Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

What cycle involves the decomposition of dead organisms returning carbon to the soil?

1. Water cycle

2. Nitrogen cycle

3. Phosphorus cycle

4. Carbon-oxygen cycle

The correct answer is: Carbon-oxygen cycle

The correct choice is the carbon-oxygen cycle because it specifically addresses the processes by which carbon, an essential element, is cycled through various forms in the ecosystem. Decomposition is a critical component of this cycle. When organisms die, decomposers like bacteria and fungi break down their bodies, releasing carbon back into the soil as organic matter or carbon dioxide. This process enriches the soil with nutrients and helps maintain the balance of carbon in the environment. The other cycles mentioned do not focus primarily on carbon. The water cycle pertains to the movement of water through evaporation, condensation, and precipitation. The nitrogen cycle involves the conversion of nitrogen in the atmosphere into forms that organisms can use, such as nitrates, and focuses on nitrogen compounds rather than carbon. The phosphorus cycle deals with the movement of phosphorus through the lithosphere, hydrosphere, and biosphere, which mainly impacts the availability of this key nutrient for plant growth but does not include carbon decomposition.