Our results suggest that the net gain of carbon in young forests with lowered nitrogen limitation is higher than in the mature ones. Land vegetation fertilized with reactive nitrogen would take up additional 0.62 Pg C/yr in 1980's and 0.75 Pg C/yr, in the 1990's assuming mature forests in the model simulations (Figure 1). The estimated carbon uptake in both decades was higher in simulations with regrowing forests. The model simulations with forests planted in the 1950 showed that land vegetation would sequester at least 1.5 Pg C/yr (140% more) in the 1980's and 1.06 Pg C/yr (40% more) in the 1990's once nitrogen deposition increased. In the simulation with young forests planted in the 1970 the additional carbon uptake of land would raise up to 2.33 Pg C/yr in the 1980's and 2.21 Pg C/yr in the 1990's or to respectively 270% and 190% of carbon uptake derived in the simulation with mature forests. A result from our model simulations is that young forest grows faster and reaches maturity earlier if amount of nutrients is sufficient to support this growth. In addition young forest generates less litter than mature one and has lower ecosystem respiration. Therefore the effect of increased nitrogen deposition on regrowing forests is considerably higher than on mature forests.
The differences between mature and young forests responses to high nitrogen inputs diminish once re-growing forest matures. Change in land carbon uptake from 1980's to 1990's had opposite trends in simulation with mature and in simulations with re-growing forests. Assuming mature forests our model estimates suggested that in 1990's land absorbed 0.13 Pg of carbon per year more than in 1980's. This increase was associated with rising deposition of reactive nitrogen only (from 80 TgN/yr in 1980's to 95 TgN/yr 1990's). In both simulations with re-growing forests the additional carbon uptake has dropped by 0.24–0.44 PgC/yr from 1980's to 1990's. This drop has occurred because growth of forests was slowing down after the initial stage of fast growth, which was accelerated by higher nitrogen inputs. As the forest ages more biomass is accumulated and ecosystem respiration is increasing. Therefore increased carbon gain in forests can not last forever.