Discrimination in Degradability of Soil Pyrogenic Organic Matter Follows a Return-On-Energy-Investment Principle

Abstract

A fundamental understanding of biodegradability is central to elucidating the role(s) of pyrogenic organic matter (PyOM) in biogeochemical cycles. Since microbial community and ecosystem dynamics are driven by net energy flows, then a quantitative assessment of energy value versus energy requirement for oxidation of PyOM should yield important insights into their biodegradability. We used bomb calorimetry, stepwise isothermal thermogravimetric analysis (isoTGA), and 5-year in situ bidegradation data to develop energy-biodegradability relationships for a suite of plant- and manure-derived PyOM (n = 10). The net energy value (ΔE) for PyOM was between 4.0 and 175 kJ mol(-1); with manure-derived PyOM having the highest ΔE. Thermal-oxidation activation energy (Ea) requirements ranged from 51 to 125 kJ mol(-1), with wood-derived PyOM having the highest Ea requirements. We propose a return-on-investment (ROI) parameter (ΔE/Ea) for differentiating short-to-medium term biodegradability of PyOM and deciphering if biodegradation will most likely proceed via cometabolism (ROI < 1) or direct metabolism (ROI ≥ 1). The ROI-biodegradability relationship was sigmoidal with higher biodegradability associated with PyOM of higher ROI; indicating that microbes exhibit a higher preference for "high investment value" PyOM.

Effects of 7 years of field weathering on biochar recalcitrance and solubility

“…All biochars in this study were found to have an "intermediate" C sequestration potential (Class B), between that of soot/graphite and uncharred plant biomass, regardless of weathering extent. These R 50 values combined with relatively low ROI values [compared to those studied by Harvey et al (2016)] indicate that the microbial decomposition of the biochars in this study would be dominated by co-metabolism. Co-metabolism is the simultaneous degradation of two substrates, in which the degradation of the secondary substrate, in this case the biochar, depends on the presence of a primary substrate (glucose, dissolved organic matter, etc.…”

“…), and is also known as the priming effect (Luo et al 2011;Zimmerman and Ouyang 2019). The ROI values associated with our biochar suggests relatively low biodegradability on "short to medium time scales" according to Harvey et al (2016). Assuming steady-state average losses of 0.9-1.2% biochar C year −1 suggests a relatively high turnover rate and low biochar residence time of 83-111 years for the pyrolysis temperature and feedstock (450 °C, mixed hardwood).…”

“…Activation energy (E a , in kJ mol −1 CO 2 ) was determined following the methods of Williams et al (2014), assuming first-order reaction kinetics during ramped combustion, and described in further detail in Williams et al (2018). The return-onenergy-investment (ROI) ratio (Harvey et al 2016) which has been demonstrated to be directly related to the biodegradability of pyrogenic organic matter was calculated for each biochar sample by dividing ED by E a . Greater ROI values correspond to greater biodegradability of pyrogenic materials.…”

“…The relatively small amounts of biochar C loss attributed to solubilization and mineralization over the 7 years of weathering, to which we attribute to the labile PyOM pool, suggest that this biochar has potential for soil C sequestration. Chemical recalcitrance (degree of condensation of the PyOM molecular structure), interactions with the soil mineral matrix, and other environmental conditions are expected to contribute to the stability of PyOM in soils (Bird et al 2015;Bruun et al 2008;Harvey et al 2016;Jindo and Sonoki 2019;Singh et al 2012;Ventura et al 2019;Zimmerman 2010). Strong inverse relationships between microbial mineralization of biochar (after 1 year) and the recalcitrance index, R 50 (which compares the thermal stability of biochar to that of graphite), were observed by Harvey et al (2012) for woody and grass biochars.…”

Editorial: Environmental Bioenergetics

“…Furthermore, Williams and Plante propose a bioenergetics framework for assessing the persistence of organic matter in soil systems. The framework is based on a return-on-investment ratio, the ratio between energy invested in relation to energy density (Harvey et al, 2016). Their framework contradicts traditional humus theory that organic matter is composed of inherently stable macromolecules; instead it supports the idea that organic matter is a continuum of progressively decomposing organic compounds (Lehmann and Kleber, 2015).…”