The SimRoot root architecture 3D model is fascinating and informative.
Though the analysis results are common sense, seeing available nitrogen removed from the uppermost soil levels is important to the capabilities of green roof plants and their capability to clean stormwater.
Watch the following youtube video of SimRoot from Penn State.
You will see the time lapse simulation of the root growth and note the concentrations of available nitrogen decreasing rapidly in the surface area.
Happy Green Roofing - Kevin