Unburnt Soil Research Articles

Trends and patterns of polycyclic aromatic hydrocarbons (PAHs) in forest fire-affected soils and water mediums with implications on human health risk assessment

Forest fires are extreme natural/artificial events releasing polycyclic aromatic hydrocarbons (PAHs), which are carcinogenic. Most of the released PAHs are trapped in burnt ash, a part of which is transported and settle on different mediums like soil and water. After strong rainfall events, PAHs enter into surface water bodies through surface runoff, thereby deteriorating water quality. Changes in PAHs levels during the post-fire duration and human health risks due to PAHs released from forest fires need attention. This study aim to explain the trends and patterns of PAHs and health risks due to exposure to soil and water contaminated with PAHs. Forest fires release a higher percentage of low molecular weight PAHs (LMW PAHs) than high molecular weight PAHs (HMW PAHs). Ash and burnt soils contain a higher percentage of LMW PAHs since biomass burning releases huge amounts of LMW PAHs. Whereas, sediments contain a higher percentage of HMW PAHs since most of the LMW PAHs are already degraded. HMW PAHs were causing higher risk to humans (both cancer and non-cancer) due to their higher oxidation potential. Exposure to water contaminated by PAHs resulted in higher health risks for both BaP equivalent and a mixture of PAHs. Exposure to sediment produced the highest health risk due to a higher percentage of HMW PAHs, followed by surface water, burnt soil, ash, and unburnt soil. Cancer and non-cancer risk due to dermal exposure was more elevated than oral exposure. The mixture of PAHs in sediment produced a higher average dermal risk for children (2.21E+00 for cancer and 7.69E+03 for non-cancer risk) and oral cancer risk for adults (7.11E-03). However, exposure to BaP equivalent in sediment produced higher oral non-cancer risk (7.01E+02) for children. Thus, effective PAHs monitoring is required in both soil and surface water mediums for ensuring proper treatment in water supply systems.

Effects of wildfire, torrential rainfall and straw mulching on the physicochemical soil properties in a Mediterranean forest

With the effects of fire, weather and post-fire management on soil properties having been studied mostly individually, there is little understanding of the combined effects of wildfire, heavy storm and straw mulching. In this study, we evaluated the changes in soil properties following a high-severity fire, post-fire soil treatment using straw mulch, and a torrential storm in a forest stand in north-western Portugal. The main physicochemical properties of the soil were evaluated in different soil conditions: a) burnt and untreated, b) burnt and mulched areas, and c) unburnt sites on three survey dates 1) soon after wildfire, 2) after the rainstorm, and 3) one year after wildfire. We found that soil water repellency strongly increased ephemerally immediately after wildfire, decrease after the storm and disappeared after one year. Fire reduced the soil organic matter (by 50%, on average), total nitrogen (by 85–90%) and available phosphorous (by about 45%) at both the mulched and untreated sites, and this effect was not changed by the ensuing rainfall. In comparison to the unburnt sites, the pH increased in the burnt soils, and the electrical conductivity decreased. Finally, the dynamics of the major cations and minor elements were affected differently by the wildfire and rainfall under the different soil conditions. One year after the fire, the most notable changes compared to the unburnt soil were detected in magnesium, potassium and almost all the minor elements. Based on these findings, instructions were given to the land managers (aim to control soil hydrophobicity following wildfire, supply more soil organic matter to the soil to avoid a decline in soil fertility, and use alternative actions to straw mulching to control the soil chemistry) that were aimed at more effective post-fire management in severely burnt areas in Mediterranean forests.

Environmental Impact of Bush Burning on the Physico-Chemistry of Mangrove Soil at Eagle Island, Niger Delta, Nigeria

Wetland soils derive their chemical composition from the sedimentary rock underneath. Among the metals produced are trace and nutrient elements which facilitate plant growth. Bush burning near mangrove forest is a common practice that has not been given much attention despite its negative impact on the environment. We thus hypothesize that the application of fire on wetland soil will negatively impact the soil chemistry. We collected soil samples from burnt and unburnt soilsat different distances from the point of burning to compare the concentration of nutrient elements (Calcium (Ca2+), iron (Fe), Magnesium (Mg), Nitrate (NO3-), Phosphate (PO43-) and Potassium (K). The result revealed that there was significant difference between metals (P 0.05). Iron had the highest overall concentration in burnt (10743.75 ± 1508.39 mg/kg) and unburnt (8854.02 ± 1734.86 mg/kg) soils. In contrast, there was no significant difference in the concentration of metals in burnt and unburnt soils (P = 0.07). The order of metal concentration in the burnt soil is Fe > Mg > PO43- > Ca2+> K > NO3- while the order of metal concentration in the unburnt soil is . There was also no correlation between distance and soil metal concentration (r = -0.05; P > 0.05). The result showed that burning does not negatively impact the wetland soil, rather it causes an increase in soil metal concentration probably due to the addition of ash from the burning process, which fertilizes the soil.

Mineral Soils Are an Important Intermediate Storage Pool of Black Carbon in Fennoscandian Boreal Forests.

Approximately 40% of earth's carbon (C) stored in land vegetation and soil is within the boreal region. This large C pool is subjected to substantial removals and transformations during periodic wildfire. Fire-altered C, commonly known as pyrogenic carbon (PyC), plays a significant role in forest ecosystem functioning and composes a considerable fraction of C transport to limnic and oceanic sediments. While PyC stores are beginning to be quantified globally, knowledge is lacking regarding the drivers of their production and transport across ecosystems. This study used the chemo-thermal oxidation at 375°C (CTO-375) method to isolate a particularly refractory subset of PyC compounds, here called black carbon (BC), finding an average increase of 11.6g BC m-2 at 1year postfire in 50 separate wildfires occurring in Sweden during 2018. These increases could not be linked to proposed drivers, however BC storage in 50 additional nearby unburnt soils related strongly to soil mass while its proportion of the larger C pool related negatively to soil C:N. Fire approximately doubled BC stocks in the mineral layer but had no significant effect on BC in the organic layer where it was likely produced. Suppressed decomposition rates and low heating during fire in mineral subsoil relative to upper layers suggests potential removals of the doubled mineral layer BC are more likely transported out of the soil system than degraded in situ. Therefore, mineral soils are suggested to be an important storage pool for BC that can buffer short-term (production in fire) and long-term (cross-ecosystem transport) BC cycling.

Fire decreases gross mineralization rate but does not alter gross nitrification rate in boreal forest soils

Wildfire has several environmental and ecological effects on soil biogeochemistry, including the nitrogen (N) cycle in forest ecosystems. In this study with 30 days of incubation, we examined the effect of a fire event on changes in soil properties, especially gross N mineralization and nitrification processes in boreal forest soils. We also tested how the addition and removal of fire-derived natural pyrogenic carbon (PyC, 1.5%) affected N transformation processes. After incubation, the concentrations of NH4+-N, NO3−-N, total N, and dissolved organic N were largely enhanced in the burnt soils compared to the unburnt soils. Fire significantly decreased gross N mineralization rates from 6.88 μg N g−1 d−1 to 0.87 μg N g−1 d−1 and NH4+ immobilization rates from 6.04 μg N g−1 d−1 to 0.55 μg N g−1 d−1 but did not significantly affect gross N nitrification rates or NO3− immobilization rates on day 30. The removal from the burnt soil or addition to the unburnt soil of PyC had no significant effect on N transformation. The abundances of nirK, nosZ I, narG, and nifH in the burnt soils were the highest across the different treatments. Our study revealed that fire decreased gross N mineralization processes, which was associated with enhanced abundances of N-cycling genes (nirK, nosZ I and narG) in the boreal forest soils.

Fire severity and post‐fire mulching effects on N transformation rates of temperate soils during the first critical winter–spring period

AbstractWildfires are increasing worldwide and, therefore, the extents of their on‐ and off‐site impacts are particularly important on soil erosion and nutrient cycles. To mitigate post‐fire erosion, mulching is increasingly being used, but its effects on the nitrogen (N) cycle are poorly known. To fill this gap of knowledge, gross N fluxes were modelled with Ntrace to compare with unburnt soils (US) the effects of intermediate (BI) and high severity (BH) burning, and post‐fire straw mulching (BIM and BHM). Two wildfires, two‐time points after the fire and 20 soil‐treatment combinations were studied. The mineralization of soil organic N to NH4+ (MSON) increased in BI (1.9–6.9×) with respect to US, with no clear trends observed in BH and mulched treatments. In most soils, NH4+ immobilisation in labile () and recalcitrant soil organic matter (SOM) () was more important than abiotic NH4+ fixation () and together were larger (3.5–11×) in BI than in US, while contrasting results were found depending on time after a fire (BH, BHM, BIM) and wildfire characteristics (BHM, BIM). Autotrophic nitrification () was included in the best model for all soils, whereas heterotrophic nitrification (OSON) was undetectable in most BH soils. Total nitrification ( + OSON) decreased with time after the fire in BH, increased in US and BI, and was differently affected by mulching depending on fire severity and time from the fire. While NO3− immobilisation () was frequently modelled only for BI soils, the dissimilatory NO3− reduction to NH4+ (DNRA) was retained in the best models for all soils and it was reduced by soil burning, whereas a short‐lived increase was induced by mulching. Denitrification (), modelled for most soils, was negatively affected by burning and positively by mulching. In summary, wildfires tended to increase MSON and , decrease DNRA and and had no clear influence on and nitrification. These effects are usually mitigated by mulching which, however, tended to enhance nitrification.Highlights Direction and magnitude of changes in soil gross N fluxes depend on fire severity. Usually, wildfires increased MSON and , and decreased DNRA and . No clear post‐fire trend was found for and nitrification. Fire effects on gross N fluxes are (partially) counterbalanced by mulching.

Rapid colonization of the post‐burn environment improves egg survival in pyrophilic ground beetles

AbstractMany insects are drawn to the heat, ash, and smoke produced by forest fires and arrive in large numbers at recent burns, often while the fire is still active. Some of these insects are pyrophilic and reproduce exclusively in the immediate aftermath of fire but are rarely, if ever, collected from unburnt habitats. Numerous observations made at active fires note an apparent preference among some pyrophilic insects to oviposit exclusively in the burnt portions of trees, raising broader questions about the adaptive benefits of reproduction in the post‐burn environment. Here, we tested whether the reproductive output of pyrophilic ground beetles (i.e., Sericoda spp.) increased in heat‐sterilized soils. In the first experiment, the eggs of Sericoda quadripunctata were reared in three types of soil collected from burnt forests: recently burnt soil (collected 1–2 weeks after the fire), soil collected 1 year after burn, and soil from an unburnt patch of forest adjacent to the fire. Daily monitoring through a dissecting microscope documented extensive predation of eggs by soil microarthropods (mites, springtails, and nematodes), especially in 1‐year‐old and unburnt soil treatments. This led to a second experiment that included the same three treatments and an additional fourth soil treatment: recently, burnt soil reheated to 100°C for 2 h (i.e., reheated soil). In this experiment, male and female pairs (n = 100) of Sericoda obsoleta were reared for 14 days in jars containing 90 g of soil corresponding to each of the four soil treatments. Reproductive output, measured as the number of larvae produced by each breeding pair, was significantly higher in the reheated soil, suggesting that heat sterilization and removal of soil‐dwelling predators improved egg survival. Our findings suggest that pyrophily in insects may have evolved as a means of increasing reproductive output in the post‐burn environment through access to heat‐sterilized ovipositing substrates. Furthermore, the abrupt disappearance of Sericoda and other pyrophilic insects following fire may be explained by suboptimal reproductive conditions as the burn is colonized by other organisms and by local emigration in favor of other recent burns on the landscape.

N cycle in burnt and unburnt soils under different vegetation covers in the Mediterranean region

The Mediterranean area is a fragile semi-arid ecosystem, characterized by a high level of biodiversity. Due to its climatic conditions, fires are frequent. Fires strongly impact the microbial community of the soil and the biogeochemical cycles of the ecosystem. Although the N cycle is crucial, limited data is available about the effects of fires on the microbial community involved in this cycle in these types of environments. The aim of this research was to evaluate the effects that fire has on the amount of microbial populations involved in the different steps of the N cycle in soils under different vegetation cover. To achieve this, surface soils were collected from unburnt and burnt soil of four plant species (holm oak, pine, black locust, and herbs) from inside the Vesuvius National Park that are typical of the Mediterranean maquis. The soils were analyzed for their main abiotic proprieties (pH, water content, concentrations of organic C, total C, total N, NH4+, NO2–, and NO3–). They were also analyzed for their amount of total DNA (DNA yield), eubacterial DNA (16S rDNA), N2-fixer, ammonia oxidizer, archaea ammonia oxidizer, and denitrifier DNA. The largest amount of microbial populations involved in the N cycle of the unburnt soil was observed in the holm oak soil. Fires slightly affected the soil abiotic properties and the DNA yield of the unburnt soil, but significantly increased the amount of eubacteria, nitrifiers, and denitrifiers. The conditions of the pine and holm oak soil encouraged a faster recovery of the amount of microbial populations involved in the N cycle.

Green islands in a sea of fire: the role of fire refugia in the forests of Alberta

Alberta wildfires vary greatly in severity, resulting in a mosaic of burnt, partially burnt, and unburnt forest. These unburnt patches (refugia) within the fire perimeter are critical for the survival of organisms during the fire and the regeneration process. We examined the literature to identify how the fire regimes and landscape features found in Alberta affect the creation and persistence of refugia, the role of refugia for the flora and fauna of Alberta, how climate change is likely to affect refugia, how humans may alter the creation and effectiveness of refugia, and management implications moving forward. Refugia can vary in scale from small areas of unburnt soil or boulders (centimetres to a few metres), to large stands of unburnt trees (many hectares) with different taxa using these refugia across all the spatial scales. Species reliant on habitat connectivity or old growth forest also benefit from refugia as they can use them as stepping-stones between intact habitats or as a lifeboat to recolonize from. The factors influencing what areas remain unburnt are complex and poorly understood but are likely tied to topography, aspect, proximity to waterbodies, weather changes (precipitation and wind direction), time of day during burning, and vegetation type. Areas with the right combination of topography, aspect, and proximity to water have cooler microclimates and higher moisture than the surrounding areas and may remain unburnt throughout multiple fire events, making them persistent refugia. Other areas may remain unburnt by a chance result of weather changes or having the fire pass through at night, making them random refugia. Many of the features that make persistent refugia unlikely to burn (cooler microclimate and higher moisture) will also buffer the effects of climate change. As a result, it is essential we manage the landscape in such a way as to protect areas that act as persistent refugia from industrial activities. In addition, we must restore fire in the landscape to maintain the mosaic of forest caused by mixed-severity fire, especially in the face of climate change, which is projected to increase the severity and frequency of wildfires in Alberta.

Concurrent and rapid recovery of bacteria and protist communities in Canadian boreal forest ecosystems following wildfire

Wildfire plays an important role in restructuring bacterial and protist taxonomic and functional communities in forest ecosystems. Yet, the recovery of bacterial and protist communities following wildfire has not been rigorously investigated across long-term wildfire chronosequences. We compared changes in bacterial and protist alpha diversity, community composition, co-occurrence networks and determinant edaphic factors between burnt and unburnt (reference) soils at 11 different sites representing a 76-year wildfire chronosequence in Canadian boreal forests. The fire increased bacterial and protist alpha diversity and altered community composition (beta diversity) as compared to the site-specific controls during the early recovery stage (<5 years since fire), but had no effect on bacterial and protist communities in the middle (8–20 years) and late (>30 years) recovery stages. Bacterial and protist communities showed similar patterns of change across the wildfire chronosequence. The post-fire recovery of bacterial communities was associated with changes in soil chemical properties (e.g., nutrient status and pH) and vegetation succession. The protist community assembly was co-influenced by changes in bacterial communities and vegetation succession/soil properties. The concurrent recovery of bacterial and protist communities, and their influencing factors of vegetation and soil properties, imply the above- and belowground living communities are strongly linked in wildfire-perturbed forest ecosystems.

Recovery patterns of soil bacterial and fungal communities in Chinese boreal forests along a fire chronosequence

Wildfire has profound and pervasive consequences for forest ecosystems via directly altering soil physicochemical properties and modulating microbial community. In this study, we examined the changes in soil properties and microbial community composition and structure at different periods after highly severe wildfire events (44 plots, 113 samples) in the Chinese Great Khingan Mountains. We also separated charcoal from burnt soils to establish the relationship between microbial community structures in soils and charcoal. We found that wildfire only significantly altered bacterial and fungal β-diversity, but had no effect on microbial α-diversity across a 29-year chronosequence. The network analysis revealed that the complexity and connectivity of bacterial and fungal communities were significantly increased from 17 years after fire, compared with either unburnt soils or soils with recent fires (0–4 years after fire). Differential abundance analysis suggested that bacterial and fungal OTUs were enriched or depleted only during 0–4 years after fire compared with the unburnt soils. In addition, soil pH, dissolved organic C and dissolved organic N were key determinants of soil bacterial and fungal communities during 17–29 years after fire. The fire-derived charcoal provided a new niche for microbial colonization, and microbes colonized in the charcoal had a significantly different community structure from those of burnt soils. Our data suggest that soil bacterial and fungal communities changed significantly during the recovery from fire events in terms of the abundance and co-occurrence networks in the boreal forest ecosystems.

Role of Seasonality and Fire in Regulating the Enzymatic Activities in Soils Covered by Different Vegetation in a Mediterranean Area

As they quickly respond to environmental conditions, soil enzymes, involved in nutrient cycles, are considered good indicators of soil quality. The Mediterranean area is a peculiar environment for climatic conditions and for fire frequency. Therefore, the research aimed to evaluate the role of seasonality and fire on enzymatic activities (i.e., hydrolase, dehydrogenase, and β-glucosidase) in soils covered by herbs, black locust, pine, and holm oak. In addition, the main soil abiotic properties that drive the enzymatic activities were also investigated. In order to achieve the aims, surface soils were collected in unburnt and burnt areas and characterized for water and organic matter contents, pH, concentrations of C and N, and available fractions of Al, Ca, Cu, Fe, Mg, Mn, Na, and Pb. The results highlighted that the soil enzymatic activities were mainly affected by seasonality more than by fire; in unburnt soils, their main drivers were nutrient availabilities, whereas, in burnt soils they were pH, water and organic matter contents, C and N concentrations, and both nutrient and metal availabilities. Finally, holm oak, as compared with herbs, pine, and black locust, conferred higher stability to soils that were affected by seasonality and fire.

Impact of Bush Burning on Chemical Properties of Some Soil in Igboora, Oyo State, Nigeria

Soil is used in agriculture as an anchor and primary nutrient base for plants, and the types of soil and available moisture determine the species of plants that can be cultivated. Bush burning, whether as result of a wildfire or a controlled burning, affects not only the appearance of the landscape, but the quality of the soil. Bush burning method of land clearing is a traditional farming system used as a means of land clearing for crop production. This method of land clearing has both beneficial and detrimental effects on soil physical and chemical properties. Therefore, this study investigated the effects of bush burning on soil chemical properties at different soil depth of 0-30 cm and 30-60 cm respectively base on the rooting depth of crop planted. The experiment was carried out in six selected farms in Igboora, Ibarapa central Local Governmental, Oyo State. The soil sampled were collected from burnt and unburnt experimental soil and analyzed using USDA standard methods for soil analysis for the selected chemical characteristics (pH, Ca2+, Mg2+ Na+. TN and P). Two samples were taken from each burnt and un-burnt locations at depth of 0-30 and 30-60 cm. Paired t-test was used to compare means value of soil chemical properties determined from burnt and un-burnt soil. ANOVA was used for significance difference between soil from burnt and un-burnt soil. pH increased from moderately acidic to slightly acidic, phosphorus content of the soil increased greatly from un-burnt soil to burnt soil at 0-30 cm and 30-60 cm depths from 6.64 to 22.21 ppm and 3.53 to 24.95 ppm, respectively. Similarly, potassium increased from 0.27 to 0.40ppm at 0-30cm depth but decreased from 0.23 to 0.17 ppm at 30-60 cm depth. Nitrogen reduced at both depths from 0.80 to 0.76% and 0.72 to 0.68% respectively. Magnesium also increased from 1.3 cmol/kg to 2.00 cmol/kg and 1.65 to 1.75 cmol/kg at both 0-30 cm and 30-60 cm depth respectively. Whereas calcium showed a reduction from 3.17 to 2.85 cmol/kg and 1.65 to 1.45 cmol/kg at both depths. The variations observed between burnt and un-burnt soil for Ca, Mg, exchangeable acidity, pH, Nitrogen, potassium was significant at p&lt;0.05 probability level. This indicates that bush burning has an impact on soil physical and chemical properties which may affect the suitability of the soil for crop production. Based on this, there is need for environmental education for farmers in the area in order to know the implications of bush burning on soil properties for soil sustainability which will boost food production.

Combined Effects of Wildfire and Vegetation Cover Type on Volcanic Soil (Functions and Properties) in A Mediterranean Region: Comparison of Two Soil Quality Indices.

Mediterranean regions are the most impacted by fire in Europe. The effects of fire on soil greatly vary according to several factors such as vegetation cover type, but they are scarcely studied. Therefore, this research aimed at evaluating the combined impacts of fire and vegetation on single soil characteristics and on the overall soil quality and functionality through two soil quality indices, simple additive (SQI) and a weighted function (SQIFUNCT). In order to reach the aims, burnt and unburnt soils were collected under different vegetation cover types (herbs and shrubs, black locust, pine and holm oak) within the Vesuvius National Park. The soils were analyzed for the main abiotic (water and organic matter content, total C, N, Ca, K, Cu and Pb concentrations, C/N ratio) and biotic (microbial and fungal biomasses, basal respiration, β-glucosidase activity) characteristics. On the basis of the investigated soil characteristics, several soil functions (water retention, nutrient supply, contamination content, microorganism habitat and activities), and the soil quality indices were calculated. The results showed that the impact of fire on soil quality and functionality was mediated by the vegetation cover type. In fact, fire occurrence led to a decrease in water and C/N ratio under herbs, a decrease in C concentration under holm oak and a decrease in Cu and Pb concentrations under pine. Although the soil characteristics showed significant changes according to vegetation cover types and fire occurrence, both the additive and weighted function soil quality indices did not significantly vary according to both fire occurrence and the vegetation cover type. Among the different vegetation cover types, pine was the most impacted one.

Quick qualitative and quantitative changes in reproductive efforts in an open cerrado community: An explorative study of fire-stimulated species

Cerrado is a tropical savanna hotspot that is impacted by frequent wildfires, but where many effects of fire still are poorly understood. This is especially true for the herbaceous strata at the community level, in which reproductive efforts are deeply affected by fire. The objectives of this study were the following: evaluate if fire qualitatively and/or quantitatively affects the production of sexual reproductive structures in an open cerrado community; describe fire-stimulated or fire-inhibited flowering and fruiting species; and evaluate the influence of water availability on flowering and fruiting. The number of species with flowers and fruits/m2 and the number of reproductive units/m2 were sampled in eight 20×20 m2 plots in Chapada Diamantina, Brazil. After the first sampling in the beginning of the rainy season (November 2009), the vegetation in four plots was burned and then the eight plots were resampled over five months. Indexes of fire-stimulated and fire-inhibited flowering and fruiting species were calculated. Non-metric multidimensional scaling and cross-correlation analyses were calculated and differences between the burnt and unburnt plots were compared with an analysis of variance. Sexual reproduction was deeply affected by fire, which changed the composition of reproducing species. Profuse reproduction of graminoids (Poaceae) and geophytic herbs classified as fire-stimulated flowering and/or fire-stimulated fruiting species concomitantly occurred with a high increase of water in the burnt soil compared to the unburnt soil in the first two months after the fires. The sexual reproduction of shrubby species was inhibited by fire. In an objective way, 21 species stimulated or inhibited by fire were recorded. Fire also changed the influence of rainfall on reproductive efforts. This study shows quick qualitative and quantitative effects of fire on open cerrado functioning and highlights the necessity of fire management for biodiversity conservation in savannas.

Impact of burning on soil organic carbon of maize-upland rice system in Mae Chaem Basin of Northern Thailand

Topsoil is the most sensitive to land management practices, and the application of fire has been used for clearing the land and burning crop residues in highlands. Thus, investigating the topsoil organic carbon density (SOCD) is crucial to improve the understanding of its impact. Four sites of highland maize-upland rice systems in the Mae Chaem Basin, Chiang Mai Province, Thailand were selected as study areas and investigated for two years . At each site, burnt and unburnt soil samples were collected separately from depths of 0–10, 10–20, and 20–30 cm. Results showed that burnt soil contained higher SOCD than unburnt soil, with average values of 75.0 and 71.4 Mg C ha−1, respectively; however, a significant difference was not detected. The SOCD of burnt soils increased in the first growing season after the burning 5 months due to the large amounts of ashes and black carbon left after the fire and decreased in the second growing season after the burning 10 months. Burning caused the fluctuation of SOCD, resulting in a slight increase of SOCD in burnt soils by 0.19 Mg C ha−1 year−1, whereas it steadily increased by 2.04 Mg C ha−1 year−1 in unburnt soils across the maize-upland rice system. This study pointed out that burning may not sustain the SOCD in the long term, especially in the highlands.

Environmental Magnetic Properties of Lateritic Soils from Southwestern India

Soil formation is influenced mainly by lithology, climate, organic matter, vegetation, topography, and time. Soil-forming processes have been well studied in temperate regions. However, there is little data on such processes in the tropics. In this work, we have studied the environmental magnetic properties of lateritic soils developed on gneiss and charnockite. We sampled surficial soils from different environmental settings (urban, agricultural, pristine, and gleyed) to determine the processes involved in their formation. A range of environmental magnetic parameters and inter-parametric ratios were investigated to determine the magnetic concentration, grain-size and mineralogy of the soil samples. Redness rating (RR) index was also calculated to estimate the reddening of tropical soils and to examine its relationship with magnetic properties. The pristine soils exhibit the highest average value for χlf and a higher value of χfd%, indicating a greater amount of ultra-fine magnetic minerals. The gleyed and agricultural soils are weakly magnetic, probably be due to waterlogging, and are characterized by a coarser magnetic grain size. A high proportion of “soft” ferrimagnetic minerals like magnetite and titanomagnetite is present in the samples (average S-ratio ~ 0.90). Hematite content is the highest in unburnt soils and the least in gleyed soils (unburnt > burnt > pristine > urban > agricultural > gleyed). The high concentration of hematite in these soils could be due to transformation of maghemite, resulting from low-temperature oxidation of magnetite. We documented a distinction between the magnetic properties of burnt and unburnt soil types as fire can significantly enhance the magnetic values of unburnt soils. The concentration of hematite (indicated by HIRM) and redness rating (RR) index are fairly well correlated for all the sample types with the exception of urban (r = 0.51; p = 0.11; n = 11) and unburnt soils (r = 0.60; p = 0.28; n = 5), which show a strong correlation. Therefore, RR index can be effectively used as an indicator of reddening of tropical soils due to pedogenesis. This study demonstrates that environmental magnetism can aid in differentiating the diverse environments of soils even though they developed on a similar rock type and under similar climatic conditions.

Habitat heterogeneity induced by pyrogenic organic matter in wildfire-perturbed soils mediates bacterial community assembly processes.

Although pyrogenic organic matter (PyOM) generated during wildfires plays a critical role in post-fire ecosystem recovery, the specific mechanisms by which PyOM controls soil microbial community assembly after wildfire perturbation remain largely uncharacterized. Herein we characterized the effect of PyOM on soil bacterial communities at two independent wildfire-perturbed forest sites. We observed that α-diversity of bacterial communities was the highest in wildfire-perturbed soils and that bacterial communities gradually changed along a sequence of unburnt soil → burnt soil → PyOM. The microbial communities reconstructed from unburnt soil and PyOM resembled the real bacterial communities in wildfire-perturbed soils in their α-diversity and community structure. Bacterial specialists in PyOM and soils clustered in phylogenetic coherent lineages with intra-lineage pH-niche conservatism and inter-lineage pH-niche divergence. Our results suggest that PyOM mediates bacterial community assembly in wildfire-perturbed soils by a combination of environmental selection and dispersal of phylogenetic coherent specialists with habitat preference in the heterogeneous microhabitats of burnt soils with distinct PyOM patches.

Influence of fire retardant and pyrogenic carbon on microscale changes in soil nitrogen and phosphorus

During wildfire season in the western US, fire retardant chemicals are dropped from aircraft in an effort to control the spread of fire. Fire retardant dropped on sites that are not actively burning results in exceptionally high soil nitrogen (N) and phosphorus (P) “fertilization” effect on wildland soils impacting terrestrial and aquatic ecosystems. Herein, we used microdialysis to evaluate the short-term spatiotemporal dynamics of soil inorganic N and ortho-P fluxes in response to wood pyrogenic carbon (PyC) on soils receiving fire retardant (Phos-Chek) in a 28-d column experiment. Retardant additions to soil induced dramatic increases in soil inorganic N and ortho-P flux rates. The addition of wood PyC to soils with retardant significantly and immediately reduced ortho-P flux rates at multiple depths (1, 5, and 10 cm) and reduced NH4+ flux rates at 10 cm while retaining flux rates at 1 and 5 cm. These effects were observed throughout the course of the experiment and were more pronounced towards the end of the experiment. By d-28, PyC significantly reduced the NH4+, NO3−, and ortho-P accumulation on ionic resins at soil column bottoms by 50%, 52%, and 43%, respectively. The application of wood PyC may be effective at buffering excess nutrient fluxes from fire retardant and reduce short-term nutrient leaching in unburnt forest soils exposed to fire retardant.

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.