Black carbon contribution in volcanic soils affected by wildfire or stubble burning

Forest wildfire and stubble burning practices in agriculture contribute to the formation of black carbon (BC), a continuum of pyrogenic carbon ranging from slightly charred degradable biomass to highly condensed refractory soot. We examined the BC contribution to Andisol after a wildfire in a pristine Araucaria-Nothofagus spp. temperate rain forest and after 17years of stubble burning on an agricultural soil. We tested the hypothesis that the severity of stubble burning and forest fire affects the quantity and composition of soil organic matter (SOM) and that fire-derived aromatic BC is the main contributor to stabilised SOM in fire-affected soil. BC contribution was analysed as the aromatic fraction of the acid dichromate oxidation residue (CORECarom). The results indicated that the BC content of agricultural soil was unaffected by the stubble burning, whereas in the forest soil, it increased with fire severity from 0.5% at an unburned site to up to 7% in the topsoil severely affected by wildfire. For both ecosystems, the total C stock correlated positively with pyrophosphate extractable Al, whereas a poor or inconsistent relationship was found with BC. We conclude that aromatic BC plays a minor role in C stabilisation in these fire-affected soils due to losses most likely following transport. Aliphatic compounds were less affected by the dichromate oxidation treatment relative to the aromatic compounds than any other functional groups, emphasising the importance of alkyl C for soil C sequestration by virtue of chemical recalcitrance.