Impact of grassland burning on soil organic matter as revealed by a synchrotron- and pyrolysis-mass spectrometry-based multi-methodological approach.

Although the continuum of pyrolysed OM is an important part of soil OM (SOM) and therefore involved in global cycles, its formation and structural composition are not sufficiently known. Gaps in knowledge originate from methodological difficulties in detecting compounds formed via heating during vegetation burning. We therefore investigated soil samples (0-4 cm depth) from 33 yr treatments of periodic vegetation burning, mulching and natural succession at two grassland sites in Germany. Samples were investigated by way of X-ray absorption near edge structure (XANES) spectroscopy at the carbon (C) and nitrogen (N) K-edges, pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), Py-single photon ionisation-MS (Py-SPI-MS) and Py-field ionisation MS (Py-FIMS). The XANES spectra differentiated the sites and indicated a relative enrichment in unsaturated C for the burned treatment at one site and substituted aromatic C and non-peptide N (nitriles and/or N in aromatic systems) at both sites. All three Py-MS methods distinguished the sampling sites via variation in content of polysaccharides, N-containing compounds and lignin derived substances. Vegetation burning depleted low molecular weight compounds (Py-SPI-MS and Py-FIMS) and formed high molecular substances (Py-SPI-MS and Py-FIMS) and some N-heterocyclic aromatic products (Py-GC-MS), which were only found in the burned samples. Furthermore, Py-FIMS revealed reaction products from m/z 614 to m/z 619 for both burned treatments. In general, all methods indicated the formation of N-heterocyclics and/or more recalcitrant SOM at almost equal total organic C and N content from periodic vegetation burning