Seasonal variations of soil CO₂ concentrations and efflux and their influencing factors in a subtropical hilly oak forest in Huainan, China

We analyzed long-term soil CO₂ concentrations, soil temperature and moisture, with simultaneous canopy microclimate measurements, in a subtropical 20 years old oak forest in Huainan, China, in the East Asian monsoon region. Daily soil CO₂ effluxes were quantified using the gradient method. We investigated the relationship between soil CO₂ concentrations and efflux, and both soil temperature and water content, and the effects of phenological stages and seasons. explored how abiotic factors (soil temperature and moisture) along with LAI influence soil CO₂ efflux. Subsurface soil CO₂ concentration and efflux showed clear seasonal variations, with an increase in spring to a small peak, reaching a maximum in summer, then a decrease in autumn but with a sharp second peak when precipitation season began, and finally a decrease to a minimum in winter. In this forest ecosystem, the calculated soil subsurface CO₂ efflux at 0.02 m averaged 1.40 µmol m⁻² s⁻¹ and ranged from 0.24 to 4.11 µmol m⁻² s⁻¹ over a whole year from September 1, 2018 to August 31, 2019. Seasonal fluctuations in soil CO₂ concentrations and efflux were primarily influenced by soil temperature, with soil moisture exerting secondary effects throughout the entire study period and in most cases. However, during warm, dry periods like summer or maturity stages, soil water content emerged as the primary factor governing CO₂ efflux. The combined influences of soil temperature and water content were different in different phenological stages, and highly regulated by drought and rains. For instance, in the phenological mature stage, soil CO₂ concentration was mainly affected by soil water content. In the mature stage, soil CO₂ efflux correlated more closely with soil water content than with soil temperature following a spring drought episode.