Deriving a proxie for soil carbon turnover using field descriptions of soil development

Pedogenesis is a first-order control over C storage and stabilization over millennial timescales through soil mineralogy such as sesquioxide accumulation and clay formation, horizon development, phase changes, hydrological controls over water storage and leaching, landscape processes, and biological responses. Using data from a variety of soil mineralogical assemblages from soil chronosequence studies, a soil development index was established from field descriptions of soil color, consistence, aggregation, texture, and pH. Chemical extractions and particle size data were also used to quantify soil development. Meanwhile soil organic carbon was fractionated into free light and heavy (specific gravity >1.8) fractions and fractions were analyzed for radiocarbon. Rubification, or the increase in hue and chroma relative to parent material were significantly correlated with dithionite extractble Fe oxides (p values <0.01) with steeper slopes in wetter climatic zones. Bulk soil radiocarbon (FM units) decreased as a function of Dithionite Fe, with shallower slopes in wetter climatic zones, reflecting a decrease in turnover with soil development. Bulk soil radiocarbon increased with increasing root abundance, reflecting higher post-bomb enrichment and retention in denser rooting zones. Last, the free light fraction was correlated with the field-based root index (p value < 0.002) and the heavy fraction was correlated with the field-based rubification index (p value <0.01). Thus, C turnover derived from fractionated SOC can be estimated from field descriptions.