On the rocks: Quantifying storage of inorganic soil carbon on gravels and determining pedon-scale variability

• Published in: Catena (Giessen) 2017-10, Vol.157, p.436-442
• Main Authors: Stanbery, Christopher A., Pierce, Jennifer L., Benner, Shawn G., Lohse, Kathleen
• Format: Article
• Language: English
• Subjects: Carbon storage; Gravel; Semi-arid; Soil inorganic carbon
• ISSN: 0341-8162; 1872-6887
• DOI: 10.1016/j.catena.2017.06.011

The storage and flux of carbon from soils, the planet's third largest carbon pool, strongly influence the global carbon cycle and are essential, but poorly constrained, parameters for global climate models. An estimated 40% of all soil carbon is stored as inorganic carbonate minerals. Despite a recognition of the importance of soil inorganic carbon (SIC) in soil carbon storage, few studies have quantified pedon-scale variability in SIC storage. We examine different stages of carbonate development and accumulation rates between gravelly and non-gravelly soils. Studies often ignore carbonate coatings on gravels in measurements of soil inorganic carbon (SIC). By quantifying and differentiating the fine (2mm) fractions of SIC in the Reynolds Creek Experimental Watershed in southwestern Idaho, we show that gravel coatings contain up to 44% of total SIC at a given site. Among the 26 soil sites examined throughout the watershed, an average of 13% of the total SIC is stored as carbonate coatings within in the gravel fraction. We measured a high level of pedon-scale field variability (up to 220%) among the three sampled faces of 1m3 soil pits. Analytical error associated with the modified pressure calcimeter (0.001–0.014%) is considerably less than naturally occurring heterogeneities in SIC within the soil profile. This work highlights and quantifies two sources of uncertainty in studies of SIC needed to inform future research. First, in gravelly sites, the >2mm portion of soils may store a large percentage of SIC. Second, SIC varies considerably at the pedon-scale, so studies attempting to quantify carbon storage over landscape scales need to consider this variability. •Soil inorganic carbon (SIC) is an understudied but significant portion of the carbon pool in arid and semi-arid regions.•Gravels contain a significant proportion of the SIC pool and should not be ignored in future work.•There are significant heterogeneities in SIC accumulation at the pedon scale.