Island Size Modulates the Effects of Multiple Global Change Factors on Greenhouse Gas Emissions and Microbial Communities

Islands are the most vulnerable ecosystems to climate change, yet our understanding of how island soil reacts to multiple global change factors (GCFs) remains limited. This study investigated the impact of warming (W), drought (D), nitrogen addition (N), warming + drought (WD), warming + N addition (WN), N addition + drought (ND), and warming + N addition + drought (WND) on greenhouse gas (GHG) emissions and microbial communities in various subtropical island soils. The results showed that ecological stochasticity of multiple GCFs was strongly regulated by the island area. Drought and the combination of WD significantly reduced dissolved organic carbon and NO₃⁻-N concentrations in terrestrial and island soils. Moreover, the amount of soil microbial phospholipid fatty acids in terrestrial land was significantly increased by interactive treatments (P < 0.05). The cumulative CO₂ emissions of WN and W significantly increased by 64.2% and 51.8%, respectively, whereas D reduced it by 49.2% in terrestrial soil relative to the control treatment (CK). In large islands, drought led to an increase in CO₂ emissions by 90% (P < 0.05). In the medium and small islands, warming and the combination of WN led to higher CO₂ emissions. Regarding N₂O, the terrestrial site acted as a sink (except for WN), whereas islands were N₂O sources. Island size directly affected various GHG emissions, while indirect effects modulated these fluxes through soil properties and microbial communities. Collectively, this study empirically demonstrate that concurrent global changes can lead to directional alterations of soil properties, GHG emissions, and microbial communities in island soils.