Distinct bioenergetic signatures in particulate versus mineral-associated soil organic matter

Physical and chemical stabilization, environmental conditions, and organic matter composition all play vital roles in determining the persistence of soil organic matter (SOM). Fundamentally, SOM stability depends on the balance of microbial bioenergetics between the input of energy needed to decompose it (i.e., activation energy; Ea) and the net energy gained (i.e., energy density; ED) from its decomposition. This relationship is complicated in soils by chemical and physical protection mechanisms, which require additional energies to overcome for decomposition to occur. In this study, we analyze the bioenergetics of soil density fractions, which vary in their degrees of organic matter-mineral association, and show that the relationship of ED and Ea has the ability to provide information about relative differences in SOM chemical composition and stability. Our results demonstrate distinct bioenergetic signatures between particulate, light (free and occluded) fractions versus mineral-associated, heavy fractions isolated from soil samples collected at two depths from a climosequence along an elevation gradient in the Sierra Nevada, California. While there were no significant differences in ED and Ea within each fraction across climates, the light fractions (LF) were characterized by larger ED and Ea values, whereas the heavy fractions (HF) were characterized by smaller ED and Ea values. Combined with CHN analyses, we conclude that SOM in HF pools is likely comprised of relatively simple organic compounds that have long turnover rates because of chemical association with soil minerals, whereas the LF pools are comprised of more chemically complex molecules with low chemical reactivity and high Ea.