An expected impact of climate change on tree leaf phenology is the extension of growth season lengths (GSL), which will affect productivity and element cycling of forests. These ecosystem processes will be further mediated by microbe-mediated plant-soil feedback (PSF). To investigate the possible consequences of interaction between GSL extension and PSF, we developed a simple mechanistic model (GSL-PSF model). The GSL-PSF model predicts that microbe-mediated PSF intensifies the negative impact of increasing temperature on the size of soil carbon stock compared to scenarios without the PSF effect. This combined effect of increasing temperature and PSF on the size of soil carbon stock occurs through enhanced microbial individual activity and increase in its population size. More importantly, the model also demonstrated that GSL extension buffers this negative impact on carbon accumulation in soil, not through increased primary production, but through intensified competition for nutrients between plants and microbes. Our model suggested that the interactive effects of GSL and PSF on C and N dynamics in forests are important for predicting future forest functions under climate change.