Why do temperate grasslands have the most fertile soil on earth?

Temperate grasslands are characterized by their deep and nutrient-rich soils, primarily due to the extensive root systems of grasses. When these grass roots decay, they add organic matter to the soil, enhancing its nutrient content and promoting overall soil fertility. This process is known as humification, where decomposed organic material contributes to the soil's structure and nutrient availability, making it highly fertile.

Additionally, the active decay of root systems contributes to a robust microbial community that plays a crucial role in nutrient cycling. This dynamic between plant growth and microbial activity enhances soil structure, water retention, and nutrient availability, making temperate grasslands particularly well-suited for agricultural practices.

Other options do not sufficiently explain the specific reasons for the high fertility of temperate grassland soils. For instance, while frequent flooding can contribute nutrients in some environments, it is not a defining feature of temperate grasslands. Similarly, although fires play a role in maintaining grassland ecosystems by clearing old vegetation, the primary source of soil fertility stems from the decay of grass roots. The proximity to rivers may provide some benefits, but this is not directly related to the unique soil characteristics that define the fertility of temperate grasslands.