Mean values of environmental factors of 130 historical sites and 147 recent sites examined in this study.

For each environmental factor, mean values of historical and recent datasets in 31 grids (see Figure 1) were compared with paired-sample t-tests. Note: A: the preceding crops in fields with wet-dry crop rotation were wet rice, while those in lands with dry crop rotation were dry crops such as soybean, corn, and cotton. “*”: P<0.05 and “NS”: not significant.