Nitrogen fertilization increases N2O emission but does not offset the reduced radiative forcing caused by the increased carbon uptake in boreal forests

Net primary production in boreal coniferous forests is generally severely limited by N deficiency. Nitrogen fertilization has thus the potential to strongly increase forest tree biomass production in the boreal region and consequently increase the biosphere uptake of atmospheric CO2. Increased N availability may though increase the production and emission of soil N2O, counteracting the climate mitigation potential from increased forest biomass production. Studies in the boreal region on the net effect on the climate mitigation potential from N fertilization are scarcer than in other biomes. Therefore, we explored how N affected soil GHG fluxes in two boreal field N-loading experiments, of which one is a long-term experiment (40 years), and the other established 6 years before investigation. We also estimated whether the increased soil N2O emission could offset the N-driven increased C sequestration by the trees. Nitrogen additions affected the soil GHG fluxes in both stands. Soil N2O emission was enhanced by N addition at every fertilization rate, though marginally compared to the reduced soil CO2 emission and the increased atmospheric CO2 uptake and biomass production. The estimated annual uptake of CH4 by soil under long-term N addition increased. The magnitude of soil CH4 uptake was on the same order of magnitude as the increase in soil N2O emissions caused by N addition, when compared as CO2 equivalents. In conclusion, forest N fertilization in boreal areas increased the GHG net uptake and, thus, provides a means to mitigate increasing atmospheric concentrations of GHG.