Charcoal and stable soil organic matter as indicators of fire frequency, climate and past vegetation in volcanic soils of Mt. Etna, Sicily

Charcoal fragments in soils are useful to reconstruct past vegetation because the level of preservation is often good enough to determine the tree genus. All forest ecosystems have the potential to burn as a result of naturally occurring or human-induced fires. Forest fires are coupled to climate and are a not-negligible factor of pedogenesis in Mediterranean areas, where they occur frequently. Furthermore, soil organic matter (SUM) is prone to undergo peculiar changes due to forest fires, both in terms of quantity and quality. A soil sequence along an elevational gradient ranging from Mediterranean to subalpine climate zones on slopes of Mt. Etna (Sicily, Italy) was investigated in respect of soil organic C and charcoal. The amount of charcoal and the identification of charred species gave an indication of the fire frequency and vegetation changes that have occurred in the past. The distribution into labile and stable organic fractions provided insight into the stabilisation and turnover mechanisms of SOM. The stable organic matter fraction was measured as the residue of a H2O2 treatment. The soils along the altitudinal sequence are variations of Vitric Andosols that developed on a single trachy-basaltic lava flow having an age of 10-15 ky BP. Maquis vegetation dominates at the lower sites of the toposequence, followed by oak- and chestnut-forests at mid elevations, and pine-forest at the highest-elevated sites. Charcoals are older at higher elevations (ages of up to 1.5 ky cal BP). Here, the vegetation type has not changed over the last >1000 years, as all charcoal pieces were identified as Pinus nigra. Charred material at the lower sites could be identified as particles of deciduous shrubs, Quercus, Castanea sativa, Lonicera implexa and Cytisus spp. with mostly a modern age up to about 300 y cal BP. A similar finding was obtained for the stable (H2O2 resistant) SOM. Very high ages for this fraction were found at the highest elevations where it had an age of up to 8.2 ky BP - an age that is close to the start of soil formation. At the lower sites, where frequent bush fires often destroyed a part of the stable fraction, the stable SUM fraction had a maximum age of 1 ky. The studied soils have recorded the signals of the interrelated factors fire frequency, climatic effects and vegetation whose role cannot always be clearly distinguished. With decreasing altitude and with a warmer climate, vegetation changes and fire frequency, org. C and especially nitrogen abundance and the amount of labile SUM increases. At the lower sites, charcoal particles reflect the more recent vegetation probably because the repeated fires here hindered their preservation. Our findings hence suggest that a high fire frequency is a powerful rejuvenating factor for soil organic matter, removing part of the old SUM and promoting plant recolonisation that is a source of young SOM. Fire frequency and intensity on Mt. Etna is, however, moderate enough even at the lowest altitudes for the organic matter pool to be high and not depleted. (C) 2011 Elsevier B.V. All rights reserved.