A Bioenergetic Framework for Assessing Soil Organic Matter Persistence

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 (E a ) 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 E a , relative to the bulk SOM.

Ramped thermal analysis for isolating biologically meaningful soil organic matter fractions with distinct residence times

“…We determined the relative percentages and ratios of chemical classes using pyrolysis-mass spectrometry-gas chromatography (Py-GC/MS) using methods described previously (Grandy et al, 2009;Kallenbach et al, 2015;Wickings et al, 2011). However, in contrast to our previous studies in which we used a single pyrolysis temperature, here we used a "ramp" or stepped approach by pyrolyzing the same sam-ple at five distinct, sequentially increasing temperatures: 330, 396, 444, 503 and 735 • C (Buurman et al, 2007;Hempfling and Schulten, 1990;Williams et al, 2014). Thus, the same sample was pyrolyzed five times, corresponding with each of these temperatures.…”

Spatial Control of Carbon Dynamics in Soil by Microbial Decomposer Communities

“…Microorganisms drive biochemical processes such as C cycling in soil (Falkowski et al, 2008). There is growing consensus that soil organic matter dynamics and stability are strongly controlled by microbial processing and associated bioenergetics constraints (Schmidt et al, 2011;Lehmann and Kleber, 2015;Williams and Plante, 2018). Yet, understanding how microbial community characteristics affect rates of biogeochemical processes remains a major research challenge.…”

Mineral and Climatic Controls Over Soil Organic Matter Stability Across the Tibetan Alpine Permafrost Region

“…Thermal analysis has been widely used to represent SOM stability due to less sample consumption but high accuracy, even with low C content (Kucerik et al., 2018; Marin‐Spiotta et al., 2014; Williams & Plante, 2018; Williams et al., 2018). In this study, thermal analysis was performed to characterize SOM stability across the Tibetan alpine permafrost region using a simultaneous thermal analyzer (449F5, Netzsch, Germany), with the medium as Ultra‐Zero air (CO 2 free, 21% O 2 ).…”

“…In both cases, the higher TG‐T 50 or DSC‐T 50 indicates the stronger SOM stability (Marin‐Spiotta et al., 2014; Merino et al., 2018). The temperature ranges of TG‐T 50 and DSC‐T 50 calculation were both from 190 to 600°C, corresponding to the exothermic range of SOM combustion (Plante et al., 2011; Williams & Plante, 2018; Williams et al., 2018). Although this temperature range could lead to the mass loss of kaolinite (Plante et al., 2011; Williams et al., 2018), this effect should be limited in our study area since the predominant clay minerals belong to illite and chlorite on the Tibetan Plateau (Chen et al., 1981).…”