Introduction Coniferous_to_broadleaved forest conversion reshapes soil carbon cycling in coastal sandy ecosystems, yet its regulation on component soil respiration and thermal sensitivity remains poorly quantified. To explore the regulatory effects and underlying mechanisms of such vegetation shift on soil carbon cycling in subtropical coastal sandy lands, we carried out this comparative field study.Methods We investigated the conversion from Pinus elliottii (coniferous forest) to Eucalyptus urophylla & times; E. grandis (broadleaved forest) using a paired adjacent plot design and two_year continuous in_situ observations. Key indicators including soil respiration components, litter properties, fine root biomass, microbial activity and soil microclimate were monitored.Results Results showed that the conversion significantly increased total soil respiration (by 25.52%), root respiration (by 62.74%), and heterotrophic respiration (by 9.01%). This promotion was driven by the coupled effects of improved litter quality (low C/N ratio and lignin content), a sharp increase in fine root biomass (by 272.5%), and enhanced microbial activity. It also notably reduced the temperature sensitivity (Q10 from 2.29 to 1.55) of soil respiration, with root respiration becoming nearly temperature_insensitive (Q10=1.25). Additionally, the explanatory power of soil temperature for respiration decreased significantly (from 76% to 37.7%), while the regulatory role of litter quality, fine root biomass, and soil microbial activity became prominent, and soil moisture did not act as a limiting factor for soil respiration throughout the study period.Discussion This conversion achieves a virtuous cycle of "high carbon turnover and high carbon sequestration", clarifying the above_belowground coupling mechanism of soil carbon dynamics. Our findings thereby provide important scientific support for the optimization of coastal protection forests and the enhancement of carbon sequestration capacity in fragile coastal ecosystems.