Question: modelling the carbon cycle using first order differential equations

The details of the tasks are explained in the maple file attached but the aim is essentially to model the carbon cycle using first order diff eqs. I'm slightly confused as to how to exactly set up the equations for part a) and b). I've set up the required constants for the equations and the initial conditions as follows: 

#### initial conditions
atmos(0):=750;
bios(0):=600;
soil(0):=1500;

##### coefficients for rate of change
terrPhoto:=110/atmos(0);
terrResp:=110/bios(0);
plantDeath:=55/bios(0);
plantDecay:=55/soil(0);

and one of the equations as :

Diff(atmos(t),t):=terrPhoto*atmos(t); 

but I've been told that I would have to accommodate the direction of flow of carbon in the rates and also reconsider how many equations I would need.  hwk16.mw