Plants use their roots to take up nitrogen available in the soil.
Animals receive it through the food they consume, such as plants which have absorbed it.
Absorption of nitrogen by plants is part of the nitrogen cycle . In the Stella model system, the above ground biomass started off low causing an initial decrease in below ground biomass. An increase in above ground biomass is due to nitrogen cycling through the system and returning to the atmosphere. In the model, it is due to the below ground biomass translocation up. Below ground biomass increased at a greater rate than the above ground biomass because of the small amount of export from the system.