Nature and reactivity of charcoal produced and added to soil during wildfire are particle-size dependent

Charcoal added to soil by wildfires is usually considered to be part of the most stable pool of soil organic matter (SOM). It consists of a continuum of slightly burned plant residues to completely charred material. We sampled the macroscopic charcoal pool produced by a moderate intensity wildfire in a pine coastal forest in Tuscany (Central Italy) with the aim of assessing the relationship between its particle size and chemical composition as well as its reactivity. Our conceptual approach included particle size separation by dry sieving and analysis of four size fractions (>2 mm, 2-1 mm, 1-0.5 mm and <0.5 mm) for elemental composition. The composition of the charcoal fractions was studied using solid-state (13)C nuclear magnetic resonance (NMR) spectroscopy, diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy, analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and CuO oxidation. The reactivity of the fractions was assessed using acid dichromate oxidation, while the morphology was observed using scanning electron microscopy (SEM). There were clear differences in chemistry and reactivity between the fractions. The different fractions are probably the result of both type of original plant material and the fire conditions. In fact, wood-derived charcoal largely prevalent in the largest fraction, while pine needles and herbs were most probably precursors of charcoal in the smallest fractions. The degree of charring was greatest for the coarse fraction, which represented 56% of the total charcoal, and decreased gradually to the finest fraction. The composition was completely aromatic for the coarsest fraction, whereas the finest fraction showed a greater contribution from aliphatic C- and N-containing components. The greatest reactivity was found for the smallest fraction, which represented 24% of the total charcoal. It is particularly rich in N and potentially susceptible to microbial decomposition. The study demonstrates that properties of charcoal added to soil during wildfires may be reliably discriminated on the basis of particle size.