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Samples from a colluvial soil rich in pyrogenic material (black C, BC) in north-west Spain were subjected to K2Cr2O7 and KMnO4 oxidation and the residual soil organic matter (SOM) was NaOH-extracted and analysed using analytical pyrolysis–gas chromatography–mass spectroscopy (Py-GC/MS) and solid-state 13C cross-polarisation magic angle spinning–nuclear magnetic resonance (13C CP MAS-NMR) in order to study the susceptibility of different SOM fractions (fresh, degraded/microbial, BC and aliphatic) towards these oxidising agents. Untreated samples that were NaOH-extracted were also analysed. The Py-GC/MS and 13C NMR indicated that KMnO4 promotes the oxidation of carbohydrate products, mostly from degraded/microbial SOM and lignocellulose, causing a relative enrichment of aliphatic and aromatic structures. Residual SOM after K2Cr2O7 oxidation contained BC, N-containing BC and aliphatic structures. This was corroborated by a relatively intense resonance of aromatic C and some signal of alkyl C in 13C NMR spectra. These results confirm that dichromate oxidation residues contain a non-pyrogenic fraction mainly consisting of aliphatic structures.

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Aim To investigate how the chemical composition of native organic matter of two contrasting soils varies with inputs of biochar and fresh material (including plant roots) and how these underlying changes influence microbial community structure. Methods Corn stover (CS) and CS-derived biochars produced at 350°C and 550°C were applied at a dose of 7.2 t C ha −1 to two contrasting soils-an Alfisol and an Andisol. After 295 days of incubation, two undisturbed subsamples from each pot were taken: (i) in one, lucerne (Medicago sativa L.) was seeded (plant study, P) and (ii) in the other, the incubation was continued without the plants (respiration study, R); all subsamples were incubated for an additional 215 days. Soils without amendments were used as controls. At the end of the incubation (510 days), their bacterial community profiles were characterised using ARISA and the molecular composition of soil organic matter (SOM) was investigated by pyrolysis-GC/MS. Results There were significant interactions between soil type, study type (P or R) and organic amendment. Organic amendments influenced overall SOM composition with microbial community response being mainly influenced by soil type but also strongly affected by the presence or absence of plants. For a specific soil type, ≥ 40 % of total variation in bacterial community ordination could be explained by the molecular composition of SOM. Conclusions The molecular composition of SOM is proposed as an important factor influencing the microbial response to organic amendments, including biochar.

The mechanisms by which lime and/or phosphate addition impacts the preservation of soil organic matter (OM) are poorly understood. We explored the changes in quantity and chemistry of water-extractable organic matter (WEOM) in the bulk soil and its heavy density fraction (>1.6 g/cm3) of an unmanaged C-rich volcanic soil caused by lime and/or phosphate application. The addition of lime or phosphate caused (i) a significant increase in the WEOM, along with a decrease in its C/N ratio and an increase in its aromaticity, and (ii) changes in the WEOM chemical composition, measured with pyrolysis-gas chromatography/mass spectrometry, this being most impacted by lime application. The combined effect of lime and phosphate addition on the quantity and chemistry of WEOM was larger than the effects of separate lime and phosphate additions. By comparing the response of the bulk soil and the heavy fraction, we infer that phosphate has a greater contribution to the destabilization of vulnerable particulate OM, while lime causes a comparable disruption in the particulate OM and that in the heavy fraction. These findings provide a mechanistic insight into the decreased OM stability after liming and/or P fertilizing Andosols. They have implications for designing climate-smart management practices for these soils.