Medium-term effects of wildfire severity on soil physical, chemical and biological properties in Pinus halepensis Mill. woodland (Southern Italy): an opportunity for invasive Acacia saligna colonization?

Wildfire frequency and severity have greatly increased in Mediterranean areas in recent decades affecting ecosystems functioning due to alteration in the above- and below-ground process. This study aimed to investigate how wildfire severity, in the medium-term (2–5 years), impacts soil properties within a Pinus halepensis woodland located in the Special Area of Conservation (SAC) of the Natura 2000 network (IT9130006 - Pinewoods of the Ionian Arch). In 2021, four years after a large wildfire in 2017, the woodland still exhibited fire effects with evidence of low, medium or high burn severity in different sites (named LBS, MBS, HBS, respectively). In addition, we observed an area burnt at medium severity that was invaded by Acacia saligna (MBSA site), a fast-growing, highly invasive, drought-tolerant nitrogen-fixing plant, thus we also examined the combined effect of wildfire and A. saligna on the soil. We compared soil properties across burnt sites with a nearby unburnt site (control). Thickness, weight and organic carbon of litter (L) and fermentation (F) layers were measured, alongside physical, chemical and biological properties in the underlying mineral soil (0–10 cm). Our results show that wildfire destroyed the organic layers and these had not recovered four years after the wildfire (except for L-layer within LBS) with a consequent loss (∼2 t C ha −1 ) of this carbon pool. In mineral soil we identified fourfold increases in N mineralization and nitrification rates across all burnt sites, regardless of the burn severity and A. saligna presence, suggesting an alteration of N-cycle processes. On the contrary, total microbial biomass and soil respiration as well as most of the physical and chemical properties of the soil were comparable between the burnt and control soils. Principal Component Analysis (PCA) highlighted that burn severity affected soil variables with lower changes in LBS than in other burnt soils. Also, the HBS soil did not show greater negative impacts compared to MBS sites. This is probably due to the increased post-fire colonization by herbaceous plants in HBS, favoured by the complete destruction of trees. In this case, waiting for natural vegetation recovery can be a valid management option, but periodic monitoring of fire-soil-vegetation interaction mainly to avoid invasive species widespread is advocated.