In which layer of the soil profile does most biological activity occur?

Most biological activity occurs in the A-horizon of the soil profile. The A-horizon is also known as the topsoil layer, and it is the layer closest to the soil surface. This layer is rich in organic matter and contains high concentrations of soil organisms such as bacteria, fungi, earthworms, and other microorganisms that play essential roles in the decomposition of organic matter and the formation of soil nutrients. The high levels of organic matter and the presence of these soil organisms make the A-horizon the most fertile layer of the soil profile, and it is the layer where most plants roots are found. As a result, the A-horizon plays a critical role in soil health and plant growth.