Fig. 4.

Mean annual cycles of key ecosystem fluxes for three time periods of the fully forced C−N case. (A) GPP, (B) net N mineralization, (C) net ecosystem exchange (NEE, positive = CO₂ source), and (D) heterotrophic respiration. Relative increase in GPP between experiments is smaller than proportional increase in N mineralization with deeper decomposition. Shift in N mineralization with enhanced deeper SOM decomposition toward autumn is due to longer decomposing than growing seasons, and phase lag of temperature in deep soils. The solid and dashed lines represent Z_τ = 0.5 m and 10 m, respectively. All cases show the mean of the geographic region in which permafrost initially occurs in the model.