Declining soil organic matter stability over the last 150 years in coral island ecosystems revealed by chemical molecular composition

Coral islands, widespread in tropical regions worldwide, have great carbon storage potential due to favorable climatic conditions for vegetation growth. However, the role of soil organic matter (SOM) in this process remains unclear. Understanding the stability of the SOM of coral islands is critical for combating climate change. Here, the sources, composition, and stability characteristics of SOM in two typical coral island soil profiles were analyzed using multi-method analyses. The stable isotope mixing model revealed a significant alteration in SOM sources from seabird guano to plant humus, reflecting the evolutionary process of island ecosystems. Results from the Fourier transform infrared spectroscopy and ¹³C nuclear magnetic resonance showed that the functional groups and chemical composition of SOM varied during different historical periods. The average proportion of carboxyl carbon decreased from 14.49 % to 8.30 %, while that of alkoxyl carbon increased from 43.45 % to 51.90 %. Pyrolysis analysis provided molecular fingerprints of SOM, where the average lignin content rose rapidly from 0 % to 12.31 % within 0–15 cm. This indicated a decrease in the contribution of guano to SOM, along with an increase in the contribution of plant sources. Combining the above results, structural equation modeling suggests that the guano- and plant-derived composition of SOM play a positive and unfavorable role in its stabilization. The source and composition of SOM and environmental conditions are critical determinants of soil carbon pool stability on coral islands. This study provides additional insights for thoroughly assessing SOM stability and maintaining soil carbon storage on coral islands under climate change.