Abstract
The emerging view of soil organic matter (SOM) persistence asserts that SOM exists as a continuum of organic material, continuously processed by the decomposer community from large biopolymers to small monomers and with increasing oxidation and solubility, protected from decomposition through mineral aggregation and adsorption. Microbial community and ecosystem dynamics regulate the exchange of both nutrients and carbon between the soil and the atmosphere through the mineralization of SOM. Because these ecosystem dynamics are driven by net energy flows, analysis of SOM bioenergetics can provide complementary constraints to SOM models as well as insight into the fundamental conundrum of why thermodynamically unstable organic matter persists in soil. Microbial substrate preference has been shown to depend on the energy status of the potential substrates in terms of energy required and energy returned. Here we propose a framework for assessing the persistence of SOM utilizing thermally determined activation energy (Ea) and energy density (ED), tested on a suite of soils that have undergone alteration in field or laboratory experiments designed to isolate persistent SOM. Comparison of these energetic parameters in this framework will determine whether a chemical or physical change during SOM decomposition resulted in a change in its environmental persistence. An expanded framework of bioenergetics changes during SOM formation, decomposition, and stabilization is proposed as persistent SOM is characterized by decreased ED and Ea, relative to the bulk SOM.
Highlights
Soil organic matter (SOM) contains more actively cycling carbon than the atmosphere and global terrestrial biomass combined (Jobbágy and Jackson, 2000; Stockmann et al, 2013), and for this reason, even small changes in the size of this carbon pool can have large impacts on the release and storage of atmospheric CO2 and other greenhouse gases (CH4, etc.; Davidson and Janssens, 2006; Bond-Lamberty and Thomson, 2010)
These microbial community dynamics are driven by net energy flows, in which soil heterotrophs oxidize organic matter, acquiring energy that was fixed during photosynthesis, and channeling this energy into storage in the form of tissues or other organic materials, or expending this energy in metabolic processes
Because these ecosystem dynamics are driven by net energy flows, analysis of SOM bioenergetics can provide complementary constraints to SOM models as well as insight into the fundamental conundrum of why thermodynamically unstable organic matter persists in soil
Summary
Soil organic matter (SOM) contains more actively cycling carbon than the atmosphere and global terrestrial biomass combined (Jobbágy and Jackson, 2000; Stockmann et al, 2013), and for this reason, even small changes in the size of this carbon pool can have large impacts on the release and storage of atmospheric CO2 and other greenhouse gases (CH4, etc.; Davidson and Janssens, 2006; Bond-Lamberty and Thomson, 2010). Microbial community and ecosystem dynamics regulate the exchange of both nutrients and carbon between the soil and the atmosphere through the mineralization of SOM (Falkowski et al, 2008). These microbial community dynamics are driven by net energy flows, in which soil heterotrophs oxidize organic matter, acquiring energy that was fixed during photosynthesis, and channeling this energy into storage in the form of tissues (i.e., microbial biomass) or other organic materials, or expending this energy in metabolic processes (i.e., respiration; Odum et al, 1962; Currie, 2003). The particulate SOM fractions were characterized by larger energy densities and activation energies, whereas the mineral-associated SOM fractions were characterized by smaller energy densities and activation energies
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