Effect of fire and retardant on soil microbial activity and functional diversity in a Mediterranean pasture

The effect of fire and retardants on functional diversity and total activity of soil microorganisms has been studied in a Mediterranean pasture. Soil samples were collected once a month during 1 year after the fire in burnt and un-burnt plots and in one treated with retardant and subsequently burnt. In all cases, samples were taken at two soil depths. Functional diversity was measured by Biolog Eco plates and microbial activity was measured by radioactive labelled thymidine and leucine incorporation technique. In the upper layer, a higher metabolic speed (kinetics of average well colour development (AWCD)) and functional diversity (Shannon's functional diversity) were found irrespective of the treatment (fire or retardant), despite the short distance between them. Results also showed that environmental factors have a greater influence than the treatments (fire and retardant), especially soil moisture. This fact was clearly shown in the seasonal variation of microbial communities throughout the year after the fire, showing a similar successional behaviour irrespective of the treatment. Nevertheless, the techniques used were able to detect changes caused by the fire and, especially, by retardant which is very interesting in public management of fire. Changes caused by the application of retardant to the soils, did not lead to any major adverse effects on the microbial community as compared to the untreated soils.Based on the results, we could hypothesise that bacterial communities studied show a noteworthy grade of fire-accommodation, which can be correlated with the inherent presence of fire in the Mediterranean ecosystems. In particular, we can assume an important resilience and resistance of Mediterranean pastures to fire at the soil bacterial level.