Soil-atmosphere fluxes of the greenhouse gases CO2, CH4 and N2O in a mountain spruce forest subjected to long-term N addition and to tree girdling

At Alptal, Switzerland, nitrogen (N) has been applied as NH4NO3 since 1995 in low doses during rain events to realistically mimic increased N deposition to a mature mountain spruce stand. Five years of measurements in a replicated plot design showed that N2O and CH4 emissions from the soil to the atmosphere increased due to the N addition. For CH4, this involved a shift from a net sink to a net source. CO2 emissions did not change significantly, although they averaged lower rates under simulated N deposition. The girdling of 40% of tree basal area, followed by subsequent felling of the girdled trees, increased emissions of N2O but reduced net emissions of CH4 from soils. CO2 and N2O emissions depended on soil temperature and water table depth. Soil temperature did not affect CH4 fluxes, whereas net CH4 production was higher when the water table was closer to the soil surface. Our data highlights the need that future investigations should focus more on the allocation of assimilates to tree roots in order to better quantify the ecosystem's C balance.