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.
