Depth-dependent effects of Ericoid Mycorrhizal shrubs on soil carbon and nitrogen pools are accentuated under Arbuscular Mycorrhizal Trees

Plant mycorrhizal associations influence the accumulation and persistence
of soil organic matter and could therefore shape ecosystem biogeochemical
responses to global changes that are altering forest composition. For
instance, arbuscular mycorrhizal (AM) tree dominance is increasing in
temperate forests, and ericoid mycorrhizal (ErM) shrubs can respond
positively to canopy disturbances. Yet how shifts in the co-occurrence of
trees and shrubs with different mycorrhizal associations will affect soil
organic matter pools remains largely unknown. We examine the effects of
ErM shrubs on soil carbon and nitrogen stocks and indicators of microbial
activity at different depths across gradients of AM versus ectomycorrhizal
(EcM) tree dominance in three temperate forest sites. We find that ErM
shrubs strongly modulate tree mycorrhizal dominance effects. In surface
soils, ErM shrubs increase particulate organic matter accumulation and
weaken the positive relationship between soil organic matter stocks and
indicators of microbial activity. These effects are strongest under AM
trees that lack fungal symbionts that can degrade organic matter. In
subsurface soil organic matter pools, by contrast, tree mycorrhizal
dominance effects are stronger than those of ErM shrubs. Ectomycorrhizal
tree dominance has a negative influence on particulate and
mineral-associated soil organic matter pools, and these effects are
stronger for nitrogen than for carbon stocks. Our findings suggest that
increasing co-occurrence of ErM shrubs and AM trees will enhance
particulate organic matter accumulation in surface soils by suppressing
microbial activity while having little influence on mineral-associated
organic matter in subsurface soils. Our study highlights the importance of
considering interactions between co-occurring plant mycorrhizal types, as
well as their depth-dependent effects, for projecting changes in soil
carbon and nitrogen stocks in response to compositional shifts in
temperate forests driven by disturbances and global change.