Fill in the Blanks

In forest ecosystems, carbon does not simply remain locked in tree trunks; it (1) moves through leaves, roots, and soil. When plants shed organic litter, (2) break it down and release carbon dioxide, but some carbon becomes (3) in soil particles. This storage can be surprisingly long-lived, depending on temperature, moisture, and the chemical structure of the material. To track these processes, scientists often use (4) as natural labels. By comparing the ratio of carbon-13 to carbon-12 in different samples, researchers can (5) whether carbon originated from recent photosynthesis or older deposits. Such measurements help refine climate models, because they reveal how quickly soils may respond to warming and how much carbon they can ultimately retain.