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Abstract:
Over timescales of soil development (millennia), the capacity of soils to stabilize soil organic carbon (SOC) is
linked to soil development through changes in soilmineralogy and other soil properties. In this study, an extensive
dataset of soil profile chemistry and mineralogy is compiled from the Cowlitz River Chronosequence (CRC),
WA USA. The CRC soils range in age from 0.25 to 1200 kyr, spanning a developmental gradient encompassing
clear changes in soil mineralogy, chemistry, and surface area. Comparison of these and other metrics of soil development
with SOC properties reveal several relationships that may be diagnostic of the long-term coupling
of soil development and C cycling. Specifically, SOC content was significantly correlated with sodium pyrophosphate
extractable metals emphasizing the relevance of organo-metal complexes in volcanic soils. The depth distributions
of organo-metals and other secondary weathering products, including the kaolin and short-range
order (SRO) minerals, support the so-called “binary composition” of volcanic soils. The formation of organometal
complexes limits the accumulation of secondary minerals in shallow soils, whereas in deep soils with
lower SOC content, secondary minerals accumulate. In the CRC soils, secondary minerals formed in deep soils
(below 50 cm) including smectite, allophane, Fe-oxides and dominated by the kaolin mineral halloysite. The
abundance of halloysite was significantly correlated with bulk soil surface area and 14C content (a proxy for
themean age of SOC), implying enhanced stability of C in deep soils. Allophane, an SRO mineral commonly associated
with SOC storage, was not correlated with SOC content or 14C values in CRC soils. We propose conceptual
framework to describe these observations based on a general understanding of pedogenesis in volcanic soils,
where SOC cycling is coupled with soil development through the formation of and fate of organo-metal or
other mobile weathering products. This framework highlights interactions between SOC and soil development,
which may be applicable to other soils where organic inputs interact with the products of chemical weathering.