Sustaining organic matter in forest soils: What we have learned and what is left

Abstract A concerted research effort over the last three decades has transformed our understanding of the processes through which soil organic matter (SOM) is formed. Although recalcitrant plant litter is important, especially for particulate organic matter, we now know that a large proportion of SOM, particularly the more persistent material associated with soil minerals, has been transformed by soil microorganisms. A major source of energy for these microorganisms is labile compounds that are exuded by plant roots and associated mycorrhizal fungi. Much of this exuded carbon (C) arises from surplus carbohydrates produced by plants growing under mild‐to‐moderate deficiencies of nitrogen, phosphorus, or water. Managing forests in a manner that sustain or enhance this flux of labile C from trees to soil would augment efforts to sequester more C in forest soils.