Final answer:
As one moves deeper into the soil, the availability of electron donors and acceptors decreases due to anaerobic conditions caused by the lack of oxygen. This affects the processes of oxidation and leads to iron reduction and organic matter accumulation.
Step-by-step explanation:
The availability of electron donors and acceptors decreases as one moves deeper into the soil. This is because as the soil gets deeper, there is less oxygen present, leading to anaerobic conditions. Oxygen is necessary for the process of oxidation, which is crucial for the availability of electron donors and acceptors. In anaerobic conditions, different processes like iron reduction and organic matter accumulation occur, affecting the availability of electron donors and acceptors.