According to Measurements Taken in the 1990s, the Levels of Inorganic

According to measurements taken in the 1990s, the levels of inorganic nitrogen in Arctic ecosystems were substantially lower than the amount of nitrogen actually taken up by plants. Which of the following is a possible explanation for this pattern?

A) Arctic trees are able to perform nitrogen fixation with enzymes in their leaves.
B) Microbes in the Arctic soil transform inorganic nitrogen into organic forms.
C) Arctic sedges can take up organic forms of nitrogen.
D) Arctic shrubs take up 60% of their nitrogen in the form of N₂.