Subsurface Carbon Contents: Some Case Studies in Forest Soils

Abstract

Abstract This article evaluates the importance of deeper soil horizons for soil C inventories in forest ecosystems. For non-Spodosols, we categorized soils as to the degree of convexity of the cumulative soil C content profile. Soils with a highly convex or asymptotic soil C content profile contained a significantly lower fraction of their total C (36 ± 8%) below 20 cm than those with less convex (nonasymptotic) profiles (51 ± 2%), even though the more convex soils were 12 cm (23%) deeper. Spodosols contained the most C below 20 cm (66 ± 3%) as a result of the presence of spodic horizons. Spodosols also contained substantially more total soil profile C than non-Spodosols even though the average depths of sampling were similar. Langmuir and logarithmic equations predicted C contents of deeper horizons fairly well for most non-Spodosol soils, whereas C content declines systematically with depth. These equations were very poor for Spodosols, however, because of the increases in soil C with depth that often occur with spodic horizons. Two case studies from the Sierra Nevada mountains suggested that C concentration varies to a greater degree than does bulk density or fine earth (% <2 mm) content, thus illustrating the importance of obtaining good estimates of the large stone content, which can offset differences in C concentration when C content is calculated in the normal fashion (i.e., ignoring the >2-mm fraction). These case studies do not support the idea of estimating bulk density from soil C concentration.