LITTER DECOMPOSITION AND NUTRIENT RELEASE IN ECTOMYCORRHIZAL MAT SOILS OF A DOUGLAS-FIR ECOSYSTEM

The ectomycorrhizal fungus Hysterangium setchellii (Fisher) forms extensive hyphal mats at the soil-litter interface with the roots of its host tree Douglas fir {Pseudotsuga menziesii [(Mirb.) Franco]}. Microbial biomass, needle decomposition rates, nutrient release from needles and exchangeable soil nutrients were measured in ectomycorrhizal mat soils and adjacent non-mat soils in a second-growth Douglas fir forest periodically throughout 1 yr. Microbial biomass and needle decomposition were 4.0 and 1.1 times higher, respectively, in mat soils than in adjacent non-mat soils during the year of sampling. After 1 yr of decomposition, needle litter concentrations of K and Mg decreased in both soil types, and release of both nutrients was greater in mats than in non-mat soils. Needle litter concentrations of Ca, Mn, Cu, Zn and Al increased during the year in both soil types, but nutrient absorption by needles tended to be highest in non-mat soils. Concentrations of N, P, Fe and B in needle litter did not change during the year, yet nearly twice as much N and P were released in mat soils compared to non-mat soils, suggesting either faster decomposition or nutrient import to mat soils. Soil solution concentrations of NH4, P, K, Mg and Zn measured by absorption onto ion-exchange resins, did not differ between mat and non-mat soils. Mat soils had higher concentrations of NO3, Fe, Cu and B but lower concentrations of Ca and Mn compared to non-mat soils. Nutrients released from the decomposing needles but not absorbed by ion-exchange resins must be rapidly removed from the soil solution either by plant roots, microbial tissue, saprophytic fungi or via ectomycorrhizal translocation to host trees. Our data suggest that the ectomycorrhizal fungus H. setchelli provides an improved microenvironment for organic matter decomposition resulting in faster release of N, P, K and Mg and more efficient removal of the nutrients from the soil solution.