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

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 (< 2 mm) and coarse (> 2 mm) 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 1 m3 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 > 2 mm 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.