Abundance and composition of free and aggregate-occluded carbohydrates and lignin in two forest soils as affected by wildfires of different severity

Organic matter is the soil component most affected by wildfires, both in terms of abundance and composition. Fire-induced alteration of soil organic matter (SOM) depends on heating intensity and duration, oxygen availability and other factors related to topography, climate, soil and vegetation features. Particularly affected by fire is the litter layer, but SOM from the uppermost mineral soil can also experience some major changes. In this study, we investigated the direct impact of fire on molecular SOM parameters in density fractions isolated from the top 2.5 cm of mineral soil in two forests that recently experienced wildfires of different severity. One, located in Tuscany, Central Italy, is a mixed forest of Downy oak and Maritime pine, developed on Acrisols formed on sandy lacustrine deposits, affected by a moderately severe fire. The other, located in Victoria, South-East Australia, is a mixed-species eucalypt forest, developed on a Cambisol formed on sandy Devonian sediments, affected by an extremely severe fire (the infamous 'Black Saturday' fire). The purpose of this study was the assessment of fire-induced changes on amount and composition of the bulk SOM and SOM associated to soil fractions having different densities. We used 1.8 Mg m(-3) as density cut-off and distinguished between free and aggregate-occluded SOM. In particular, the analyses focused on abundance and composition of two major SOM components, proposed as molecular indicators of fire severity: the non-cellulosic neutral sugars, digested by trifluoroacetic acid (TFA), and the lignin-derived phenolic monomers, released by cupric oxide (CuO) oxidation. The chemical structure of both bulk SOM and SOM fractions were analysed by solid-state C-13 nuclear magnetic resonance spectroscopy. In contrast to the moderately severe fire affecting the Italian site, the extremely severe fire at the Australian site caused substantial loss of SOM from the top mineral soil. Both fires had major effects on SOM composition. In spite of the evident impact they experienced, neither hydrolysable sugars nor lignin phenols resulted to be reliable indicators of fire severity. Moreover, both fires apparently broke up soil aggregates, hence promoting the release of some occluded organic matter. The fire-induced changes of SOM observed have implications for the C cycle, so highlighting the critical role of wildfire occurrence and severity in climate change.