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INTRODUCTION
Wildfires are affecting large areas of forests and shrublands in the Mediterranean region every year, with large economic and ecological impacts. In the past years, fire regimes have been shifting to large devastating fires, with higher intensity and frequency (Pausas, 2015). Nonetheless, wildfires are an intrinsic process of the mediterranean landscape and vegetation dynamics.
Soil is considered as the largest carbon terrestrial pool (Lal, 2004). Previous studies carried out in Mediterranean ecosystems highlighted a wide variability of the fire effects on soil properties and hydrological processes (Certini, 2005), contributing to soil degradation, including carbon loss.
Fire effects on soils depend on many factors such as fire severity and duration, vegetation type, soil type and pre-fire status, with the strongest impacts on the superficial layer (0-5 cm) (Hrelja et al., 2020). However, the fire can reach beyond this superficial layer and affect the deeper soil layers (Úbeda and Outeiro, 2009). For example, a soil layer of up to 30 cm is used to estimate soil carbon stock (FAO, 2019), indicating the importance of knowing fire effects and post fire recovery of soils beyond the superficial layer. Furthermore, there is little knowledge of fire effects on cork oak forests, despite the high ecological and economic importance of these ecosystems.
This study aimed to asses the long-term effect of wildfires in the soil chemical characteristics of cork oak forests and post-fire soil resillience.
MATERIAL AND METHODS
Study area
The study was conducted in Serra do Caldeirão (Algarve, southern Portugal). Serra do Caldeirão is a mountainous region, with a hilly landscape dominated by cork oak forests and shrublands of the Cistaceae and Ericacea families. The climate is Mediterranean, classified as Csa according to the Köppen classification (Köppen, 1936). The soils were developed on schist and greywackes, included in the Mira Formation (Oliveira, 1982), being classified mainly as Leptosols (WRB, 2014) although some Cambisols can also occur.
The area was affected by an extreme wildfire in 2004, which caused a severe loss of forest area (≈13600 ha). In the study area 37 plots were selected (each plot with a total area of approximately 441 m2), based on forest inventory, for soil sampling: 12 plots located in unburned area and 25 plots located in the area burned in 2004 (hereafter named as scenarios). In November of 2020, three soil samples were collected at two depths (0-5 cm and 5 cm to the maximum depth), at each plot.
After air-drying and homogeneization, soil samples (fraction <2 mm) were analysed for: pH and electrical conductivity (1:2.5 m:V), cation exchange capacity (extraction with ammonium acetate 1 mol/dm3), extractable P (Olsen and Sommers, 1982), total N (Kjeldahl method) and organic C by wet digestion method (Springer and Klee, 1954).
A T-test was performed to evaluate significant differences of soil chemical caracteristics analysed (separately for each depth) between burned and unburned plots.
RESULTS AND DISCUSSION
Most of the soils presented a maximum depth up to 47 cm. Soil pH values were slightly acid to neutral, while electrical conductivities (EC) were low (Table 1), independently of the scenario and depth. There were no significant differences between burned and unburned areas for these parameters. Although the increase of soil pH after fire is reported due to ash-bed deposits on the surface (Chambers and Attiwill, 1994), this effect was not detected in the study area, 16 years after the fire.
Similar results for pH and EC between burned and unburned areas were reported by Fonseca et al. (2017) in Umbric Leptosols from Pinus pinaster forests, three years after fire. For soil developed on limestone of Quercus coccifera forests, pH values for the superficial layer (0-5 cm) also showed no significant differences between burned and unburned areas, nine years after fire, and regardless of fire severity, but EC increased (Alcañiz et al., 2016).
Table 1 Chemical characteristics of soils collected, at different depths, in unburned and burned areas (mean ± SD). Values with * and ** indicate significant differences in soil parameters for the same depth at p<0.05 and p<0.01, respectively
| Scenarios | Unburned | Burned | ||
| 0-5 cm of depth | ||||
| pH | 6.07 ± 0.44 | 6.02 ± 0.41 | ||
| EC (ds/m) | 0.09 ± 0.04 | 0.08 ± 0.03 | ||
| Ntotal (g/kg) | 1.84 ± 0.49** | 2.19 ± 0.67** | ||
| Pextractable (mg/kg) | 3.74 ± 2.27* | 5.00 ± 2.34* | ||
| Corganic (g/kg) | 27.68 ± 10.95 ** | 40.12 ± 13.71** | ||
| CEC (cmolc/kg) | 6.51 ± 2.70 | 7.51 ± 2.77 | ||
| 5 cm - maximum depth | ||||
| pH | 5.74 ± 0.30 | 5.69 ± 0.36 | ||
| EC (ds/m) | 0.04 ± 0.01 | 0.05 ± 0.02 | ||
| Pextractable (mg/kg) | 2.14 ± 1.15 | 2.40 ± 0.18 | ||
| Ntotal (g/kg) | 1.28 ± 0.40 | 1.38 ± 0.31 | ||
| Corganic (g/kg) | 11.53 ± 6.00** | 18.12 ± 6.54** | ||
| CEC (cmolc/kg) | 3.53 ± 1.37 | 3.89 ± 1.78 | ||
Cation exchange capacity (CEC) varied from low to very low (5-10 and <5 cmolc/kg) with the depth, being Ca the dominant cation in the exchangeable complex. No significant differences were obtained in CEC values for both scenarios and depths (Table 1). CEC can decrease after fire due to the loss of organic matter. Nonetheless, the signficantly different organic C concentrations found between burned and unburned areas did not match with obtained values for CEC, sugesting a significant contribution of clays fraction to the CEC.
Contrarily to all other parameters analyzed, organic C and extractable P concentrations were significantly different beween burned and unburned plots at the study area.
Organic C concentrations decreased with depth, reaching always higher values in burned areas. This can be explained by the higher plant development of autochthonous species in burned areas in Serra do Caldeirão, such as Cistus, Erica and Ulex genus (data not shown). In fact, it was shown that frequent fires in the study area lead to the encroachment of persistent shrublands (dominated by Cistus ladanifer) (Acácio, 2009). High organic C amounts were also obtained in soils collected in Pinus halepensis and Pinus pinaster forests burned three years after fire, indicating that potsfire recovery of soil organic C depends on the interaction among plant recovery, fire severity and years after fire (Moya et al., 2019). Thus, the pattern of organic C recovery in the study area seems to be influenced by the soil-plant interaction.
Extractable P concentrations were very low, independently of the scenario, and showed a similar tendency to organic C concentration only the superficial layer, (higher in the burned areas; Table 1). These results are in concordance with the data obtained for a Quercus coccifera forest with similar shrub composition, where significant increases of extractable P in burned areas were identified nine years after fire (Alcañiz et al., 2016).
On the other hand, similar extractable P concentrations from superficial soil layer (0-5 cm) were obtained for burned and unburned areas, three years after fire on shrublands dominated by Cistus, while P increased on deeper layers (5-20 cm) of burned areas (Fonseca et al., 2017). The plant colonization of the burned area and consequent input of litter can be a possible explanation for P increase, as well as the effect of root systems on deeper levels leading to more plant uptake (Guerrero et al., 2005). Plant-soil interaction also plays a major role in the recovery of this soil parameter after fire. Moreover, Moya et al. (2019) showed that P can be affected by the interaction between fire severity and fire year but concentrations can increase three years after fire.
CONCLUSIONS
Sixteen years after the wildfire, soils in burned cork oak forests showed similar chemical characteristics to the unburned areas, independently of the soil depth. Important exceptions were obtained for organic C concentrations for all depths and extractable P only for superficial layer and can be related to the high plant development in burned areas. In fact, plant-soil interaction plays an important role in the soil recovery after fire.
In general, the edaphic processes in the burned cork oak forests from Serra do Caldeirão seem to be relatively dynamic, allowing the natural recovery of the system. The high resilience of this ecosystem and the consequent stimulation of post-fire plant development contributed to the increase of organic matter and its decomposition in soils from burned areas.
