Large-scale forest girdling shows that current photosynthesis drives soil respiration

The respiratory activities of plant roots, of their mycorrhizal fungi and o f the free-living microbial heterotrophs (decomposers) in soils are signifi cant components of the global carbon balance, but their relative contributi ons remain uncertain(1,2). To separate mycorrhizal root respiration from he terotrophic respiration in a boreal pine forest, we conducted a large-scale tree-girdling experiment, comprising 9 plots each containing about 120 tre es. Tree-girdling involves stripping the stem bark to the depth of the curr ent xylem at breast height terminating the supply of current photosynthates to roots and their mycorrhizal fungi without physically disturbing the del icate root-microbe-soil system. Here we report that girdling reduced soil r espiration within 1-2 months by about 54% relative to respiration on ungird led control plots, and that decreases of up to 37% were detected within 5 d ays. These values clearly show that the flux of current assimilates to root s is a key driver of soil respiration; they are conservative estimates of r oot respiration, however, because girdling increased the use of starch rese rves in the roots. Our results indicate that models of soil respiration sho uld incorporate measures of photosynthesis and of seasonal patterns of phot osynthate allocation to roots.