Soil organic carbon storage in grazing pasture converted from forest on Andosol soil

In order to clarify the effect of land‐use change from forest to grazing pasture on the organic carbon storage in Andosol soil, the Rothamsted carbon turnover model for volcanic soil was applied to a pasture situated at the National Livestock Breeding Center (37°09′N, 140°03′E). The top 25‐cm soil layer was considered to be an active soil carbon pool. The carbon storage in the soils of native forest surrounding the pastures ranged 111–163 t C ha −1 with an average of 133 t C ha −1 , which was adjusted according to an equivalent soil weight of pasture. The pasture soil carbon stocks ranged 88–135 t C ha −1 , with variations according to site and/or pasture age. The carbon inputs to the soil through the above‐ and below‐ground dead material from pasture plants and cattle feces were estimated to be 1.1, 1.8 and 0.9 t C ha −1  year −1 , respectively. As the model outputs of 14 C content of the soil, which is an index of carbon dating corresponding to nuclear weapons testing, showed a relatively close agreement with the observations, the modeling was acceptable for the purpose of predicting the turnover of organic carbon in Andosol soil. The model simulation demonstrated that, in order to maintain the average forest carbon level, 3–4 t ha −1  year −1 of the organic carbon input would be needed. These inputs would be provided in a grazing pasture producing 8–9 t ha −1  year −1 of above‐ground dry matter.