<Fig. 4>
The spatial distribution maps of the SOC contents derived by the OK method were presented in Fig. 4, with a kriging error range of 1.16-3.67 g kg-1, 1.11-4.41 g kg-1, and 1.57-2.29 g kg-1 for SOC1980, SOC2000, and SOC2015, respectively (Figure S2). Overall, the SOC distribution patterns on the three sampling dates were similar, with high SOC contents in the southeast (mainly surrounding Taihu Lake) and low SOC contents in other parts of the study area. However, differences in the local details of the spatial patterns of SOC still existed among the three sampling dates. The high-value areas with SOC1980 exceeding 18 g kg-1 were mainly distributed in the southeast of the study area, while the low-value areas with SOC1980< 10 g kg-1 were scattered along the southern side of the Yangtze River and the areas near the western boundary of the study area. The SOC1980 in most parts of the study area ranged from 10 to 18 g kg-1. However, in 2000, the high-value areas with SOC > 18 g kg-1 expanded significantly across the study area; these areas were located throughout the whole southeastern parts of the study area and in several small areas in Jiangyin, Yixing, and Jintan city. The areas with moderate values of SOC2000 between 10-18 g kg-1 were observed in the northwest (Dantu, Danyan), in the southwest (Liyang, Jintan), and near the Yangtze River (Zhangjiagang, Taicang). In 2015, the SOC contents in most parts of the study area varied from 10 g kg-1 to 18 g kg-1, while the high-value areas with SOC2015 > 18 g kg-1 were very limited and were only distributed in Yixing and southwestern Changshu.

3.3 Temporal changes in SOC