plant pools (i) leaf, wood, fine root, coarse root

• Add CN parameters for each pool
• calculate dN_i/dt = dC_i/dt * CN_i for each pool (eq 12)
• calculate total plant N demand (eq 20, F^N_demand = sum dN_i/dt), and subtract from N_min
  • calculate all the N demand and N supply terms in a time step, then for a particular N stock calculate k = (N demand) / (N stock + N supply), then if k < 1 each demand gets k*Ndemand
• N limitation
  • F^N_uptake = min(N_min/dt, F^N_demand)
  • New indicator variable I_N = boolean(F^N_demand > N_min/dt)
  • if(I_N): NPP = 0; this might be done by setting GPP = R_A