The study looked at how adding extra CO2 to the air affects different parts of soil humus in paddy fields. They found that the amount of organic carbon in the soil increased with more CO2, especially in the top layer of soil. Water-soluble organic carbon also increased in the top layer. However, the humic acid content decreased, while humin was not affected much. Overall, the results suggest that elevated CO2 levels can lead to more organic matter in the soil and changes in the composition of soil humus fractions.