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Topsoil (0-20 cm) data of 25 997 sites were analyzed across five land uses in the Jiangsu Province of China. The results showed that the residential/industrial land stored the highest amount of soil organic carbon (SOC) among all the land uses. The polynomial model can best describe the relationship between SOC density (SOCD) and pH, showing that SOCD increases over the pH range of 4.2-6.5, decreasing sharply in the range of 6.5-9.2. The reason may be that microbial activity tends to be maximal at intermediate soil pH levels (pH 6-7). The relationship between SOCD and pH was found to vary substantially with the land use. Soil pH can explain 4.3-63.8% of the variation in SOCD for all land uses, suggesting that this variable should be introduced in SOC models to better predict SOC stock changes in the future. [ABSTRACT FROM AUTHOR]