Nodes of different colours represent the following:
Solid arrows point from a submodule to the (sub)module which it is
descended from. Dashed arrows point from a module or program unit to
modules which it uses.
Where possible, edges connecting nodes are
given different colours to make them easier to distinguish in
large graphs.
write all organic carbon for the soil profile
write total carbon for the soil profile, plants, and residue
write all carbon, organic n and p, and mineral n and p for the soil profile, plants, and residue