The Impact of Buried Horizons and Deep Soil Pedogenesis on Soil Carbon Content and Vertical Distribution

Abstract:

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 35 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 as the frontier of soil formation, as an important resource for plant growth, and as a repository of diffuse but significant carbon storage. On average, there was 188.1 Mg C ha^-1 total across all 35 sites, of which 76.3 Mg ha^-1 (40.5%) was found below 0.5 m and 44.4 Mg ha^-1 (23.6%) was found below 1 m. There was substantial variability in the vertical distribution of C with as little as 8.0% and as much as 58.0% of total C below 1 m. 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 that can significantly impact total soil carbon stocks as well as 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, and limits mechanistic understanding of carbon sequestration and mobilization in soil.