Refer to the following information to answer the next few questions.
Researchers decided to test the hypothesis that if the 2-m tall Polytrichum gametophyte-sporophyte plants had acted as a physical buffer,then they would have reduced water's ability to erode the soil and carry away its nitrogen.They began with four equal-sized areas where Polytrichum mosses grew to a height of 2 m above the soil surface.One of the four areas was not modified.In the second area,the mosses were trimmed to a height of 1 m above the soil surface.In the third area,the mosses were trimmed to a height of 0.5 m above the soil surface.In the fourth area,the mosses were trimmed all the way to the ground,leaving only the rhizoids.Water,simulating rainfall,was then added in a controlled fashion to all plots over the course of one year.The figure below presents four graphs that depict potential results of this experiment.

-Why should we expect the soil's nitrogen not to be contained solely within the rhizoids of the Polytrichum mosses?