Wildfires are a recurrent and increasing threat in mainland Portugal, where over 4.5 million hectares of forests and scrublands have burned over the last 38 years. These fire-affected landscapes have ...suffered an intensification of soil erosion processes, which can negatively affect soil carbon storage, reduce fertility and forest productivity, and can become a source of pollutants. The main objective of the present study is to produce a post-fire soil erosion risk map for the forest and shrubland areas in mainland Portugal and assess its reliability. To this end, the semi-empirical Morgan-Morgan-Finney erosion model was used to assess the potential post-fire soil erosion according to distinct burn severity and climate scenarios, and the accuracy of the predictions was verified by an uncertainty analysis and validated against independent field datasets. The proposed approach successfully allowed mapping post-fire soil erosion in Portugal and identified the areas with higher post-fire erosion risk for past and future climate extremes. The outcomes of this study comprise a set of tools to help forest managers in their decision-making for post-fire emergency stabilization, ensuring the adequate selection of areas for mitigation to minimize the economic and environmental losses caused by fire-enhanced soil erosion.
•The stocks of humus and POM were examined in relation to elevation and tree species.•Elevation strongly affected the early stages of decay (represented by humus layers).•Tree species was a major ...driver of the buildup of POM.•Calcium negatively related to the buildup of organic matter within individual layers.
Forests of European beech (Fagus sylvatica) and Scots pine (Pinus sylvestris) in the Iberian Peninsula contain some of the southernmost localities of these species and thus may display highly sensitive responses to environmental variations as a result of subjection to particular climate stresses. In this study, we investigated the effects of elevation and dominant tree species (beech vs. pine) on the buildup of organic matter (OM) and nutrients within the humus (O layers) and particulate organic matter (POM) of the topsoil (Ah layer) in centennial Scots pine forests compared to that in natural beech forests along an altitudinal gradient from 1100 to 1600 m.a.s.l. in the Moncayo Massif (northern Spain). The contents of nonstructural, hemicellulose, cellulose, lignin-like, soluble ash and free lipid fractions and of C and major nutrients (N, P, S, K, and Ca) were analyzed and related to the buildup of OM within individual layers, which we used as an inverse surrogate for decomposition rates. The stocks of the studied variables in the humus and the POM were examined in relation to elevation and tree species. The highest storage values were found in the Scots pine sites at higher elevations in the form of thick layers of strongly decomposed organic matter (OH layers) and in the POM. Climate, as it related to elevation, was found to be the major driver of the early and intermediate stages of decomposition (represented by humus) but showed little effect on POM, which was more strongly affected by the tree species. The buildup of OM has its largest correlation (negative) with the content of Ca (which was higher under beech vegetation and decreased with increasing elevation), suggesting an effect of Ca in improving habitat conditions for biological activity.
The dense thicket Echinospartum horridum (Vahl, Rothm) is expanded in secondary pastures of the Central Pyrenees (NE-Spain). The control of this grassland encroachment is attempted through prescribed ...burnings, trying to minimize its direct effects on the soil. But the structural changes on the new soil surface, burned and bare, are unknown in the medium-term. To check it, soil aggregate stability (SAS), mean weight diameter of the aggregates (MWD), water repellency (WR), unsaturated hydraulic conductivity (k), and soil organic carbon (SOC) were measured in the surface (at 0–1, 1–2, 2–3, and 3–5 cm) in both unburned and 1-yr burned soils, after verifying that it suffered no direct damage. We also used the digital images of thin sections, obtained from undisturbed and oriented topsoil samples, to detect potential changes in soil microstructure.
No significant changes were found in SAS, MWD and SOC for any thickness of soil studied. Nevertheless the WR, which was high before and just after burning, decreased significantly in the upper soil cm after 1-yr burning. WR decrease coincides with the 6-fold increase of the unsaturated hydraulic conductivity (k) and the presence of cappings on the burned topsoil. Cappings are coatings poor in organic matter and composed by fine sand-sized particles of angular quartz, mixed with charcoal, covering irregularly the original topsoil. The formation of cappings seems to derive from the impact of raindrops on the bare soil surface, hence its irregular spatial distribution. Summarizing, removing bushes by means of a low-intensity fast-moving prescribed burning caused the formation of discontinuous cappings without worsening significantly the rest of the measured properties.
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•Soil aggregation size, stability and organic C do not change in cm-measurements.•Repellency reduction (0–1 cm) coincides with the 6-fold increase of infiltration.•Cappings are detected in thin sections on soil surface and infilling pores.•Patterned cappings are formed by splash erosion on 1 yr-burned and bare soils.
•Prescribed fire caused considerable C losses at the 0–1 cm depth, mostly from POM.•Uneven combustion resulted in few qualitative changes in the remaining organic matter.•Bioavailable N showed ...transient increases, while S and P gradually declined after fire.•Steadier nutrient inputs were derived from the breakdown of particulate charcoal.
High-mountain soils are rich in partially decomposed organic matter, which is highly sensitive to mineralization and fire. Prescribed burning is performed in the Pyrenees to keep subalpine grasslands open for grazing. The compositions of the ash, litter and duff layers, and the particulate organic matter (POM) of the topsoil in the 0–1, 1–2, 2–3, and 3–5 cm depths were analyzed in relation to the nutrient availability after the prescribed burning of a stand encroached by erizón (Echinospartum horridum). The concentrations of C, N, P, and S and organic components (nonstructural, hemicellulose, cellulose, and lignin-type) were determined before the prescribed burn and 0, 6, 12, 18, and 24 months after the prescribed burn. The fire consumed the aboveground biomass, the litter and part of the duff layer, and the most thermostable (i.e., lignin-type) components and the least volatile elements (P, S) were selectively preserved in the resulting ash. Prescribed burning caused significant losses of organic-C and N only in the 0–1 cm depth (–72% and –68%, respectively). The organic-C loss was mostly (82%) from the POM, whereas the N loss was from more similar proportions of the POM (57%) and the nonparticulate organic matter (NPOM) (43%). However, few changes were observed in the composition of the organic matter, which pointed to a largely uneven combustion that resulted in a substantial part of the organic matter remaining largely untouched by the fire. After 6 months, the duff layer was depleted in hemicellulose by 32% compared to immediately after the burn, and fragmentation of the POM into the NPOM was observed. During the second spring, N- and P-rich charred POM were incorporated into the top 1 cm, while C-rich charcoal particles underwent fragmentation and vertical transport into the deeper soil. The preburn ecosytem was limited by P, and likely also by S. The plant available N showed transient increases of 1.5–2.1 times the immediate postburn levels for nitrate-N at 12 months after burning, and of up to 10–20 times for ammonium-N at 18 months. In contrast, the concentrations of plant-available P and S gradually declined to 1.8–3.3 and 1.8–4.0 times, respectively, lower at 24 months after the burn. Results indicated that fire-induced increases in the nutrient availability can be short-lived in high-mountain habitats, but steadier and likely more persistent nutrient inputs can derive from the gradual breakdown of charred organic matter.
Wildfires directly emit 2.1 Pg carbon (C) to the atmosphere annually. The net effect of wildfires on the C cycle, however, involves many interacting source and sink processes beyond these emissions ...from combustion. Among those, the role of post‐fire enhanced soil organic carbon (SOC) erosion as a C sink mechanism remains essentially unquantified. Wildfires can greatly enhance soil erosion due to the loss of protective vegetation cover and changes to soil structure and wettability. Post‐fire SOC erosion acts as a C sink when off‐site burial and stabilization of C eroded after a fire, together with the on‐site recovery of SOC content, exceed the C losses during its post‐fire transport. Here we synthesize published data on post‐fire SOC erosion and evaluate its overall potential to act as longer‐term C sink. To explore its quantitative importance, we also model its magnitude at continental scale using the 2017 wildfire season in Europe. Our estimations show that the C sink ability of SOC water erosion during the first post‐fire year could account for around 13% of the C emissions produced by wildland fires. This indicates that post‐fire SOC erosion is a quantitatively important process in the overall C balance of fires and highlights the need for more field data to further validate this initial assessment.
Carbon (C) redistribution associated with fire‐enhanced soil erosion is speculated to affect the global C cycle, yet remains unquantified. Here we estimate the magnitude of post‐fire soil organic carbon (SOC) erosion by water and assess its capacity to act as a C sink or source. Results indicate that post‐fire SOC erosion is a quantitatively important process in the overall C balance of fires; and that the C sink ability of SOC water erosion alone during the first post‐fire year could account for around 13% of the C emissions produced by wildland fires.
Wildfires are known to be one of the main causes of soil erosion and land degradation, and their impacts on ecosystems and society are expected to increase in the future due to changes in climate and ...land use. It is therefore vital to mitigate the increased hydrological and erosive response after wildfires to maintain the sustainability of ecosystems and protect the values at risk downstream from the fire-affected areas. Soil erosion mitigation treatments have been widely applied after wildfires but assessment of their effectiveness has been limited to local and regional-scale studies, whose conclusions may depend heavily on site-specific conditions. To overcome this limitation, a meta-analysis approach was applied to investigations of post-wildfire soil erosion mitigation treatments published in peer-reviewed journals.
A meta-analysis database was compiled that consisted of 53 and 222 pairs of treated/untreated observations on post-fire runoff and erosion, respectively, extracted from 34 publications indexed in Scopus. The overall effectiveness of mitigation treatments, expressed as the quantitative metric ‘effect size’, was determined for both the runoff and erosion observations, and further analyzed for four different types of treatments (cover-based, barriers, seeding, and chemical treatments). The erosion observations involving cover-based treatments were analyzed for differences in effectiveness between 3 different types of mulch materials (straw, wood-based, and hydromulch) as well as between different application rates of straw and wood materials. Finally, the erosion observations were also analyzed for the overall effectiveness of post-fire year, burn severity, rainfall amount and erosivity, and ground cover.
The meta-analysis results show that all four types of treatments significantly reduced post-fire soil erosion, but that only the cover and barrier treatments significantly reduced post-fire runoff. From the three different cover treatments, straw and wood mulches were significantly more effective in mitigating erosion than hydromulch. In addition, the effectiveness of both straw and wood mulches depended on their application rates. Straw mulching was less effective at rates below than above 200 g m−2, while mulching with wood materials at high rates (1300 to 1750 g m−2) produced more variable outcomes than lower rates. Results also suggest that the overall effectiveness of the treatments was greatest shortly after fire, in severely burned sites, providing or promoting the development of ground cover over 70%, and that it increased with increasing rainfall erosivity.
It can be concluded that, in overall terms, the application of the studied post-fire erosion mitigation treatments represented a better choice than doing nothing, especially in sites where erosion is high. However, the meta-analysis highlights under-representation of studies on this topic outside of the USA, Spain and Portugal. It was also observed that most of the studies were conducted at hillslope scale and tested mulching (namely straw, wood and hydromulch) and/or barriers, while larger scales and other treatments were scarcely addressed. Further efforts need to be made in testing, from field and modeling studies, combinations of existing and/or emerging erosion mitigation treatments to ensure that the most adequate measures are applied after fires.
•High soil moisture content attenuates the heat transmission within burned topsoil.•Soil thickness affected by burning is mainly limited to the first cm, either wet or dry.•Fire leads to a C loss of ...50% in dry soil and 25% in wet soil for up to the 1st cm.•SWR is tightly linked with fire, soil moisture content and soil depth.
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The aim of this work is to investigate the topsoil thickness affected by burning under contrasting soil moisture content (field capacity versus air-dried conditions). A mollic horizon of an Aleppo pine forest was sampled and burned in the laboratory, recording the temperature continuously at the topsoil surface and at soil depths of 1, 2, and 3cm. Changes in soil properties were measured at 0–1, 1–2, 2–3, and 3–4cm.
Both the maximum temperature and the charring intensities were significantly lower in wet soils than in air-dried soils up to 3cm in depth. Moreover, soil heating was slower and cooling faster in wet soils as compared to dry soils. Therefore, the heat capacity increase of the soil moistened at field capacity plays a more important role than the thermal conductivity increase on heat transfer on burned soils. Burning did not significantly modify the pH, the carbonate content and the chroma, for either wet or dry soil. Fire caused an immediate and significant decrease in water repellency in the air-dried soil, even at 3cm depth, whereas the wet soil remained hydrophilic throughout its thickness, without being affected by burning. Burning depleted 50% of the soil organic C (OC) content in the air-dried soil and 25% in the wet soil at the upper centimeter, which was blackened. Burning significantly decreased the total N (TN) content only in the dry soil (to one-third of the original value) through the first centimeter of soil depth. Soluble ions, measured by electrical conductivity (EC), increased after burning, although only significantly in the first centimeter of air-dried soils. Below 2cm, burning had no significant effects on the brightness, OC, TN, or EC, for either wet or dry soil.
Prescribed burning has been recently readopted as a management practice in the Central Pyrenees (NE-Spain) to stop shrub encroachment processes and recover pasturelands. The immediate effects of ...prescribed burning on soil C stocks and related biological properties and their evolution in the short-to mid-term after burning were assessed. The study was conducted during three autumnal prescribed burnings in the Central Pyrenees in the municipalities of Buisán, Asín de Broto and Yebra de Basa. At each site, the topsoil Ah horizon was sampled at soil depths of 0–1, 1–2 and 2–3 cm immediately before and immediately after burning. Additionally, seasonal samplings were conducted every 6 months up to one year in the case of the Asín and Yebra sites and up to 24 months at the Buisán site. The total soil organic C stock (SOCS) total N stock (NS), microbial biomass C (MBC), soil basal respiration (SR) and β-D-glucosidase activity were analyzed. The maximum temperatures recorded at the soil surface were 438 °C (Buisán), 768 °C (Asín) and 595 °C (Yebra). At the Buisán site, burning significantly decreased the SOCS (−52%), NS (−44%), MBC (−57%), SR (−72%) and glucosidase activity (−66%) at 0–1 cm depth, whereas fire had no direct effects on soil at the Asín and Yebra sites. The contrasting effects of burning on soil that were observed among sites were found to be related to differences in fire residence time. The prescribed fire at the Buisán site was on a plain slope under slow winds (<8 km h−1) at a burning rate of 0.64 ha h−1, which produced greater impacts on the soil properties than the burnings at the Asín and Yebra sites, where fire spread rapidly (2.72 and 1.43 ha h−1, respectively). At the Buisán site, the SOCS and NS recovered to the unburned values 24 months after burning. One year after burning, the SOCS at Asín were 60% higher than those of the unburned soils at 0–1 cm depth. At all sites a decreasing trend in soil biological activity in the short- and mid-term was observed. From the results it can be concluded that: 1) the direct effects of burning on soil are highly dependent on the environmental conditions, 2) in the mid-term, the reduction in soil biological activity and the incorporation of ashes and charred plant remains led to an increase in the SOCS of the burned soils.
•Prescribed burning is used to remove shrubs and restore pastures.•The effect on soil C stocks and related biological properties is studied.•Slow prescribed burning severely affected soil properties at 0–1 cm depth.•Soil properties were not affected at sites where the fire spread rapidly.•Soil biological activity decrease occurred in the short- and mid-term.
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