Looking at the model source code, I am not able to locate that exact equation. In APEX, the bulk density changes according to several aspects such as tillage operations, biological activity, and soil settling after rainfall.
Also, several things in APEX1501 are different from what is reported in the theoretical documentation of the old version of APEX. It is still a good reference of the main structure of the model, but now some processes might be simulated with an evolution of the original way or with an improved new approach.
Let us know if you need specific information about the model and how the processes are simulated.