Fig. 4. Cumulative CO₂–C respired during the course of a long-term laboratory incubation of soils at 25°C. An equation is fit to the data C_t = C_l (1 – e^–k_lt) + k_rt, where C_t is the known cumulative amount of C respired at sampling period t, C_l is the size of the labile C pool, k_l is the decay constant for the labile pool, and k_r is the mineralization rate for the recalcitrant pool, which is set at 3 µg g^–1 soil d^–1. For this sample, C_l = 1544 kg ha^–1, k_l = 0.012 d^–1.