Soils contain more carbon than all of Earth's atmosphere and vegetation combined. Understanding the factors that enhance or suppress soil carbon content is critical to understanding the overall carbon budget, and some kinds of fungi, known as mycorrhizae, grow inside tree roots and affect the carbon content of soil. Many forest trees form mycorrhizal associations, and some types of associations have greater soil carbon content than others. Forest Service scientists and their research partners discovered that the difference for the different amounts of soil carbon content was due to the increased activity of decomposers. Their research suggests that the mycorrhizal association of a given tree may promote different communities of decomposers and this ultimately leads to a lower rate of decomposition and therefore a higher content of soil carbon under some trees. These findings provide new information to refine existing estimates of how forest landscapes store carbon in soils and may be crucial to predictions of soil carbon storage under novel assemblages of tree species in forest ecosystems.