Oxidation products of betulin: New tracers of abiotic degradation of higher plant material in the environment

In order to fill the need for specific and stable tracers for monitoring the degradative state of particulate organic matter (OM), betulin and its degradation products were selected to trace biotic and abiotic degradation processes affecting terrestrial higher plant-derived OM in riverine environments. Samples of Quercus ilex leaves and suspended particulate matter were collected from the Marseille Luminy area and the Rhône River, respectively, and analyzed in order to validate the tracer potential of betulin oxidation products identified during in vitro simulations. Three degradation products were selected as tracers: lup-20(30)-ene-3β,28,29-triol, lupan-20-one-3β,28-diol and the 20R and 20S epimers of 3β,28-dihydroxy-lupan-29-oic acid. They were deemed sufficiently stable for tracing the different degradative processes in aquatic systems affecting OM of terrestrial higher plant origin. All were found in riverine suspended particulate matter (SPM), evidencing the advanced degradation state of riverine particulate OM (POM), as well as the importance of autoxidation in the degradation. Lup-20(30)-ene-3β,28,29-triol and lupan-20-one-3β,28-diol were also found in senescent leaves of Q. ilex, attesting to the involvement of photo- and autoxidation in the degradation of plant leaves. Alongside existing tracers, these compounds provide a better insight into the degradation state of riverine OM, as well as into the degradative processes at play, a knowledge that will be a necessary basis for further studies of the degradative state of particulate marine OM and sediments.