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

Abstract

Abstract. In this work, we assess whether or not ramped thermal oxidation coupled with determination of the radiocarbon content of the evolved CO2 can be used to isolate distinct thermal fractions of soil organic matter (SOM) along with direct information on the turnover rate of each thermal fraction. Using a 30-year time series of soil samples from a well-characterized agronomic trial, we found that the incorporation of the bomb spike in atmospheric 14CO2 into thermal fractions of increasing resistance to thermal decomposition could be successfully modeled. With increasing temperature, which is proportional to activation energy, the mean residence time of the thermal fractions increased from 10 to 400 years. Importantly, the first four of five thermal fractions appeared to be a mixture of fast- and increasingly slower-cycling SOM. To further understand the composition of different thermal fractions, stepped pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS) experiments were performed at five temperatures ranging from 330 to 735 ∘C. The Py-GC/MS data showed a reproducible shift in the chemistry of pyrolysis products across the temperature gradient trending from polysaccharides and lipids at low temperature to lignin- and microbe-derived compounds at middle temperatures to aromatic and unknown compounds at the highest temperatures. Integrating the 14C and Py-GC/MS data suggests the organic compounds, with the exception of aromatic moieties likely derived from wildfire, with centennial residence times are not more complex but may be protected from pyrolysis, and likely also from biological mineralization, by interactions with mineral surfaces.

Direct evidence for the role of microbial community composition in the formation of soil organic matter composition and persistence

“…1A-C). This is consistent with the theory stipulating that high growth efficiency is ultimately associated with greater soil carbon retention [12][13][14]. Communities depleted of fungi were characterized by low growth efficiency and low biomass in these soils, and also had some of the most thermally labile SOM (Fig.…”

Evidence linking calcium to increased organo-mineral association in soils

“…The thermal stability of OM has also been proposed as a proxy for SOC dynamics (Plante et al 2009 ; Sanderman and Grandy 2020 ). Among thermal analysis techniques, Rock-Eval® analysis can provide insights into OM quality changes (Disnar et al 2003 ; Matteodo et al 2018 ; Poeplau et al 2017 ).…”

Limited Presence of Permafrost Dissolved Organic Matter in the Kolyma River, Siberia Revealed by Ramped Oxidation

“…Thus, CO 2 generated across these temperature intervals relates to its thermostability, a proxy for biological reactivity and the theoretical barrier to energy acquisition (i.e., activation energy [ E ]). In particular, thermal parameters have shown to be a useful measure of aromaticity, where low H/C compounds are preferentially found at higher temperature values (De la Rosa et al., 2008, 2018; Peltre et al., 2013; Sanderman & Grandy, 2020). Microbes degrade and uptake a range of DOM, though relatively high H/C material is thought to be more biolabile (D'Andrilli et al., 2015; Šantl‐Temkiv et al., 2013; Spencer et al., 2015), and biolability has been shown to positively correlate with the abundance of aliphatic compounds (Textor et al., 2019).…”