## 3 Results

Table 3: **Abiotic properties of unburnt and burnt soils under different vegetation covers. ** Mean values (\(n=12\), \(\pm\)s.e.) of pH, water content (WC, expressed as % d.w), C and N concentrations (expressed as % d.w.), organic matter content (OM, expressed as % d.w.), and Al, Ca, Cu, Fe, K, Mg, Mn, Na, and Pb available fractions (expressed as \(\mu\)g g\({}^{-1}\) d.w.) in unburnt (UB) and burnt (B) soils collected inside the Vesuvius National Park under different vegetation covers (P: pine; HO: holm oak; BL: black locust; H: herbs). Different letters indicate statistically significant differences in each property between unburnt and burnt soils under the same vegetation cover (_t_-test, at least, \(p\) < 0.05). The dehydrogenase (DHA) activity ranged from 0.01 to 3.65 mmol TFF g\({}^{-1}\) d.w. h\({}^{-1}\) in soils covered by pine (P), from 0.46 to 9.06 mmol TFF g\({}^{-1}\) d.w. h\({}^{-1}\) in soils covered by holm oak (HO), from 0.69 to 6.47 mmol TFF g\({}^{-1}\) d.w. h\({}^{-1}\) in soils covered by black locust (BL), and from 0.01 to 10.6 mmol TFF g\({}^{-1}\) d.w. h\({}^{-1}\) in soils covered by herbs (H). DHA statistically varied among the seasons only in soils covered by P with values higher in spring and lower in winter (Figure 1). The \(\beta\)-glucosidase (\(\beta\)-GLU) activity ranged from 63.8 to 506 mmol PNP g\({}^{-1}\) d.w. h\({}^{-1}\) in soils covered by pine (P), from 74.6 to 1200 mmol PNP g\({}^{-1}\) d.w. h\({}^{-1}\) in soils covered by holm oak (HO), from 62.1 to 901 mmol PNP g\({}^{-1}\) d.w. h\({}^{-1}\) in soils covered by black locust (BL), and from 61.4 to 871 mmol PNP g\({}^{-1}\) d.w. h\({}^{-1}\) in soils covered by herbs (H). \(\beta\)-GLU did not statistically vary among the seasons in soils under the same vegetation cover (Figure 1). The enzymatic activities (HA, DHA, and \(\beta\)-GLU) measured in unburnt (UB) and burnt (B) soils under the same vegetation cover are reported in Figure 2 and they did not statistically vary between UB and B soils (Figure 2). Figure 1: Mean values (\(n=6\), \(\pm\) s.e.) of the enzymatic activities (hydrolase: HA; dehydrogenase: DHA; \(\beta\)-glucosidase: \(\beta\)-GLU) measured in surface (0–10 cm) soils collected inside the Vesuvius National Park (Southern Italy) under different vegetation covers (P: pine; HO: holm oak; BL: black locust; H: herbs) in summer (S) and fall (F) 2018, and winter (W) and spring (Sp) 2019. Different letters indicate statistically significant differences in enzymatic activities among different seasons under the same vegetation cover (\(t\)-test, at least, \(p<0.05\)). The results of the PCA performed on soils collected under pine highlighted that the first two axes accounted, respectively, for 26% and 18% of the total variance (Figure 3A). The first axis clearly separated the soils according to the season (Figure 3A). Soil water content and available fractions of Al, Fe, Mn, and Na were positively correlated to the first axis; whereas, HA, DHA and available fractions of Ca and K were positively correlated to the second axis (Figure 3B). The results of the PCA performed on soils collected under holm oak highlighted that the first two axes accounted, respectively, for 29% and 19% of the total variance (Figure 3B). Soil C and N, available fraction of Mn, \(\beta\)-GLU, and DHA were positively correlated to the first axis (Figure 3B). Available fractions of Al, Cu, and Fe were positively correlated to the second axis; whereas, available fractions of K and Mn concentrations were negatively correlated to it (Figure 3B). The results of the PCA performed on soils collected under black locust highlighted that the first two axes accounted, respectively, for 34% and 22% of the total variance (Figure 3C). Available fractions of Ca, Mg, and Mn and HA were positively correlated to the first axis (Figure 3C). Available fractions of Al and Cu and water content were positively correlated to the second axis; whereas, C and available fraction of K were negatively correlated to it (Figure 3C).