Since the 1970s, increase in fire frequency has been observed in all European Mediterranean regions. The objectives of this study were (1) to determine the effects of wildfire frequency on the recovery at short‐ and long‐term of soil chemical and microbial properties and (2) to identify the mechanisms underlying the recovery of these sites properties. Soils from 17 plots (Maures mountains range, Var, France) were classified into 5 wildfire regimes (i.e. not burned since at least 57 years ago, infrequently and frequently burned‐with time since fire between 4 and 17 years). Soil samples from these plots were analysed for their nutrient content, chemical functions of soil organic matter (SOM) using FT‐MIR spectroscopy and microbial mineralising activities. Our results showed that the frequent wildfire regime slowed down the recovery in the short term of SOM spectroscopic properties and nutrient availability. Both low quantity and low quality (i.e. high percentage of aromatic and phenolic organic forms) of soil organic matter were found to be related to soil microbial recovery at 4 years after frequent wildfires. The frequent wildfires improved the recovery in net nitrification and nitrate content, leading to an increase in catabolic evenness and a recovery in microbial C‐substrate utilisation profiles between 4 and 17 years. However, frequent wildfires slowed down the recovery of hydrolytic enzyme pool (i.e. FDA hydrolases) and phenol oxidase activity, both involved in soil C cycling. Overall, our observations suggest that 4 fires in 50 years is a threshold beyond which soil quality may be endangered. Copyright © 2011 John Wiley & Sons, Ltd.

Summary1. Over a century of agricultural abandonment across the Mediterranean region has favoured the installation of the pioneer expansionist species Aleppo pine (Pinus halepensis Miller). This species synthesizes a wide range of secondary metabolites that are partially released during needle decomposition, and which can thus affect the 'brown food chain'. Litter decomposition is a key process connecting ecosystem structure and function, and involving microbial and faunal components. 2. The goal of this study was to determine the effect of chemical compounds from Aleppo pine needles on the litter decomposition process along a gradient of Mediterranean forest secondary succession. Using in situ litterbags, we compared the dynamics of decomposers, particularly the relative contributions of fungal and mesofauna biomass to litter mass loss (calculations based on the measured decomposer biomass, published fungal growth efficiency and mesofauna feeding rate), against the dynamics of secondary metabolites associated with decomposed needles in three successional stages (early, middle and late, i.e. pinewoods that were aged 10, 30 and over 60 years old). 3. Our first key finding was that fungi accounted for the largest portion of overall litter mass loss (60-79%) and detritivorous mesofauna contributed to 8-12%. In the early stage of succession, fungal biomass after 6 months of decomposition was lower than in middle and late stages, and may be responsible for the delay in litter colonization by mesofauna. We linked this result to a clearly longer residence time for phenolic compounds in young pine forest, leading to an overall slowdown in the decomposition process. 4. Synthesis. Litter phenolic content emerged as a key functional trait for predicting litter decomposition, delaying the colonization of litter by decomposers in Mediterranean forest ecosystems. Another key finding is that the relative contributions of fungi and detritivores to needle mass loss were different between the successional stages. From the food-web perspective, the organic matter available for higher trophic levels thus remains unchanged beyond 30 years after pine colonization.