What is primarily responsible for the cycling of carbon and nitrogen in ecosystems?

The cycling of carbon and nitrogen in ecosystems is primarily driven by primary productivity and decomposition. Primary productivity refers to the process by which autotrophs, mainly plants, convert sunlight into chemical energy through photosynthesis, fixing carbon from the atmosphere into organic forms. This not only contributes to the carbon cycle but also supports the entire food web by providing energy for herbivores and subsequently for higher trophic levels.

Decomposition is equally important, as it involves the breakdown of dead organic matter by decomposers such as fungi, bacteria, and other microorganisms. This process returns carbon and nitrogen back into the soil and atmosphere, transforming organic materials back into inorganic forms that can be reused by plants and other producers. Through these interconnected processes, carbon and nitrogen can continuously cycle through different reservoirs in the ecosystem, maintaining ecological balance and supporting life.

In contrast, other options such as secondary productivity, soil erosion, and fertilization do not primarily focus on the fundamental processes that recycle these critical nutrients in the ecosystem. While they may influence nutrient dynamics, the core cycling is fundamentally linked to the activities of primary producers and decomposers.