Immediate effects of prescribed burning in the Central Pyrenees on the amount and stability of topsoil organic matter

•Prescribed burning is performed in the Pyrenees for pasture management.•We evaluated the immediate impact of prescribed burning on soil in the upper 3 cm.•Most properties were fire-affected in terms of extent of the effect and affected depth.•Prescribed fire had a stronger impact on topsoil properties than usually reported.•The choice of soil sampling depth can be relevant for detecting immediate effects of fire.

Prescribed burning is the deliberate application of fire under selected conditions to accomplish predetermined management objectives. It is generally accepted that controlled use of fire has neutral or even positive effects on soils due to its lower temperature, intensity and severity compared to wildfires. However, very few studies have examined the effects of prescribed burning of shrub vegetation in humid mountain areas on soil properties. The objective of this work was to determine the immediate effects of prescribed burning on the quality and biochemical stability of soil organic matter (SOM) in areas encroached by shrubs in the Central Pyrenees (NE Spain). Soil samples were sampled in triplicate immediately before and after burning from the Ah horizon at 0-1, 1-2 and 2-3 cm depths. We quantified the variations as a direct result of burning in (1) the SOM content, (2) the content and mineralization rates of labile and recalcitrant C pools as inferred from incubation assays (141 days), and (3) the soil biological activity related to C cycling (microbial biomass C and β-D-glucosidase activity). Nearly all the soil properties studied were significantly affected by fire, varying in terms of extent of the effect and the soil depth affected. The total soil organic C (SOC), C/N ratio, β-D-glucosidase activity, C-CO2 efflux and estimated content of labile SOC decreased significantly up to 3 cm depth. The total N and microbial biomass C were significantly affected only in the upper cm of the soil (0-1 cm). These results describe a short-term stronger impact of the prescribed fire on topsoil properties than usually reported. However, comparing these findings to other studies should be performed with caution because of the different environments considered in each case, as well as the differing soil thicknesses found in the literature, typically between 5 and 15 cm, which can lead to a dilution effect associated with the actual impacts of fire on soil properties. In this sense, the choice of a suitable soil thickness or sampling just after burning can be relevant factors in the detection of the immediate effects of fire. Short- and medium-term monitoring of the soils is needed to assess the suitability of this practice for pasture maintenance and for adapting the frequency of prescribed fires in order to minimize its impact on soil.

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