AMS ¹⁴C dating with biochemical characterization by pyrolysis-combustion and infrared technology reveal terrestrial organic carbon cycling dynamics

Terrestrial systems hold twice as much organic carbon (OC) as the atmosphere and vegetation combined. The long term stability of this carbon pool influences ecological resilience and helps mitigate global climate change. Here, we couple an updated pyrolysis combustion system with online infrared gas analyzer (IRGA) and Fourier transform infrared spectroscopy (FTIR) to partition OC into thermally labile, recalcitrant, and stable fractions, enabling the analysis of their biochemical structures, origins, and AMS 14C ages. We find that the labile recalcitrant and stable OC fractions from fluvial, eolian, and farmland ecosystems consistently exhibit younger to-older age patterns over timescales ranging from 102 to 104 year. These trends exhibit a first order reaction rate temperature dependance as predicted by Arrhenius equation. This pattern verifies selective preservation of OC compounds by minerals under microbial respiration. In contrast, labile recalcitrant and stable OC fractions from rock varnish and flooding sediments in karst cracks and sinkholes exhibit reverse (older younger) or identical age patterns, confirming protection of OC by minerals in the absence of intensive respiration. In high organic carbon stock ecosystems, thermally labile recalcitrant and stable OC fractions yield identical modern ages in the rhizosphere and younger to-older age patterns in deeper zones, quantitatively exemplifying the process by which unprotected OC progressively attaches to mineral associated forms. The CO2 thermograms from IRGA and the OC functional groups from FTIR analysis provide biochemical evidence for assessing the origin of terrestrial OC compounds, thereby improving our understanding of their age patterns.