Fig. 1. One example each of incubations from 25°C (A and B) and 4°C (C and D). Monitored over time were CH₄ uptake, CO₂ production (A and C), and the ¹³C of each of these gases (B and D). Solid symbols represent CH₄ while open symbols represent CO₂. In every incubation (n = 7 at 4°C and n = 10 at 25°C), the CH₄ uptake rate was greater than the CO₂ production rate. From the isotopic composition of methane oxidized (calculated with Eq. [5]) and the measured ¹³C of soil organic matter, we were able to partition the CO₂ produced from these two sources (Eq. [6]) to determine the apportionment of CH₄ to methanotrophic biomass and respiration, respectively ("carbon utilization", Table 6). Note the different times scales on the x axes