Effects of fertilization on soil CH4 and N2O fluxes in young Norway spruce stands

Climate change mitigation strategies have increased the demand for wood products, resulting in an urgent need to increase wood production. One approach is to fertilize forest land, but this can influence greenhouse gas (GHG) fluxes within the ecosystem. The aim of this study was to examine the effects of forest N fertilization on soil CH4 and N2O fluxes in young Norway spruce (Picea abies (L.) Karst.) stands in southern Sweden. The gas fluxes were measured using flow-through non-steady-state dark chambers. In the first, long-term, experiment, half of the stand was fertilized twice (once in 2014 and once in 2016) with 150 kg ha(-1) of N, and gas flux measurements were taken throughout 2014-2017. In the second, dose, experiment, 0, 150, 300, or 450 kg ha(-1) of N was added to the stand in April 2016, and gas flux measurements were taken during April-December 2016. The dose experiment showed that the sink strength of CH4 decreased with increasing amounts of N; the long-term experiment indicated that repeated fertilization decreased the CH4 sink strength over time. Additionally, the long-term experiment indicated that, while significantly higher N2O emissions were recorded in the fertilization years, this was not detected in subsequent years, suggesting the effect to be short-lived. In the dose experiment, fertilization tended to increase the N2O emissions relative to the amount of fertilizer. However, despite the significant effects of fertilization on these GHGs, the summed fluxes were a fraction of the net uptake of C at the sites, as recorded in another study. These findings suggest that fertilizing forest land with commercial NP or NPK fertilizers corresponding to 150 kg ha(-1) of N, the level used in operational forestry in Sweden today, can be conducted without changing CH4 and N2O fluxes to any great extent.