Time-dependent radiative forcing effects of forest fertilization and biomass substitution

Here we analyse the radiative forcing implications of forest fertilization and biomass substitution, with explicit consideration of the temporal patterns of greenhouse gas (GHG) emissions to and removals from the atmosphere (net emissions). 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 substitution benefits and carbon stock. The radiative forcing of the 2 stands is identical for the first 15 years, followed by 2 years during which the fertilized stand produces slightly more radiative forcing. After year 18 the instantaneous and cumulative radiative forcing are consistently lower for the fertilized forest system. Both stands result in long-term negative radiative forcing, or cooling of the earth system. 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. This suggests that forest fertilization and biomass substitution are effective options for climate change mitigation, as climate change is a long term issue.