Fire effects on biochemical properties of a semiarid pine forest topsoil at cm-scale

Forest fires can greatly affect soil properties and processes. In the study of the fire effects on soil, the soil thickness affected by heat depends on the characteristics of the fire and soil itself, but also on the attribute to be measured. The objective of this work is to know to what thickness (up to 1, 2 or 3 cm) various sensitive soil properties are immediately affected by a controlled burning. To achieve this aim, unaltered fresh topsoil (mollic horizon) of a fire-prone Aleppo pine forest in the semiarid Ebro Valley (NE-Spain) were sampled and, without destroying their original structure, burned from the surface in an outdoor combustion tunnel in triplicate. Biological properties are measured, including basal and normalized soil respiration (bSR and nSR), β-D-glucosidase (GLUase) and phosphomonoestarase (PHOase) activities, and related parameters, such as total organic matter (TOM), oxidizable organic C (OxC), nonhydrolyzable carbon (NHC), P-Olsen, pH, soil moisture and soil water repellency (WR). In the unburned soil, most of these properties showed a decreasing gradient with depth which is modified after burning, in some cases inverted (as enzymatic activities and WR), in others intensified (P-Olsen) and in most, truncated, with a maximum value in the second cm. The depth of the soil in which changes were recorded varied according to the attibute considered; thus, burning significantly decreased only up to the first cm: bSR (73 %) and TOM (81 %), up to 2 cm: PHOase (89 %), OxC (17 %) and WR (96 %) and up to 3 cm depth GLUase (58 %), NHC (24 %) and moisture (73 %). However, P-Olsen and pH both increased after burning up to 1 and 3 cm soil depths, with increases of up to 240 % and 11 %, respectively. In conclusion, fire effects on soil are depth dependent, and this dependency is not uniform across soil properties.