulase activity, driven by bacterial–fungal interactions, regulated ROC accumulation and preservation throughout biocrust development. Over the five-year developmental period, the levels of semi-labile and ROC in the calcareous sand increased quadratically with biocrust chlorophyll a (Chl a ). The maximum preserved ROC content occurred at a Chl a content of 30.8 mg/kg, corresponding to 6.85 g/kg (dw) of ROC that could be sustainably stored in the calcareous sand. The preserved ROC effectively improving the carbon preservation potential of biocrusts within tropical coral island ecosystems. Overall, this study underscores the importance of understanding biocrust formation and development and its influence on the carbon cycle, providing valuable insights into the management of tropical coral island ecosystems.