The impact of trees, ectomycorrhiza and potassium availability on simple organic compounds and dissolved organic carbon in soil

The presence of tree roots and symbiotic mycorrhizal fungi is recognized to have a substantial impact on carbon dynamics in soils. In this study the effect of Pinus sylvestris seedlings and the ectomycorrhizal fungus Hebeloma crustuliniforme on a number of biogeochemical variables, mainly related to labile carbon pools was investigated. The impact of K limitation as a potential regulatory factor was also examined. Columns filled with E horizon ±plants and ±mycorrhizal fungi were incubated for 18.5 months. The results demonstrate that plants, as well as mycorrhizal fungi, significantly increased the concentrations of some simple organic acids, including oxalate, in soil solution. Observations for dissolved organic carbon were slightly contradictory but the cumulative amount found in drainage water was ∼20% higher in planted versus non-planted columns. Soil from planted treatments also showed more rapid mineralisation kinetics for oxalate. However carbon utilization (mineralisation vs. biomass) of oxalate and glucose by the soil microbial biomass was less influenced by plants. At harvest a component integration study of soil autotrophic and heterotrophic respiration was performed which revealed that both plant and mycorrhiza had a positive effect on the heterotrophic respiration. Potassium omission had little effect on the variables studied with the exception of the maximum mineralisation rate for oxalate, which increased when K was withdrawn. The results are discussed in the context of the dynamics of labile soil carbon pools and ecosystem C fluxes.