Net ecosystem exchange of CO2 and its driving mechanisms in an island forest nature reserve in southern China

• The subtropical island forest is a stronger CO 2 sink in the wet season than in the dry season. • During the wet season, PAR and Tair primarily drove NEE. • In the dry season, SWC was the key driver, with low SWC significantly suppressing diurnal carbon sequestration. Carbon sequestration in island forests is a significant component of the global carbon sink. Monsoon island forests experience unique land − sea interactions, including seasonal variations in hydrothermal conditions. However, the seasonal changes in their carbon exchange and response mechanisms to environmental changes remain unclear. This study measured the net ecosystem exchange of carbon dioxide (CO 2 ) (NEE) during 2020–2022 using the eddy covariance technique in an island forest ecosystem in the Nanji Islands National Marine Protected Area. During the study period, the island forest acted as a significant CO 2 sink, with annual NEE, gross primary productivity (GPP), and ecosystem respiration (Re) of −548.6, 1881.5, and 1332.9 g C m −2 , respectively. The NEE, GPP, and Re fluxes exhibited seasonal variations, with the mean GPP and Re in the wet season being significantly higher than in the dry season ( p < 0.001). The mean diurnal NEE was −2.0 g C m −2 day −1 in the wet season and −1.0 g C m −2 day −1 in the dry season. This disparity was due to the higher sensitivity of GPP to radiation and temperature during the wet season compared to Re, resulting in a significantly greater increase in GPP than Re during the wet season ( p < 0.001). The primary driving factors of GPP, Re, and NEE during the wet season were photosynthetically active radiation (PAR) and air temperature (Tair), while soil water content (SWC) impacted most during the dry season. Specifically, there was an exponential decrease in PAR regarding to diurnal CO 2 fluxes, and the low SWC suppressed diurnal carbon sequestration. Tair was linearly and positively correlated with the nocturnal CO 2 fluxes, and a high SWC promoted nocturnal carbon emissions only during the dry season. The results of this study will improve our understanding of the response of monsoon island forests to land − sea interactions and climate change.