Ectomycorrhizal fungi introduced with exotic pine plantations induce soil carbon depletion

Exotic pine plantations are promoted for their presumed capacity to provide a net sink of atmospheric C. Millions of hectares worldwide will be subjected to conversion into plantations during the next decades. However, pine introductions are known to result in a marked depletion of soil C, a phenomenon which has remained unexplained. We studied plantations in paramo grasslands of Ecuador, where the effect of the exotic introduction of radiata pines (Pinus radiata) and their accompanying ectomycorrhizal fungi can be studied in isolation from other ecosystem disturbances. We suggest that ectomycorrhizal fungi can extract C previously accumulated by paramo grasslands based on (a) a drastic simplification of the ectomycorrhizal community shown by direct DNA identification, (b) a loss of up to 30% soil C within <20 years of plantation, (c) stable C isotope values in fungal fruitbodies which are closer to grassland than pine values, and (d) radiocarbon dating of fruitbodies indicating relatively old C sources for fruitbody formation. Species number in the ectomycorrhizal guild drops to only three fungal species per plantation compared to approximately 100 in comparable native pine stands. Our results provide evidence for a dynamic role of ectomycorrhizal fungi in soil C processing, and question the strategy of introducing pine plantations as a general solution to reduce mounting atmospheric CO2 levels.