Organic Carbon as a Major Differentiation Criterion in Soil Classification Systems

Abstract

Soil is the largest terrestrial pool of organic carbon (OC), however the spatial distribution of the estimated 1.5–2.3 Tt organic carbon content of the soil cover has great variation horizontally and vertically. As organic carbon is one of the most important soil constituents, governing important functions and properties of soils, it is one of the major differentiation criteria in internationally used soil classification systems, such as Soil Taxonomy (ST) and the World reference base for soil resources (WRB), and most national systems. Several soil units at the highest level of the systems, such as surface and subsurface horizons, modifiers and qualifiers are defined on the basis of presence and amount of OC and/or depth and thickness of OC rich soil layers. As a result of the evaluation of current definitions of ST and the WRB, and analyses of international soil data sets (US NASIS and the ISRIC WISE) it was concluded that there are sufficient number of categories for expression of the amount, kind and vertical distribution of OC, however several of the definitions are very complicated, and not always consistent throughout the systems. This makes interpretation and also computation of categories difficult. With globally available data and mathematical tools, there is an opportunity to improve definitions and adjust arbitrarily set limits. An improved set of organic carbon related diagnostics could serve the needs of a future global classification system, mapping and monitoring OC, and also modeling of related estimations and processes.