What is the primary impact of land-use change on the biosphere?

The primary impact of land-use change on the biosphere is its effect on the biosphere's ability to act as a carbon sink. Land-use changes, such as deforestation, urbanization, and agricultural expansion, can significantly alter the natural processes of carbon sequestration that forests, grasslands, and wetlands provide. By transforming these ecosystems, the capacity for these areas to absorb carbon dioxide from the atmosphere is diminished.

Forests, for instance, play a critical role by taking in carbon dioxide during photosynthesis and storing carbon both in biomass and soil. When these forests are cleared or degraded, not only is this carbon absorption capacity reduced, but the stored carbon is often released back into the atmosphere, exacerbating climate change. Consequently, the ability of the biosphere to regulate atmospheric carbon levels is compromised, leading to broader environmental implications.

The other options relate to aspects of land-use change but don't represent its primary impact. While enhanced carbon absorption, increased biodiversity, and soil degradation can occur as a result of land-use changes, they are secondary effects rather than the main consequence regarding the carbon sink functionality of the biosphere.