Effects of Nitrogen Fertilization of Boreal Forest Land on Greenhouse Gas Flows

Forest growth on mineral soils in boreal regions is often limited by a low availability of nitrogen (N), and fertilization has shown particular promise in increasing yields in productive boreal forests. In this study we analyze the greenhouse gas (GHG) implications of increasing forest biomass production through N fertilization and using the increased production for bioenergy and biomaterials in place of non-renewable fuels and materials. We begin with a stand-level analysis of the radiative forcing implications of forest fertilization and biomass substitution, with explicit consideration of the temporal patterns of GHG emissions to and removals from the atmosphere. We model and compare the production and use of biomass from a hectare of fertilized and non-fertilized forest land in northern Sweden. We calculate the annual net emissions of CO2, N2O and CH4 for each system, over a 225-year period with 1-year time steps. We calculate the annual atmospheric concentration decay of each of these emissions, and calculate the resulting annual changes in instantaneous and cumulative radiative forcing. We find that forest fertilization can significantly increase biomass production, which increases the potential for material and energy substitution. The average carbon stock in tree biomass, forest soils and wood products all increase when fertilization is used. The additional GHG emissions due to fertilizer production and application are small compared to increases in carbon stock and substitution benefits. By the end of the 225-year simulation period, the cumulative radiative forcing reduction of the fertilized stand is over twice that of the non-fertilized stand. We then consider a steady-state landscape-level scenario where 10% of Swedish forest land is fertilized. We estimate the primary energy use and GHG emissions from forest management including production and application of N and NPK fertilizers. Based on modelled growth response, we then estimate the net GHG benefits of using biomaterials and biofuels obtained from the increased forest biomass production. The results show an increased annual usable biomass production of 8.3 million t dry matter, of which 37% is large-diameter stemwood. The resulting annual net GHG emission reduction is 12.7 million or 19.5 million tCO2e if the avoided fossil fuel is fossil gas or coal, respectively, corresponding to 19% or 30% of the total Swedish GHG emission in 2007. An average annual application of 21 kg N per ha results in an average annual production increase of about 4 t oven-dry biomass per ha, leading to a total annual emission reduction of up to 8.5 t CO2e per ha. A significant one-time carbon stock increase also occurs in wood products and forest tree biomass. These results show that relatively small quantities of N can leverage large changes in GHG flows, suggesting that forest fertilization and biomass substitution may be effective options for climate change mitigation.