Insufficient O2 concentration in the matrix pores, which is adjusted by air-immobile regions in compost piles, is a main factor in forming anaerobic cores in compost particles and then generating harmful off-gases during composting. However, it is unclear how the change of air-immobile regions affects temporal variation of O2 in the pores during the whole composting process and after turning. In this study, we first used a tracer-inverse calculation protocol to obtain feature parameters (proportional coefficient of gas in the air-immobile region, φ; the first-order mass transfer coefficient, α) of the air-immobile regions in the matrix pores before and after turning during whole composting process, and then predicted the temporal variation of O2 in the pores based on two-region model with these measured parameters. The φ values in compost piles before/after turning for initial-material, temperature-increasing, thermophilic, and curing phases were 0.38/0.40, 0.42/0.40, 0.46/0.46, and 0.41/0.45, respectively, while the corresponded α values were 0.002/0.001, 0.001/0, 0.004/0, and 0.005/0.001 min-1, respectively. Air-immobile regions were higher in the temperature-increasing and thermophilic phases than that in curing. The air-immobile regions caused difference of predicted O2 concentrations between two regions, and the difference was enhanced during the composting mainly by the reaction rate of organic-matter biodegradation. Turning piles slightly decreased φ in the temperature-increasing phase and had little change in thermophilic phase, while increases in other phases. The α declined after turning in the whole composting process. These findings provide support for reducing the production of harmful off-gases in composting
