Rhizosphere processes are increasingly recognized as a crucial component of the overall functioning of belowground systems. Understanding the role of rhizosphere processes in controlling soil organic carbon decomposition, or the rhizosphere effect, is essential since roots and rhizosphere processes are integral part of the soil. Studies have shown that rhizosphere effect can accelerate the rate of soil organic matter decomposition by as much as 380% or inhibit decomposition by as much as 50%. However, most published data so far come from experiments using herbaceous plant species. It remains an open question as of how tree rhizosphere effect may change soil carbon decomposition. Since it is commonly known that forests constitute a major portion of the global carbon cycle, understanding tree rhizosphere effect may bear more relevance to the issue of terrestrial carbon sequestration or loss at the global scale. This presentation summarizes results from three greenhouse experiments using natural 13C labeling and tracing methods. When cottonwood and ponderosa pine seedlings were grown in a tallgrass prairie soil for a period of 70 days, the rhizosphere effect accelerated the rate of soil organic carbon decomposition by 70% and 35% for cottonwoods and ponderosa pines, respectively. In a year-long greenhouse experiment, the rhizosphere effect of cottonwood trees greatly accelerated or suppressed the mineralization of soil organic C, depending upon the time of year. Averaging over the whole course of the experiment, the rhizosphere effect doubled soil organic C decomposition rate. In the 395-day greenhouse study with Ponderosa pine and cottonwood trees grown in three different soils, the rhizosphere effect increased soil organic carbon decomposition by as much as 225%. This rhizosphere effect persisted throughout the experiment. These results indicate that rhizosphere effect is an important mechanism in controlling soil carbon dynamics in forest ecosystems similarly as in grassland or agricultural ecosystems.