Changes in morphological and physical characteristics of biochar after long-aging in soil.

There is increasing attention on the potentials of using biochar as a climate mitigation strategy. The mitigation potential of biochar is associated to the fact that carbon is not easily returned back to the atmosphere, even after very long incubation time into the soil. In a recent study, Woolf et al. (2010) quantified the theoretical carbon sequestration potential of biochar following its incorporation into agricultural soils as 1.8 Gt of carbon per year. Nevertheless much remains to be understood on the effective long-term decomposition rate of biochar: is the carbon contained in biochar lasting for decades, centuries or millenia? In this study, fragments of biochar were extracted from the soil of a charcoal burning site in the Eastern Alps (Trentino, Val di Pejo), exactly dated at 1859 by dendroanthracological approach and identified as Larix decidua from the morphological structure. We investigated biochar decomposition in those soils and reliably calculated the fraction of carbon that was lost over 155 years. Then, we focused the morphological and physical characterization of several fragments, using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF). Such study enabled the identification of peculiar morphological features of tracheids, which were tentatively associated to a differential oxidation of the structures that were created during carbonization from lignine and cellulose. In order to assess the effect of soil-aging we compared the old-biochar with a modern one obtained from the same feedstock and with similar carbonization process. XRD and XRF analysis were performed on both old and modern biochar, in order to study the multiphase crystalline structure and chemical elements found. We observed mineralization and a fossilization of old biochar samples respect to the modern ones, with accumulation of several mineral oxides and a substantial presence of quartz. A graphene structure was also found, indicating weak bonds in the carbon structures, explained by inter-molecular Van der Waals forces. Furthermore, we have detected a graphite oxide structure responsible of the bending effect in the tracheid, revealed in SEM images. We consider that those results may contribute to the ongoing debate on the best, most suitable geo-engineering strategies that can potentially enable effective and sustainable carbon sequestration in agricultural soils, using biochar.