Increased nitrous oxide emissions from a drained organic forest soil after exclusion of ectomycorrhizal mycelia

The aim of this study was to determine how roots and their ectomycorrhizal symbionts affect the fluxes of nitrous oxide (N2O) from nutrient-rich drained organic forest soils. Specifically, the relative impacts of roots and mycorrhizal mycelia on N2O fluxes were investigated using two different trenching treatments, excluding (a) roots or (b) roots and mycorrhizal mycelia, from the soil. N2O fluxes were measured at the soil surface, for 1 year before and 2.5 years after trenching, within the two trenching treatments and on untreated controls. While the exclusion of roots alone did not affect N2O emissions, the simultaneous exclusion of roots and mycorrhizal mycelia doubled N2O emissions, compared to the control plots. Two probable explanations for the increased fluxes were identified: (1) a decreased uptake of nitrogen (N) from the soil, through the mycorrhizal fungi, which increased N availability for the N2O-producing microorganisms, and (2) a decreased uptake of water from the soil, through the mycorrhiza, which increased the soil water content and thus the N2O emissions from denitrification. If the trenching reduced any potential stimulation of N cycling, through rhizodeposition, this mechanism did not outweigh the effects of a discontinued mycorrhizal N and/or water uptake on N2O fluxes. The results of the study emphasise the importance of ectomycorrhiza in regulating N2O emissions from forested organic soils.