The soil organic carbon content in Hebei Plain has been increasing over the past 25 years. Different types of soil, like rice soil and bog soil, have varying abilities to store carbon based on factors like water content and soil texture. Soils with higher water and clay content can store more carbon, while those with lower water and clay content store less. Overall, most soil types in the area have seen an increase in organic carbon storage, except for certain types like brown soil and bog soil, which have experienced a decrease.