Soil organic carbon (SOC) is an important component of the global carbon cycle, acting as a huge reservoir. Thus, its response to global change may initiate a long-term feedback between the atmosphere and the biosphere. The dynamics of SOC have been the focus of various ecological studies, such as comparison of matter flows among a variety of ecosystems. However, there remain several uncertainties about the estimation of SOC storage on a global scale, chiefly because of the paucity of information about the heterogeneity of SOC accumulation. I first review the literature related to estimation of global SOC stocks and flows, in order to clarify the role of SOC in the global carbon cycle. Several major problems responsible for estimation uncertainty are revealed. The present paper places considerable emphasis on the modeling of SOC dynamics, because the development of process-based models will become increasingly important for global-scale estimation of ecosystem processes and functions. Both database- and model-derived estimations of global SOC storage are summarized. Spatio-temporal variations in SOC budget are discussed to clarify mechanisms determining the amount of SOC. Also, I discuss the possible impact of global environmental change on the soil carbon budget, which may have a positive or negative feedback effect on the human-induced climate change. Based on these discussions, a number of urgent issues, which ecologists need to consider, are addressed.
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