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Impact of ectomycorrhizas on the concentration and biodegradation of simple organic acids in a forest soil

Simple organic acids seem to be important in many soil processes including nutrient uptake in the rhizosphere and long‐term pedogenic processes such as podzolization. The factors regulating the concentrations of these acids in soil, however, remain poorly understood. We have investigated one set of factors, namely the impact of ectomycorrhizal (Paxillus involutus) and non‐mycorrhizal Picea abies seedlings and humic acid on the concentration and dynamics of organic acids in soil solution. We did so over 10 months in laboratory columns containing soil from the E horizon of a sandy forest soil. Several organic acids were identified in the solution extracted from the root zone including oxalic, citric, malonic, succinic, acetic, formic and lactic acids at concentrations ranging from <0.1 to 2.3 µm. Both plants and ectomycorrhizas had significant effects on the concentration of organic acids in soil solution. In contrast, omitting P from the irrigation water appeared to have little effect on the concentrations. The microbial mineralization kinetics of oxalate conformed well to a single Michaelis–Menten equation. Further, the soil with the mycorrhizas had a significantly faster mineralization of oxalic acid over a wide concentration range than did the soil without ectomycorrhizas and without plants. We conclude that the oxalate flux through the soil with both trees and mycorrhizas is much faster than is evident from measurements of solution concentration at steady state. Humic acid had little effect on the concentrations of organic acids or dynamics in the soil solution. Oxalic acid concentrations in the soil solution were correlated with hyphal length, rate of microbial mineralization, soil respiration, and shoot to root weight ratio. We conclude that both mycorrhizas and plants have a large impact on organic acid cycling in forest soils.