*4.2. Effect of F–T Cycles on the Speciation of HMs*

The main reasons for this phenomenon are as follows. Through the continuous action of the F–T cycle, the porosity ratio in the soil keeps increasing, making it easier for carbon dioxide to enter the soil. Some studies have shown that the carbonization process of soil has adverse effects on the S/S of HMs. The pH value of the soil keeps decreasing with the carbonization, causing hydrolysis of the hydration products of many adhesives, and further activation of more HMs.

This is due to the fact that the carbonate-bound fraction of HMs is very sensitive to the change of pH. Under alkaline conditions, the carbonate-bound fraction is relatively stable. With the continuous decrease of soil pH under the action of the F–T cycle, the HM compounds in the carbonate-bound fraction decompose, while the rate of decomposition is associated with the change speed of pH because the degree of influence of carbon dioxide on soil Ph gradually decreases over time. During this time, the process of S/S of HM is still underway but does not play a dominant role.

As the F–T cycle progresses, the relative content of the humic-acid-bound fraction, iron and manganese oxidized bond fraction, strong organic bonding fraction, and residue fraction in the stable form increases continuously. However, due to their small content, they have little influence on the overall fraction of HMs, so the relative content of the stable form of HMs is still in a downward trend as the F–T cycle progresses. It may also be noted that these four HM forms are not sensitive to pH.
