Why do many grassland soils have a dark upper layer rich in organic material?

Grassland soils often feature a dark, organically rich upper layer primarily due to the nature of native grass vegetation and its pattern of decomposition. Unlike forest soils, which accumulate a surface layer of rotting leaf litter, grasslands produce a significant amount of organic matter through their extensive root systems.123

As these grass roots die and decompose within the soil profile, they contribute to a substantial accumulation of humus, which gives the uppermost horizon its characteristic dark color. This process is further supported by the activity of soil organisms, such as earthworms, which help mix organic material into the soil matrix. These soils, known as Mollisols, are highly fertile and are shaped by the specific climatic conditions and biological cycles inherent to grassland ecosystems.124