We observed no significant difference between burned and unburned. Therefore, prescribed fire did not significantly alter the physical properties of the soil. This is in line with our hypothesis.
Like our study, the majority of previous studies indicated the absence of significant differences between burned (prescribed fire) and unburned areas (Table 1). The main reasons for such lack of diferences are as follows: (a) the absence of rainfall that contributed to the maintenance of aggregate stability (Ebel et al., 2022b; Spera et al., 2000); (b) maintenance of soil organic matter content that acts as a soil aggregating agent and, consequently, favors the maintenance of infiltration (Granged et al, 2011b; Strydom et al., 2019); (c) the low intensity of fire that prevents the recombination of soil particles which, as a consequence, assists in the maintenance of porosity (Chief et al., 2012), and (d) action of soil mesofauna (e.g. earthworms and oligochaetes) that cause soil aeration and favor water flow (Fischer et al., 2015; Mataix-Solera et al., 2011). All these explanations might apply to our study area. Thus, more studies are needed to pinpoint which of theses causes are at work. On the other hand, some studies found significant differences between burned and unburned sites (Table 1) which were attributed to: (a) at reduction of soil litter (biomass) that rain splash whcih, in turn, favor erosion (CAWSON et al., 2016) and (b) increased water repellency, which reduces infiltration and favors runoff (EBEL, 2020; CAWSON et al., 2016).
Table 1
Mean infiltration capacity found in different studies under the burned and unburned treatments.
Infiltration Capacity (mm/h) | | | | |
Unburned | Burned | | Infiltration Method | Vegetation | Authors |
20.5 | 4.1 | * | Constant Positive Charge Infiltrometer | Eucalyptus Dry Forests | Cawson et al., 2016 |
79.8 | 0.7 | * | Mini Disk Infiltrometer | Conifers | Ebel (2020) |
77 | 60 | NS | USDA Swing Nozzle Rainfall Simulator | Conifers | Robichaud (2000) |
78 | 49.2 | NS | Double Ring Infiltrometer | Savanna Forests | Savadogo et al. (2007) |
33.9 | 26.6 | NS | Portable swing-arm rain simulator | Artemisia | Pierson et al. (2008) |
119.4 | 158.7 | NS | Mini Disk Infiltrometer | Srhub | González-Pelayo et al. (2010) |
61 | 31 | NS | Tension disk infiltrometer | Savanna | Strydom et al. (2019) |
8.6 | 8.84 | NS | Mini Disk Infiltrometer | Pines | Wittenberg et al. (2020) |
14.2 | 14.8 | NS | Mini Disk Infiltrometer | Pines | Lucas-Borja et al. (2023) |
626.05 | 481.05 | NS | Mini Disk Infiltrometer | Neotropical Savanna | Presente estudo |
Asterisks (*) denote significant differences between unburned and burned area, while NS denotes no significant differences between unburned and burned área.
In the present study, we also observed the absence of significant differences between treatments regarding water repellency. Such result differs from previous studies that have demonstrated increased water repellency after prescribed fires (Table 2), due to: (a) the formation of a subsurface (3 to 5 cm deep) water repellent layer (derived from the condensation of organic substances volatilized during the fire) (Robichaud and Hungeford, 2000; Hubbert et al., 2006; Jordán et al., 2010; Zavala et al., 2010; Mataix-Solera et al., 2011; Granged et al., 2011a; Granged et al, 2011b) and; (b) short time interval between prescribed fire event and sampling, since the greater the time interval between fire event and sampling, the greater the possibility of recovery to water repellency pre-fire levels (Robichaud, 2000; Hubbert et al., 2006; Savadogo et al., 2007; Keeley, 2009; González-Pelayo et al., 2010; Malkinson and Wittenberg, 2011; Plaza-Álvarez et al., 2019; Ebel, 2020; Carrá et al., 2021; Carrá et al., 2022; Ebel et al., 2022b; Fardo-Cantos et al., 2023). On the other hand, other studies have already demonstrated a reduction in water repellency, due to: (a) prescribed fires that reached temperatures above 250 ºC which completely consume hydrophobic compounds present in the first soil layers (Robichaud and Hungerford, 2000; Zavala et al., 2010); (b) duration of the prescribed fire which, when above 5 minutes, consumes the hydrophobic compounds of the first soil layers (De Bano, 1981; Zavala et al., 2010) and; (c) lack of oxygen during prescribed fire, which makes the temperature thresholds for the dissipation of water repellency higher (Bryant et al., 2016). In this sense, possible explanations for our results include: (a) the present study focused on the surface soil layer (0 cm) and not on greater depths that, generally, usually present these significant differences right after prescribed fire and (b) sampling was carried out 45 days after prescribed fire. Therefore, prescribed fire can both favor maintenance and cause significant changes on water repellency, as previous studies have noted (Table 2).
Table 2
Mean and standard deviation of soil water repellency found in different studies under the unburned and burned treatments.
Water Drop Penetration Time (s) | | | |
Unburned | Burned | | Vegetation | Authors |
0 ± 0 | 173 ± 226 | * | Artemisia | Pierson et al., 2008 |
45 ± 29 | 1591 ± 1567 | * | Eucalyptus Forest | Granged et al., 2011a |
95.6 ± 55.1 | 83 ± 63.6 | NS | Eucalyptus Forest | Zavala et al., 2010 |
82.9 ± 109.38 | 139 ± 120.7 | NS | Pines | Malkinson; Wittenberg, 2011 |
4.8 ± 8.2 | 3.9 ± 5.5 | NS | Pines | Lucas-Borja et al., 2023 |
106.95 ± 122.15 | 91.9 ± 122.58 | NS | Neotropical Savanna | Presente estudo |
Asterisks (*) denote significant differences between unburned and burned area, while NS denotes no significant differences between unburned and burned area.
Finally, unlike what we found in the present study, previous studies documented that prescribed fire can also promote both positive (e.g. enhancing infiltration capacity, reducing both soil resistance to penetration and water repellency) or negative (raising soil bulk density and water repellency and reducing infiltration) significant changes (Robichaud and Hungeford, 2000; Zavala et al, 2010; Redin et al., 2011; Alcañiz et al., 2018; Li et al., 2022). When positively associated with soil physical properties, these significant differences occur, mainly due to: (a) the growth of root biomass, which causes the reduction of soil bulk density through the formation and stabilization of large granular aggregates and which can also favor water-repellency processes if the fire reaches ideal temperatures (Brye, 2006; Granged et al, 2011a); (b) the soil meso and macrofauna that contribute to the creation of preferential water flow paths that favor infiltration, in addition to the decline in soil bulk density (Brye, 2006; Li et al, 2022); (c) to prescribed fires of moderate and high intensities (> 350 ºC), which completely eliminate water repellency and favor strong aggregation of soils in the subsurface (Granged et al., 2011a; Granged et al, 2011b; Mataix-Solera et al., 2011), and (d) to the decrease of evapotranspiration through the consumption of aboveground biomass, which maintains soil wettability and prevents the increase of water repellency (Atchley et al., 2018). However, when negatively associated, the significant differences are explained because of: (a) increased water water repellency that tends to decrease infiltration (Cawson et al., 2016; Plaza-Álvarez et al., 2018; Chen et al., 2020; Lucas-Borja et al., 2023); (b) increased soil bulk density caused by notable reduction of roots, soil biota, and organic matter (Phillips et al., 2000).