Plant roots and their mycorrhizal symbionts can both stimulate soil carbon (C) losses by supplying microbes with the energy needed to decompose organic matter and suppress soil C losses by competing with microbial decomposers. However, it is unclear the extent to which roots and mycorrhizal fungi influence decomposition of below-ground litter, which can account for 20-50% of litter inputs in forests. We aimed to test how tree species that associate with either ectomycorrhizal (EcM) or arbuscular- (AM) mycorrhizal fungi alter the decomposition of fungal and root litter and in turn, how decomposing litter influences the foraging behavior of AM and EcM associated roots.
We placed fungal necromass and root litter from either Acer saccharum (AM) or Quercus alba (EcM) trees inside and outside root exclusion trenches (15.2 x 30 cm PVC cores affixed with 1-micron mesh bottoms) in forest plots differing in their abundance of AM and EcM-associated trees. Outside of trenches, litters were decayed inside fabric root-bags containing proliferating mycorrhizal roots. Thus, our design allowed us to compare necromass and root litter mass remaining in soils where roots were excluded (trenches) and actively growing (root bags). Morphological measurements were taken on roots from root-bags with or without litter additions.
Results/Conclusions
Mass remaining of root litter did not differ between tree species but was significantly lower in root exclusion treatments (F2,27 = 6.01, P<0.01). This increase in decomposition was not attributable to differences in soil moisture, as volumetric water content did not vary between PVC trenches and the soil contained within root bags. Fungal litter mass loss did not differ between root exclusion treatments or when decomposed in the presence of A. saccharum vs. Q. alba roots. After three months, root proliferation was significantly higher in root-bags containing litter, compared to root-bags without litter additions (χ2 = 4.94, P = 0.03). Average root diameter and branching intensity (number of root tips per cm root length) did not differ between roots grown with or without litter additions but differed between tree species. On average, A. saccharum absorptive roots had larger diameters and lower branching intensities. The one-year harvest will occur in May 2019 and anticipated analyses include soil enzyme activity in the presence and absence of A. saccharum and Q. alba roots. This study demonstrates the potential for suppressed saprotrophic decomposition of root litter by living roots and mycorrhizae in mixed hardwood forests containing both AM- and EcM- associated tree species.