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.