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.
Highlights
Soil organic matter (SOM) consists of a spectrum of material from labile, rapidly cycling compounds to mineral-stabilized molecules that resist degradation for centuries
A greater incorporation of the bomb spike in 14C was found in the lowertemperature fractions, with no modern carbon seen in the highest-temperature fraction (Fig. 2)
The single-pool steady-state model to trace the incorporation of the bomb spike in atmospheric 14CO2 into the thermal fractions indicated that turnover times increased from 37 to 386 years with increasing temperature (Table 2)
Summary
Soil organic matter (SOM) consists of a spectrum of material from labile, rapidly cycling compounds to mineral-stabilized molecules that resist degradation for centuries. This spectrum of turnover rates is due to a combination of organic matter composition with varying reactivity, various degrees of interaction between organic and mineral phases and greatly varying microclimates more or less suited to microbial activity (Lehmann and Kleber, 2015; Ruamps et al, 2013; Schmidt et al, 2011). Fractionation schemes – employing various physical, chemical, biological or thermal methods – are generally used to reduce the inherent complexity found in SOM.
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