A simplified method for the quantification of pyrogenic organic matter in grassland soils via chemical oxidation

The elucidation of the impact of pyrogenic organic carbon (PyOC) on soil ecological functions and its role within geobiochemical cycles requires a quantification that is not restricted to a certain window of the Black Carbon continuum. In the present study the method of chemical oxidation with acid dichromate was modified to allow for the determination of the whole range of PyOC in soil systems frequently subjected to prescribed fires with a fairly homogenous intensity distribution. After confirming the reproducibility of the technique, top and subsoils of Leptosols and Umbrisols from a grassland (Campo) landscape in the Planalto region, Southern Brazil were demineralized with hydrofluoric acid and subsequently the residues were chemically oxidized. Solid-state 13C NMR spectroscopy was applied to distinguish between chemical oxidation resistant elemental C (COREC) of pyrogenic organic carbon (PyOC) and lipid components surviving the harsh treatment because of their hydrophobic properties. Correlation of the aromatic COREC content of the samples with their aromatic C content resulted in a positive correlation with R2 = 0.74 for the topsoils and R2 = 0.53 for the subsoils. The correlation functions allowed the development of an equation that yields in a correction factor f, which is necessary to account for the C-losses of PyOC caused by the chemical oxidation and thus for the estimation of its original contribution to the soil. For the topsoils this correction factor was comparable to that determined for a model char derived from the Campo vegetation after charring for 4 min at 350 °C. Applying this factor to Campo plant-char/soil mixtures resulted in recoveries between 93 and 125%, indicating that it should be possible to elucidate the PyOC content of soils by the determination of CORECarom and subsequent multiplication with f. In soil systems with char input of fairly consistent chemical composition, as they occur in prescribed grassland fires, f may be directly obtained from model chars produced from the respective vegetation cover.