Low-molecular weight organic acid production by arbuscular and ectomycorrhizal conifers

Temperate rainforests along the North American Pacific coast typically have podzolized soils with high acidity and depletion of base cations. Low-molecular weight organic acids (LMWOA) play a potentially important role in rainforest soils by enhancing the biotic weathering of minerals for cation release, but whether ectomycorrhizal (EM) fungi and arbuscular (AM) fungi differ in this trait is uncertain. We compare LMWOA concentrations in soils and directly produced from individual root tips by an EM conifer (Picea sitchensis) and AM conifer (Thuja plicata) in a mature field trial. Soil water extracts were dominated by tartaric acid, particularly in the organic horizon, and concentrations were 10 fold higher on average under Thuja plicata. The effect of tartaric acid on cation availability was highlighted by positive correlations with calcium in soil solution, but at a significantly lower rate of release under Thuja plicata compared to Picea sitchensis. We also found podzolization attributes elevated under Thuja plicata, with reduced base saturation, lower pH near the soil surface, and greater concentrations of exchangeable aluminum. A short-term laboratory assay of excised roots demonstrated 50% higher production of total LMWOA (primarily formic acid) by distal roots of Thuja plicata compared to EM fungal colonies. Several findings deserve further investigation, including whether these organic acids protect roots from aluminum toxicity, and the possible role of mycorrhizosphere bacteria in LMWOA production. When considering the whole-tree response, we suggest Thuja plicata, through LMWOA production, would be well adapted to exploiting the base-poor, podzolized soils of the North American Pacific coast