If trees get their mass from the soil, we can predict that the soil will weigh less at the end of the five-year experiment. This is because as trees grow, they absorb nutrients and water from the soil, which are used to build new tissues like leaves, branches, and roots. The carbon that trees use to build their tissues comes from carbon dioxide in the air, but the other elements and minerals needed for growth typically come from the soil. As trees grow, they continue to take up more and more nutrients from the soil, which may eventually become depleted.
Therefore, if the trees in the experiment have been growing for five years, we can assume that they have been absorbing nutrients and water from the soil for that entire time, and that the soil has likely become depleted to some extent. As a result, we would expect the soil to weigh less at the end of the experiment than it did at the beginning.
However, it's worth noting that this is a simplified view of how trees get their mass. Trees also get some of their mass from the carbon dioxide they absorb from the air during photosynthesis, and they can store carbon in their tissues for many years. Additionally, trees shed leaves and branches, which decompose and return nutrients to the soil. So while we might expect the soil to weigh less over time, the actual outcome could be more complex and variable depending on the specifics of the experiment.