Unburned Soils Research Articles

Modeling the Soil Response to Rainstorms after Wildfire and Prescribed Fire in Mediterranean Forests

The use of the Soil Conservation Service-curve number (SCS-CN) model for runoff predictions after rainstorms in fire-affected forests in the Mediterranean climate is quite scarce and limited to the watershed scale. To validate the applicability of this model in this environment, this study has evaluated the runoff prediction capacity of the SCS-CN model after storms at the plot scale in two pine forests of Central-Eastern Spain, affected by wildfire (with or without straw mulching) or prescribed fire and in unburned soils. The model performance has been compared to the predictions of linear regression equations between rainfall depth and runoff volume. The runoff volume was simulated with reliability by the linear regression only for the unburned soil (coefficient of Nash and Sutcliffe E = 0.73–0.89). Conversely, the SCS-CN model was more accurate for burned soils (E = 0.81–0.97), also when mulching was applied (E = 0.96). The performance of this model was very satisfactory in predicting the maximum runoff. Very low values of CNs and initial abstraction were required to predict the particular hydrology of the experimental areas. Moreover, the post-fire hydrological “window-of-disturbance” could be reproduced only by increasing the CN for the storms immediately after the wildfire. This study indicates that, in Mediterranean forests subject to the fire risk, the simple linear equations are feasible to predict runoff after low-intensity storms, while the SCS-CN model is advisable when runoff predictions are needed to control the flooding risk.

Soil Water Content and Soil Respiration Rates Are Reduced for Years Following Wildfire in a Hot and Dry Climate

AbstractIncreasing fire severity and frequency may stress ecosystems also impacted by climate change. We studied the physical limitations to regeneration after fire in an ecosystem that already experiences high summer temperatures and drought and is therefore a possible analog of the future. We compared soil respiration as an indicator of microbial activity in burned and unburned forest soils in central Texas, where two recent wildfires have occurred (2011 and 2015). We also measured soil temperature, water content, soil water δ18O and δD values, total C, N, pH, and δ13C values of total organic matter. Burned soils had lower total C and N than unburned soils; however, lab‐based respiration measurements, which controlled for temperature and water content, suggest that microbial activity in burned and unburned soils are similar. Conversely, field measurements show that during hot and dry months respiration rates in burned soils were much lower than they were in unburned soils due to differences in soil temperature and water content. Soil temperature at 5 cm reached 60°C in burned soils due to the removal of canopy cover, the removal of organic matter insulation, and the deposition of black ash on the soil surface. Higher temperatures increased evaporation as indicated by significantly lower deuterium excess of water in soils burned in 2015 than unburned soils. If the disturbance of vegetation by fire is substantial enough, the resulting perturbations to soils persist for years, most importantly increased heat absorption which results in lower water contents and ultimately reduced microbial activity.

Increased colluvial hollow discharge and subsequent recovery after a low intensity wildfire in the Blue Ridge Mountains, USA

AbstractWildfires in mountainous regions have been documented to enhance water repellent soils which can increase runoff, erosion, and sedimentation during subsequent rain events. However, the extent of soil hydrophobicity and water repellency varies significantly with burn severity and between ecosystems, and the southern Appalachians remain an understudied region. Here we examine the impact of the low severity Chestnut Knob Fire, which occurred in the fall 2016, on soil properties and runoff in South Mountains State Park. To examine these impacts, we installed crest‐stage gauges in burned (n = 10) and unburned (n = 8) colluvial hollows to compare peak runoff. Results from the 2017 field season indicated that burned locations produced significantly higher peak discharges than unburned sites. From July 2019 to January 2020, we repeated the experiment and found that burned areas produced runoff comparable to unburned areas. Examination of soil profiles during the summer of 2017 found high variability in hydrophobicity in both the burned (n = 10) and unburned (n = 2) soils. Further, we found that burned soils had significantly deflated organic surface horizons compared with unburned soils. We interpret the differences in runoff in 2017 to be the result of a combination of increased hydrophobicity and decreased soil moisture storage capacity in organic rich surface soils. While the recovery we observed here was relatively fast, it is important to understand that increased runoff immediately after a fire likely increases the chances of sediment mobilization and debris flow occurrence.

Utility and optimization of LANDSAT-derived burned area maps for southern California

ABSTRACT Maps of burned area derived from Land Remote-Sensing Satellite (LANDSAT) system imagery may be more reliable than fire perimeter records based on ground observation or manual cartographic delineation. This study evaluates accuracy of LANDSAT-derived burned area maps associated with 19 fires (65 to 86,776 ha) within shrublands of southern California in the period 1996–2018. High spatial resolution aerial images collected soon after these 19 fires were used to verify burned fractions within LANDSAT ground resolution elements. Validated burned fractions were used to optimize classification thresholds applied to burn severity metrics based on LANDSAT Normalized Burn Ratio (NBR), Normalized Burn Ratio 2 (NBR2), and Normalized Difference Vegetation Index (NDVI). The burn severity metrics included post-fire spectral index data (e.g., NBR), pre-fire to post-fire differences (e.g., dNBR), and relative differences (e.g., RdNBR). Optimized classifications of dNBR had the greatest overall accuracy (94%) compared to dNBR2 (93%) and dNDVI (92%). Classifications based on standard difference metrics yielded burn area maps that were 1% more accurate than those based on relative-difference metrics for each spectral index. Classification products based solely on post-fire imagery were 85% to 90% accurate, depending on the spectral index utilized. In comparison, perimeter data from the California Fire and Resource Assessment Programme (FRAP) and Monitoring Trends in Burn Severity (MTBS) substantially overestimated burned extent (by about 44%). The MTBS severity maps excluding the ‘unburned/low severity’ class slightly overestimated burned extent (11%). Commission error in the FRAP data set was attributed to low cartographic detail and inclusion of internal unburned patches. Site-specific differences in unburned soil and vegetation fractions (within partly-burned areas) correlated strongly with overestimation of burned area in the optimized LANDSAT-derived maps; areal overestimates of 10%–15% resulted from data grain size rather than map commission error. The main innovation presented in this study is an empirical method to predict dNBR thresholds based on local (pre-fire) NDVI mean and variance, which produced burned area maps of 90% median accuracy. This LANDSAT-based method could support an efficient reconstruction of fire history in recent decades, which would be more comprehensive and accurate than available fire records for southern California.

Impacts of prescribed burning on urban forest soil: Minor changes in net greenhouse gas emissions despite evident alterations of microbial community structures

Knowledge of the impacts of prescribed burning on ecosystem functions could help to improve forest management practices. However, relative to forest safety and aesthetic values, changes in soil properties and processes after prescribed burning are not well understood. This study aimed to quantify changes in soil microbial communities and greenhouse gas emissions following prescribed burning in a subtropical suburban forest. The soil microbial communities were determined with the Illumina sequencing methods. Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) emissions were quantified under three different soil water contents, and net greenhouse gas emissions were derived from emissions of the three gases after 7 days of incubation and expressed in CO2-equivalent. Prescribed burning had negligible impacts on soil bacterial and fungal community diversities but significantly changed soil microbial community structures. Compared to the unburnt soil, the relative abundances of Chloroflexi increased by 86% and 119% one month and three months after the prescribed burning, respectively. However, the relative abundances of Mortierellomycota decreased by 94% one month after prescribed burning. At different soil water contents, The CO2 and N2O emissions during a 7-day laboratory incubation did not differ consistently or significantly between the soils collected before/after the prescribed burning. Simultaneously, there were no significant changes in net greenhouse gas emissions from soil in association with the prescribed burning. The results revealed that relative to the unburned soils, prescribed burning caused minor alterations in net soil greenhouse gas fluxes, despite short-term evident changes in soil microbial community structures.

Characteristics of soil organic matter 14 years after a wildfire: A pyrolysis-gas-chromatography mass spectrometry (Py-GC-MS) study

Severe wildfires combust most above ground vegetation and detritus layers, altering the content and chemical composition of soil organic matter (SOM). To evaluate the lasting effects of wildfire on SOM and the recovery of burned soils, we sampled surface (Oa horizon) and mineral soils (0−5 and 5−15 cm depths) in unburned areas and areas burned at moderate and high severity 14 years after the 2002 Hayman Fire, in Colorado, USA. We characterized SOM using Pyrolysis Gas Chromatography Mass Spectrometry (Py-GC–MS) and identified 106 pyrolysates within eight chemical classes [aromatic hydrocarbon (ArH), carbohydrate (Carb), lignin compound (LgC), nitrogen containing compound (Ntg), polyaromatic hydrocarbon (PAH), phenol compound (PhC), saturated hydrocarbon (SaH), and unsaturated hydrocarbon (UnSaH)]. Burned soils had greater total quantified peak areas (TQPA) for the pyrogenic C indicator (PyC) benzene, compared to unburned soils; however, other common PyC markers were not abundant in burned relative to unburned soils. Factor analysis on the individual pyrolysates suggests that factors 1 and 2 correlated with pyrolysate aromaticity and hydrophobicity, respectively. Sample factor scores potentially suggest that SOM aromaticity increases with fire severity, though difference between moderate and high severity was slight. Factor analysis also indicates that the ratio of [ArH + Ntg] / [PhC + LgC] may serve as index of PyC content in SOM. This study shows that wildfire effects on SOM character may persist for more than a decade of ecosystem recovery and that Py-GC–MS coupled with factor analysis has utility for evaluating how disturbance alters SOM and PyC in complex environments.

Fates and fingerprints of sulfur and carbon following wildfire in economically important croplands of California, U.S.

Sulfur (S) is widely used in agriculture, yet little is known about its fates within upland watersheds, particularly in combination with disturbances like wildfire. Our study examined the effects of land use and wildfire on the biogeochemical “fingerprints,” or the quantity and chemical composition, of S and carbon (C). We conducted our research within the Napa River Watershed, California, U.S., where high S applications to vineyards are common, and ~ 20% of the watershed burned in October 2017, introducing a disturbance now common across the warmer, drier Western U.S. We used a laboratory rainfall experiment to compare unburned and low severity burned vineyard and grassland soils. We then sampled streams draining sub-catchments with differing land use and degrees of burn and burn severity to understand combined effects at broader spatial scales. Before the laboratory experiment, vineyard soils had 2–3.5 times more S than grassland soils, while burned soils—regardless of land use—had 1.5–2 times more C than unburned soils. During the laboratory experiment, vineyard soil leachates had 16–20 times more S than grassland leachates, whereas leachate C was more variable across land use and burn soil types. Unburned and burned vineyard soils leached S with δ34S values enriched 6–15‰ relative to grassland soils, likely due to microbial S processes within vineyard soils. Streams draining vineyards also had the fingerprint of agricultural S, with ~2–5 fold higher S concentrations and ~ 10‰ enriched δ34S-SO42− values relative to streams draining non-agricultural areas. However, streams draining a higher fraction of burned non-agricultural areas also had enriched δ34S values relative to unburned non-agricultural areas, which we attribute to loss of 32S during combustion. Our findings illustrate the interacting effects of wildfire and land use on watershed S and C cycling—a new consideration under a changing climate, with significant implications for ecosystem function and human health.

KANDUNGAN C-ORGANIK TANAH DAN TOTAL NITROGEN PADA TANAH VERTISOL YANG TELAH DIBAKAR DAN TIDAK DIBAKAR

The use of fire in agriculture land preparation may result in the change of various soil properties. Therefore, a study was conducted in the village of Oeteta, Kupang to determine soil total organic-C and total nitrogen of burned and unburned Vertisols. Randomized complete block design with two factors was applied. The first factor was burning treatment with two levels: burned and unburned. The second factor was soil depth with three levels: 0-10cm, 11-20cm and 21-30cm; all together made up 54 experimental units. Results showed that in all depths there was no significant difference in total soil organic-C between burned and unburned soils. However total soil organic-C contents in both burning treatments were considered high. Similarly, the was also no significant difference in total soil nitrogen between burned and unburned plots. Total soil N was considered in high and medium levels for burned and unburned soil respectively. In summary, burning does not affect soil total organic-C contents but reduces soil total N contents.

Wildfire effects on soil bacterial community and its potential functions in a permafrost region of Canada

Boreal forests in permafrost zone store significant quantities of carbon that are readily threatened by increases in fire frequency and temperature due to climate change. Soil carbon is primarily released by microbial decomposition that is sensitive to environmental conditions. Under increasing disturbances of wildfire, there is a pressing need to understand interactions between wildfires and microbial communities, thereby to predict soil carbon dynamics. Using Illumina MiSeq sequencing of bacterial 16S rDNA and GeoChip 5.0K, we compared bacterial communities and their potential functions at surface and near-surface permafrost layers across a chronosequence (>100 years) of burned forests in a continuous permafrost zone. Postfire soils in the Yukon and the Northwest Territories, Canada, showed a marked increase in active layer thickness. Our results showed that soil bacterial community compositions and potential functions altered in 3-year postfire forest (Fire3) comparing to the unburned forests. The relative abundance of Ktedonobacteria (Chloroflexi) was higher in Fire3 surface soils, while Alphaproteobacteria and Betaproteobacteria (Proteobacteria) were more abundant in unburned ones. Approximately 37% of the variation in community composition can be explained by abiotic variables, whereas only 2% by biotic variables. Potential functional genes, particularly for carbon degradation and anammox, appeared more frequent in Fire3 than in unburned soils. Variations in functional gene pools were mainly driven by environmental factors (39%) and bacterial communities (20%; at phylum level). Unexpectedly, wildfire solely altered bacterial communities and their functional potentials of the surface layers, not the near-permafrost layers. Overall, the response of bacterial community compositions and functions to wildfire and the environment provides insights to re-evaluate the role of bacteria in decomposition.

Effects of Microbial Biomineralization Surface Erosion Control Treatments on Vegetation and Revegetation along Highways

Microbial induced calcite precipitation (MICP) has been widely studied in laboratories to test changes to soil strength and density. Rarely studied is the biogeotechnology’s influence on real-world conditions. Consideration for the natural environment coexisting with treated soil is important, particularly vegetative responses to biochemical and physical changes from treatments. In this factorial designed study, vegetative response from one-time biochemical surficial treatments is observed in four soil growth mediums: two variants burned soil, unburned side slope construction soil, and Ottawa sand. Treatment objectives are to create a light crust that provides short-term erosion control, protects concurrently applied seeds and provides a beneficial plant environment (BPE). The crust creates a BPE through increased soil water retention and shear soil strength allowing better root and plant stability. An overly dense crust prevents root penetration and is avoided because established root systems are crucial in long-term erosion control. This study successfully created such a crust in all soil types treated. Also studied were influences of solution components on germination rates. Component influence proved highly dependent on soil type as calcium chloride inclusion was highly detrimental to seedling success in clean sand, somewhat detrimental to burned soil with ash layer, insignificant in unburned soil, and beneficial to burned soil without ash layer. These results give an indication of the complex biochemical soil reactions occurring from MICP treatment. This study gives evidence that a one-time application of a seeded biochemical solution has real-world potential as a balanced short-term and long-term erosion control technology for burned and construction soils.

Wildfire effects on soil quality: application on a suburban area of West Attica (Greece)

On the occasion of the July 2018 wildfire in the West Attica region (Kineta area, Geraneia Mountain) this study aimed to the assessment of soil quality in Kineta area and the effect of fire on soil properties and the bioavailable forms of copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn), which is a topic poorly documented. Topsoil (0–5 cm depth) and subsoil (5–25 cm depth) samples were collected from burned sites and unburned control sites two months after the wildfire event. The area is mainly covered by pine trees (Pinus halepensis). Statistical analysis and Geographical Information System (GIS) database elaboration proved successful tools for this research. The type of methodology applied in this study consists of Diethylene-Triamine-Penta-Ace-tic acid (DTPA) chemical extraction, pH, Cation Exchange Capacity (CEC), Electric Conductivity (EC) and soil organic matter (OM) evaluation, factor analysis, GIS database and comparison of soil parameters with quality standards and levels given from the literature. Results of this study indicated that wildfires significantly (p < 0.05) increased pH values, as well as, OM, FeDTPA and MnDTPA higher contents in topsoil at burned sites than topsoil in unburned locations. This higher availability of elements in the soil may be associated with the addition of ash during a fire. At the same time, relationships between various parameters have been identified from factor analysis and reflect genetic association. A GIS elaboration showed the spatial relationship among high positive factor scores, lithology, Mn-oxides and soil condition (burned or unburned soil). In particular, ZnDTPA contents in Kineta soils constitute a case of lithogenic contamination, while high concentrations of MnDTPA demonstrate the role of wildfire in enhancing levels of available elements in topsoil.

Effects of wildfire and logging on soil functionality in the short-term in Pinus halepensis M. forests

Salvage logging is thought to have negative impacts on soil functionality because it may increase soil compaction and reduce vegetation cover and soil organic matter content. We investigated whether and to what extent burning and subsequent logging initially altered soil functionality of a Mediterranean forest of Pinus halepensis M. Soil functionality indicators (e.g. soil enzyme activities, basal soil respiration, glomalin-related soil protein, and microbial carbon) were measured in March and October 2017 in unburned forest plots, nearby plots severely burned by wildfire in July 2016, and nearby burned plots severely burned by wildfire and then logged in December 2016 using a lightweight agricultural tractor. The results showed significant differences among three groups: unburned soils sampled in spring (1) and autumn (2), and burned soils (not subject or subject to logging) sampled in spring and autumn. In unburned plots, seasonality had a significant effect, which disappeared in burned plots regardless of whether they had been logged. The burned plots had higher content of organic matter and total nitrogen than the unburned soils but they were not correlated to higher soil respiration or microbial biomass. There were not any differences in any of the soil functionality indicators between the unlogged and logged burned plots. In addition, the burned plots had a higher glomalin-related soil protein content than the unburned soil in the autumn measurement. Overall, the results suggest a short-term wildfire impact of soil properties whereas logging using a lightweight tractor produced no significant impacts in this sparse Mediterranean pine forest.

Microbial quality of surface water and subsurface soil after wildfire

Runoff from wildfire affected areas typically carries high concentrations of fine burned residues or eroded sediment and deposits them in surface water bodies or on subsurface soils. Although the role of wildfire residues in increasing the concentration of chemical contaminants in both environments is known, whether and to what degree wildfire residues may affect microbial contaminants is poorly understood. To examine the effect of wildfire residues on growth and die-off of Escherichia coli (E. coli) —a pathogen indicator, we mixed stormwater with E. coli and suspended particles from the pre- and post-wildfire area in batch reactors and monitored E. coli concentration. E. coli grew initially in the presence of all particles, but the relative E. coli concentration was 10 times lower in the presence of wildfire residues than in natural soil from unaffected areas. Wildfire residues also decreased the persistence of E. coli during a 15-day incubation period. These results indicate that the growth or persistence of E. coli in surface water in the presence of wildfire residues was less than that in the presence of unburned soil particles, potentially due to depletion of nutrient concentration and/or loss of viability of bacteria in the presence of wildfire residues. To examine the transport potential of wildfire residues and their ability to facilitate the transport of E. coli in the subsurface system, suspensions containing wildfire residues and/or E. coli were injected through unsaturated sand columns—a model subsurface system. Transport of wildfire residues in sand columns increased with decreases in the depth and increases in the concentration of particles, but increased transport of wildfire residues did not result in the increased transport of E. coli, suggesting wildfire residues do not facilitate the transport of E. coli. Overall, the results indicate that wildfire residues may not increase the risk of the microbial contamination of surface water or groundwater via subsurface infiltration.

Functional stability of mycorrhizal interactions in woody natives and aliens facing fire disturbance

Changes in soil biota following fire and its successive influence on their interactions with plants may have direct implications on the invasion success of woody alien species in fire-prone ecosystems. In this work, we experimentally addressed if soil biota from burned and unburned areas differentially affect growth and nutrition of two dominant woody natives (Lithraea molleoides and Vachellia caven) and three woody invaders (Gleditsia triacanthos, Ligustrum lucidum and Pyracantha angustifolia) that occur on fire-prone ecosystems of central Argentina. Our results indicate that growth and P content in tissues of both woody natives Lithraea and Vachellia were generally benefited by arbuscular mycorrhizal fungi (AMF) and that these effects did not strongly differ between AMF communities from burned and unburned soils. In the case of alien species, differences in growth and nutrition in response to AMF were related more to the identity of the invader than on the provenance of the soil biota. This study provides evidence on the important role that soil biota may have in the success of woody aliens in these ecosystems and on the functional stability of AMF communities facing disturbances.

Fire Affects Asymbiotic Nitrogen Fixation in Southern Amazon Forests

AbstractIn this study, we investigate the biogeochemical consequences of fire in seasonally flooded Amazon forests, where recent declines in forest cover have been linked to increases in fire frequency and severity. Previous studies have hypothesized that a quasi‐permanent state‐shift transition from typical Amazon forests to open savannas can occur when fire results in further depletion of already impoverished soil nutrient pools. Asymbiotic N2 fixation (ANF) is an essential pathway for fire‐affected forests to acquire nitrogen (N) after disturbance, but ANF response to fire has yet to be quantified in Amazonia. Here, we quantify ANF through field sampling and laboratory incubations using 15N‐labeled dinitrogen (15N2) and measurement of 14 biogeochemical parameters in surface (0–10 cm) and subsurface (10–30 cm) soils. Our data represent burned and unburned replicated sampling sites, across five stands, spanning a gradient from infrequent (once in 13 years) to frequent (five times in 13 years) fire occurrences. ANF did not vary with fire frequency but was, on average, 24% lower in burned than in unburned surface soils across all stands. Burned and unburned subsurface soils had similar ANF rates. About 58% of ANF variance was explained by the joint effect of carbon (C):N ratio and available phosphorus (P) in burned and unburned soils. ANF increased linearly with C:N and P availability in unburned soils, but a highly non‐linear relationship was observed in burned soils. Our findings show that fire alters soil C‐to‐nutrient stoichiometry, which resulted in lower N inputs via ANF into burned relative to unburned tropical forest soils.

Soil water repellency after wildfires in the Blue Ridge Mountains, United States

It is not well understood if wildfires induce soil water repellency in broadleaf deciduous forests, such as those endemic to the Blue Ridge Mountains of the eastern United States. In 2016, widespread wildfires provided an opportunity to study soil water repellency in this region. We selected sites in four locations with low to moderate burn severities, along with unburned controls. We estimated soil water repellency using water drop penetration time measurements from the surface (i.e. ash or organic) layer to ~5cm within the underlying mineral layer. Two months after the fires, water repellency was detected in all locations and was greater in more severely burned sites. One location had the greatest water repellency in surface ash (frequency of occurrence: 68–74%), whereas the other locations showed greatest repellency at the ash–mineral interface (40–96%). Unburned soils rarely showed repellency (0–18%). Burned soils also exhibited water repellency 1 year post fire. The study results suggest that combustion of non-resinous foliage within litter layers can cause water repellency in deciduous forests, meaning that this condition is not exclusive to coniferous and dryland forests. The duration of impact depends on fire severity, and may enhance overland flow and sediment transport in affected landscapes.

Impact of bonfires on soil properties in an urban park in Vilnius (Lithuania)

Lighting bonfires in urban parks is a widespread practice. However, few studies have examined their impact on soil properties. The aim of this study was to analyze the impact of bonfires on the soil properties of an urban park in Vilnius, Lithuania. The properties studied were soil water repellency (SWR), aggregate stability (AS), soil organic matter (SOM) content, total nitrogen (TN), inorganic carbon (IC), pH, electrical conductivity (EC), extractable calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), aluminum (Al), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), boron (B), chromium (Cr), available phosphorus (P), silicon (Si) and sulfur (S). Soil ratios calculated were carbon/nitrogen ratio (C/N), Ca + Mg/(Na + K)1/2 (SPAR), Ca:Al and Ca:Mg. Three areas were studied: Site 1) Pinus sylvestris L. and Quercus robur L.; Site 2) Aesculus glabra Wild.; and Site 3) Pinus sylvestris L. and Acer plantanoides L. At each site, 20 samples were collected (10 within the bonfire area, 10 from a control area). The results showed significantly higher values of SOM, IC, pH, EC, Ca, Mg, Na, K, P, Al, Zn, Cu, Cr, S, C/N ratio, Ca:Al ratio and Ca:Mg ratio in bonfire soils than in control unburned soils. In bonfire soils, significantly lower values were recorded of SWR, AS, TN, SOM, Al, Mn, Fe, Cr, S and SPAR comparing to control soils. Most affected area by the bonfire was Site 1, which presented a marked increase in heavy metal content comparing to the control. The impact of soil heating was evident in AS, IC, pH, extractable Ca, Mg, Na, K, P, Al, Zn, Cu, Cr, S, and in its ratios. Protective measures are needed to limit bonfires and prohibit the burning of hazardous materials.

Effect of soil water‐repellent layer depth on post‐wildfire hydrological processes

AbstractSoil water repellency induced by wildfires can alter hydraulic properties and hydrologic processes; however, the persistence and vertical position (i.e., depth) of water‐repellent layers can vary between systems and fires, with limited understanding of how those variations affect infiltration processes. This study occurred in two forested locations in the south‐central Appalachian Mountains that experienced wildfires in late 2016: Mount Pleasant Wildfire Refuge, Virginia, and Chimney Rock State Park, North Carolina. In each location, sites were selected to represent unburned conditions and low to moderate burn intensities. At each site, we measured the soil water repellency at the surface (ash layer or O horizon) and ~2 cm below the surface (A horizon) using the water drop penetration time method (n= 10–14). Soil water content was also measured over the upper 10 cm of the soil (n= 10), and infiltration tests were conducted using a tension infiltrometer (n= 6–8). The results showed that soil repellency was highest in the surface layer at the Mount Pleasant location and was highest in the subsurface layer at the Chimney Rock location. Soil water content was lower in unburned soil than in burned soil, especially for measurements taken immediately postfire, with soil water content negatively correlated with water repellency. Water repellency in the surface layer significantly reduced relative infiltration rates (estimated as differences between initial and steady‐state rates), whereas subsurface water repellency did not affect relative infiltration. As a result, water repellency persisted longer in sites with surface as opposed to subsurface water repellency. Finally, differences between burned and unburned sites showed that although the wildfires increased the occurrence of water repellency, they did not alter the underlying relationship between relative infiltration and water repellency of the surface soil.

Multiple, Compounding Disturbances in a Forest Ecosystem: Fire Increases Susceptibility of Soil Edaphic Properties, Bacterial Community Structure, and Function to Change with Extreme Precipitation Event

The intensity and frequency of ecosystem disturbances are shifting with climate change, and multiple disturbances in close succession have the potential to compound their independent effects and strongly alter ecosystem structure and function. In this paper, we examine the effects of an extreme precipitation event on a montane forest landscape that was previously decimated by wildfire (37 months prior) relative to an unburned site in the same ecosystem. We assessed responses in soil edaphic properties, bacterial community composition and assembly, and soil enzyme activities involved in carbon (C) and nitrogen (N) acquisition. Our research reveals that previously burned landscapes are susceptible to a subsequent extreme precipitation event via significant increases in soil pH where unburned soils are not. Beta- and Delta-proteobacteria associated with early succession increased and shifts were observed in N- vs. C-acquiring extracellular enzymes within burned soils after the extreme precipitation event. Finally, we connected variation in ecological selective pressures on bacterial communities associated with pH change to these differences in microbial mediated soil enzyme activity. Thus, this research demonstrates how multiple, compounding disturbances drive distinct changes relative to systems experiencing a single disturbance and suggests that changes in bacterial community assembly process with disturbance may underlie this response.

Burn Effects on Soil Aggregate Stability and Water Repellency of Two Soil Types from East and North Tunisia

ABSTRACTTo date, evidence of the potential effects of burning practices on soil properties in Tunisia is limited. In order to address this issue, we carried out laboratory investigations of the effects of burning on soil aggregate stability (AS) and water repellency (WR) of a clayey and a sandy loam Fluvisol soils. The treatments included low (100°C, LT), medium (300°C, MT) and high (600°C, HT), heating temperatures. Unburned (0°C, UB) soil samples were used as a control. Two breakdown mechanisms, fast wetting (FW) and mechanical breakdown (MB), were used for the measurement of AS. The latter is expressed by calculating the mean weight diameter (MWD). The water drop penetration time (WDPT) was used to evaluate the soil WR. The results showed that the unburned clayey and sandy loam soils are poorly aggregated. The HT and MT treatments significantly (p < 0.05) increased MWD of both soils, compared to UB samples, following the FW stability test. A lesser increase of MWD was observed with the MB test. The LT treatment did not significantly (p < 0.05) affect the soils AS. For the unburned clayey soil, the FW and MB tests gave significantly (p < 0.05) different MWDs . In contrast, the unburned sandy loam soil had similar MWDs under both tests. As for water repellency, the sandy loam soil was initially wettable and the clayey soil slightly water repellent. Burning treatments did not affect the sandy loam soil behavior but caused a decrease of clayey soil WR.