The Physical Chemistry of Recalcitrance - What is Stable Organic Carbon?

The historic concept of recalcitrance was informed by humus theory, i.e. by the notion that a humification process transforms labile organic biomolecules into thermodynamically "stable" = recalcitrant humic macromolecules. A contrasting view defines recalcitrance not in a thermodynamic sense, but as a physical property: the "molecular- level characteristics of organic substances that influence their degradation by microbes and enzymes". In thermodynamics, stability is expressed as energy content and measured in Joule. A stable bond is a short bond: electrons are on low energy levels close to their nuclei. The resulting molecule has less energy than the individual atoms. Unless energy is supplied to the molecule, it will "stably" remain in its current state. In biogeochemistry, a long tradition refers to carbon that persists for a prolonged time within a biogeochemical system as being "stable". Consequently, biogeochemical stability is expressed as residence time and measured in time units (years). The obvious question here is: Does thermodynamic stability (low energy content) lead to biogeochemical stability (long residence time)? This talk attempts to reconcile the "time-concept" with the "energy-concept" by showing how energy rich "labile" organic molecules can persist for very long times, while relatively energy-poor and "stable" organics may be decomposed very rapidly. In the last years experimental data have become available which show that reduced carbon is never "stable" (over time) in any aerobic environment. It appears that resistance against decomposition or "recalcitrance" can not be parameterized as a constant, measurable and specific property of a given organic compound, but results from a delicate interplay between aqueous solubility, oxygen availability, separation from enzymes, sorptive protection and various other controlling factors. It follows that the concepts of "stable humic substances" or "recalcitrant organic matter" need to be revised before they are incorporated in future models of organic matter turnover.