(a) Soil in central Iowa with dark, organic-matter-rich topsoil. Iowa's average topsoil depth decreased from 35-45 cm to 15-20 cm during the 20th century due to farming. (b) Severe gully erosion ...

a product that often gives carbon-rich soils their characteristic dark color (Fig. 2). These various forms of SOC differ in their recalcitrance, or resistance to decomposition. Humus is highly ...