Lessons in Buried Horizons and Pedogenesis from Deep Forest Soils.

The lower boundary of soil has been a point of contention among soil scientists for decades. Recent evidence suggests that soil is much deeper than is measured by many ecological studies and that arbitrary definitions of maximum soil depth unnecessarily exclude important regions of the soil profile. This paper provides illustrated examples of soil profiles that have important deep soil characteristics or buried horizons. Soil pits were excavated with a backhoe to at least 2.5 m depth at 36 sites throughout the Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) ecoregion of the Pacific Northwest. These soils cover four orders – Andisol, Inceptisol, Alfisol, and Ultisol – and highlight the hidden diversity of subsoil characteristics throughout the region. The roots of trees and understory species often extended deep into the C horizons of soil. Despite experiencing less pedogenic development than surface horizons, C horizons are important both as the frontier of soil formation and as an important resource for plant growth. In some cases, B horizons are far deeper than the 1 or 2 m depth arbitrarily assumed to represent the whole soil. In other cases, subsoil hides buried profiles and other pedogenic features that can significantly impact aboveground plant growth. These buried horizons are important repositories of nutrients and carbon that are poorly understood and rarely sampled. Ignoring subsoil precludes incorporating soil burial or deep soil processes into biogeochemical and global carbon cycle models.