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 degrees C) by gasification (BCGS) and measured the C-13 abundance of density and particle size fractions. We quantified the BCGS effects on distinct native C-3-SOM pools of a grassland topsoil one year after BCGS amendment. The chemical composition was analyzed with solid-state C-13 CPMAS NMR, whereas information on the nanostructure of BCGS were obtained by Raman microspectroscopy measurements. Our aim was to assess BCGS induced chemical changes of SOM and physical fractions and to validate the accuracy of BCGS detection by C-13 NMR spectroscopy. Quantification by isotopic measurements and C-13 NMR spectroscopy for aromatic C yielded similar estimates of BC in soils. Of the total BCGS, 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 BCGS was detected in the clay fraction. After one year of field exposure, the surface of the BCGS 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 BCGS amendment. With respect to soil functionality, the BCGS 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. (C) 2017 Elsevier Ltd. All rights reserved