A multi-technique approach to assess the fate of biochar in soil and to quantify its effect on soil organic matter composition.

Differentiation of biochar and native soil organic matter (SOM) is required to assess the effect of biochar amendment on in-situ changes of SOM. Therefore, we used C 4 biochar produced at high temperature (1200 °C) by gasification (BC GS ) and measured the 13 C abundance of density and particle size fractions. We quantified the BC GS effects on distinct native C 3 -SOM pools of a grassland topsoil one year after BC GS amendment. The chemical composition was analyzed with solid-state 13 C CPMAS NMR, whereas information on the nanostructure of BC GS were obtained by Raman microspectroscopy measurements. Our aim was to assess BC GS induced chemical changes of SOM and physical fractions and to validate the accuracy of BC GS detection by 13 C NMR spectroscopy. Quantification by isotopic measurements and 13 C NMR spectroscopy for aromatic C yielded similar estimates of BC in soils. Of the total BC GS , 52% were recovered as free particulate organic matter (POM) and 33% were located in aggregated soil structures isolated as occluded POM particles. Around 4% of the total BC GS was detected in the clay fraction. After one year of field exposure, the surface of the BC GS particles decreased in unordered graphitic-like structures. The higher ordered BC residue is supposed to be more recalcitrant. The native SOC stock increase ( p = 0.06, n = 4) in the clay fraction indicated increased sequestration of organic matter as mineral-bound SOM due to BC GS amendment. With respect to soil functionality, the BC GS amendment induced a tremendous shift from a soil system dominated by organo-mineral associations to POM−dominated OC storage, resulting in increased soil air capacity. [ABSTRACT FROM AUTHOR]