Ectomycorrhizal mats alter forest soil biogeochemistry

Dense hyphal mats formed by ectomycorrhizal (EcM) fungi are prominent features in Douglas-fir forest ecosystems, and have been estimated to cover up to 40% of the soil surface in some forest stands. Two morphotypes of EcM mats have been previously described: rhizomorphic mats, which have thick hyphal rhizomorphs and are found primarily in the organic horizon, and hydrophobic mats, which occur in the mineral horizon and have an ashy appearance. This study surveyed EcM mat and non-mat soils from eight early and late seral conifer forest stands at the H.J. Andrews Experimental Forest in western Oregon. EcM mats were classified by morphology and taxonomic identities were determined by DNA sequencing. A variety of chemical and biochemical properties, including enzymes involved in C, N, and P cycling were measured. Analysis was confined to a comparison of rhizomorphic mats colonizing the organic horizon with non-mat organic soils, and hydrophobic mats with non-mat mineral soils. Both the organic and mineral horizons showed differences between mat and non-mat enzyme profiles when compared on a dry weight basis. In the organic horizon, rhizomorphic mats had greater chitinase activity than non-mat soils; and in the mineral horizon, hydrophobic mats had increased chitinase, phosphatase, and phenoloxidase activity compared to the non-mat soil. The rhizomorphic mats had 2.7 times more oxalate than the non-mats and significantly lower pH. In the mineral horizon, hydrophobic mats had 40 times more oxalate and significantly lower pH than non-mat mineral soils. Microbial biomass C was not significantly different between the rhizomorphic mat and non-mat organic soils. In the mineral horizon, however, the hydrophobic mats had greater microbial biomass C than the non-mat soils. These data demonstrate that soils densely colonized by EcM fungi create a unique soil environment with distinct microbial activities when compared to non-mat forest soils.