Carbon sequestration processes in temperate soils with different chemical properties and management histories

A variety of biochemical, physical, and chemical processes protect organic carbon from decomposition in soils; however, the influences of land use and soil properties on the relative importance of these processes are not well known. In this study, amounts of organic C in unprotected, physically protected, and 10 chemically and biochemically protected pools in agricultural and forest soils and soils with different mineralogical composition were investigated using a combination of organic C mineralization and sequential chemical extraction techniques. Results from the mineralization procedure with intact and disrupted aggregates showed that unprotected, physically protected, and biochemically/chemically protected organic C made up 1 to 7%, 0.1 to 2%, and more than 90% of soil organic C, respectively. The most important biochemically and chemically protected organic C pools, as revealed by the sequential extraction procedure, were (i) acid-hydrolyzable organic C, representing fulvic acid, amino sugars, and polysaccharides (34-58% of soil organic C), (ii) pyrophosphate-extractable organic C, representing compounds complexed with Fe and Al cations and oxyhydroxides and mineral surfaces (12-21%), (iii) nonextractable organic C, representing lignin and humin (7-29%), and (iv) base-hydrolyzable organic C, representing humic acids (3-21%). Cultivation of soils significantly decreased the amount of organic C in all pools except for those in the physically protected, humic acid and mineral-intercalated fractions. Results from the study showed that organic C in these soils was protected primarily by a combination of biochemical and chemical processes and that these were dictated by land management practices (organic matter inputs and quality) and soil chemical properties (levels of Fe and Al cations and oxyhydroxides).