Spatiotemporal variability of fire effects on soil carbon and nitrogen: A global meta-analysis

Synergistic interplay of management practices and environmental factors in shaping grassland soil carbon stocks: Insights into the effects of fertilization, mowing, burning, and grazing

Grasslands, which account for over 40 % of the Earth's terrestrial area, play a vital role in mitigating global change and biodiversity loss. These ecosystems serve as critical carbon sinks, regulating the global carbon cycle and supporting diverse flora and fauna. However, their ability to sustain these functions is threatened by land use change and climate disruption. Current challenges revolve around understanding how key management practices such as grazing, mowing, burning, and fertilization, interact with environmental factors to influence grassland soil carbon stocks. This study presents a meta-analysis of the effects of these management practices and environmental factors, such as soil type, depth, texture, temperature, precipitation, and their synergistic interplay. It evaluates how management intensity, duration, and frequency interact with these environmental variables to influence soil carbon storage, providing valuable insights into optimizing grassland management for enhanced soil carbon stock and broader ecosystem stability. The findings reveal that grazing, particularly at high intensity, tends to reduce soil carbon stocks (-0.412, p < 0.001), with the most pronounced effects observed in shallow soils and temperate climates. Mowing also negatively affected carbon stock (-0.416, p = 0.013), especially when carried out frequently and over long durations. On the other hand, burning had mixed results with an overall positive effect (0.340, p = 0.078). Short-term burns promoted carbon accumulation, while frequent burning led to carbon loss. Fertilization, especially with nitrogen and phosphorus, proved beneficial for increasing soil carbon stocks (0.712, p < 0.001), particularly in nutrient-poor soils and semi-arid climates. This study introduces a systems-based approach to grassland management, providing a framework for optimizing carbon-focused strategies. By integrating the role of management practices, particularly their frequency, intensity, and duration, along with soil characteristics and climate, these findings provide actionable insights for policymakers, land managers, and researchers. They guide the development of sustainable management strategies that not only enhance soil carbon stocks but also support ecosystem health and resilience.

Impact of post-anthropogenic forest fire on soil carbon dynamics and physicochemical properties in tropical dry deciduous Sulia Reserve Forest, Odisha

Anthropogenic forest fire is an emerging cause for forest degradation, which primarily alters soil physicochemical characteristics and carbon content. For assessing these effects, the current study compares soils of unburned (as CON), managed (as MAN) planted locations (Shorea robusta, Tectona grandis), and burned (as BUR) locations of Sulia Reserve Forest of Nayagarh, Odisha. Soils were collected from below surface litter (BSL) (0 to 5 cm), top soil (TS) (5 to 15 cm), and sub-soil (SS) (15 to 30 cm) randomly from selected patches. Samples were analysed for colour, texture, pH, bulk density (BD), water holding capacity (WHC), electrical conductivity (EC), potassium (K+), and nitrogen (N). Also, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and Fourier transform infrared spectroscopy (FTIR) were utilised to study the soil mineralogical and surface properties. In comparison to unaffected locations (CON and MAN), the BUR soil showed increased levels of EC, OC (oxidisable carbon), TOC (total organic carbon), SOM (soil organic matter), K+, and available nitrogen (AN). However, suppression in total nitrogen (TN) was observed for BUR locations in BSL samples. In BUR, the range of EC (0.12-0.29 S/m), OC (1.8-2.8%), TOC (2.5-3%), and SOM (4-6%) demonstrated variability in soil physicochemical properties. The BUR soils reflect significant changes in feldspar, haematite, and kaolinite. The results also highlight the C alteration and enhancement of nutrients (K+ and N) availability in BUR soil. The assessment demonstrates complex behaviours of soil properties due to forest fire within a dry deciduous forest and emphasises a location specific management plan to conserve the natural resource.

Thermal Compensatory Response of Soil Heterotrophic Respiration Following Wildfire

Frequent wildfires pose a serious threat to carbon (C) dynamics of forest ecosystems under a warming climate. Yet, how wildfires alter the temperature sensitivity (Q10) of soil heterotrophic respiration (Rh) as a critical parameter determining the C efflux from burned landscapes remains unknown. We conducted a field survey and two confirmatory experiments in two fire-prone regions of China at <1, 3, 6, and 12 months after wildfires (n = 160 soil samples). We found that wildfire generally reduced the Q10 for soil organic and mineral horizons within the first year after wildfire mainly due to substrate depletion, which was confirmed by a uniform inoculation experiment. Mineral protection of organic matter in the mineral horizon rich in iron/aluminum (hydr)oxides and a near-neutral pH in organic horizons of postfire soils further suppressed the Q10. Decreased Q10 persisted in organic horizons even after removing substrate limitation, reflecting the dominance of a thermally adapted, r-strategist microbial community in postfire soils. Moreover, fire-induced low C quality increased Q10, which supported the C quality-temperature hypothesis, but a C-limited condition restricted this stimulatory effect. This study illustrates that a thermal compensatory response of Rh will help maintain C stocks in forest ecosystems after wildfires in a warming world.

Recoupling of Soil Carbon, Nitrogen, and Phosphorus Cycles along a 30 Year Fire Chronosequence in Boreal Forests of China

The biogeochemical coupling of soil carbon, nitrogen, and phosphorus (C-N-P) cycles is crucial for maintaining the ecological balance of boreal forests. Yet, the current understanding of wildfire disturbance is only based on changes in elemental contents, thereby lacking any within-ecosystem corroboration of biogeochemical coupling. Here, we conducted a field survey of microbial functional associations for 53 genes related to soil C-N-P cycling from 17 locations spanning a 30 year succession period after high-severity forest fires in the Greater Khingan Mountains (Inner Mongolia-China). We found that bacteria proliferated and dominated the competition with fungi by encoding genes for recalcitrant C decomposition, N fixation, and inorganic N cycling during the postfire early succession. Wildfire prominently decoupled the microbial functional associations of soil C-N-P cycling, particularly in organic-inorganic N turnover. However, over the 30 year succession period, these functional associations recoupled in both soil organic and mineral horizons. Notably, the decoupling of microbial functional associations recovered from a wildfire disturbance faster than the soil C-N-P imbalance. This strong resilience of the microbiome will aid in the recovery of the soil elemental balance and ecosystem stability from the increased intensity of wildfires projected for the boreal forests of China.

Forest Wildfire Increases the Seasonal Allocation of Soil Labile Carbon Fractions Due to the Transition from Microbial K- to r-Strategists

Promoting the formation and accumulation of soil carbon (C) is one of the natural solutions to address climate change, but frequent wildfires increase its uncertainty and challenge. This two-year study deciphered the driving pathways of seasonal and vertical patterns in a soil C pool following a wildfire from a microbial perspective. Results showed that total organic C concentration and stock postfire decreased by 29.9 and 17.5% on average compared with the unburned control, respectively, whereas the allocations of labile C increased by 25.1-45.7%. Fire-induced alterations in labile C fractions were complicated due to their significant seasonality and respective sensitivities. Nonetheless, we emphasized that microbial life-history traits were the decisive mediators of variations and that significant positive linkages existed between labile C and microbial r-selected communities. Fire stimulated lower bacterial and fungal copiotroph/oligotroph ratios and higher ribosomal ribonucleic acid operon copy number, shifting microbes from K- to r-strategists. From integrated soil C pool management indices, fire can be concluded to reduce C stability and accelerate C cycling, but whether the recaptured prevalence of K-strategist over time will modify C processes remains unknown. This study provided a stepping stone for future efforts in accurate C predictions and reasonable C management.

Fire-driven disruptions of global soil biochemical relationships

Fires alter the stability of organic matter and promote soil erosion which threatens the fundamental coupling of soil biogeochemical cycles. Yet, how soil biogeochemistry and its environmental drivers respond to fire remain virtually unknown globally. Here, we integrate experimental observations and random forest model, and reveal significant divergence in the responses of soil biogeochemical attributes to fire, including soil carbon (C), nitrogen (N), and phosphorus (P) contents worldwide. Fire generally decreases soil C, has non-significant impacts on total N, while it increases the contents of inorganic N and P, with some effects persisting for decades. The impacts of fire are most strongly negative in cold climates, conifer forests, and under wildfires with high intensity and frequency. Our work provides evidence that fire decouples soil biogeochemistry globally and helps to identify high-priority ecosystems where critical components of soil biogeochemistry are especially unbalanced by fire, which is fundamental for the management of ecosystems in a world subjected to more severe, recurrent, and further-reaching wildfires.

Fire Reduces Soil Nitrate Retention While Increasing Soil Nitrogen Production and Loss Globally

Elucidating the response of soil gross nitrogen (N) transformations to fires could improve our understanding of how fire affects N availability and loss. Yet, how internal soil gross N transformation rates respond to fires remains unexplored globally. Here, we investigate the general response of gross soil N transformations to fire and its consequences for N availability and loss. The results showed that fire increased gross N mineralization rate (GNM; +38%) and ammonium concentration (+47%) as a result of decreased soil C/N ratio but decreased microbial nitrate immobilization (INO3; -56%), resulting in increased nitrous oxide (N2O; +50%) and nitric oxide (+121%) emissions and N leaching (+308%). Time since fire affected soil N cycling and loss. Fire increased GNM, ammonium concentration, and N2O emission, and decreased INO3 only when time since fire was less than one year, while increased N leaching in the short (one year) terms. Thus, the consequences of fire were a short-lived increase in N availability and N2O emissions (lasting less than one year) but with persistent risks of N loss by leaching over time. Overall, fire increased the potential risks of N loss by stimulating N production and inhibiting nitrate retention.

Spatial heterogeneity of soil respiration after prescribed burning in Pinus koraiensis forest in China

Soil respiration (RS) is crucial for releasing carbon dioxide (CO2) from terrestrial ecosystems to atmosphere. Prescribed burning (a common forest management tool), along with its important by-product pyrogenic carbon (PyC), can influence the carbon cycle of forest soil. However, few studies explore RS and PyC spatial correlation after prescribed burning. In this study, we investigated the spatial pattern of RS and its influencing factors by conducting prescribed burnings in a temperate artificial Pinus koraiensis forest. RS was measured 1 day (1 d) pre-prescribed burning, 1 d, 1 year (1 yr) and two years (2 yr) after prescribed burning. Significant decrease in RS were observed 1-2 yr After burning (reductions of 65.2% and 41.7% respectively). The spatial autocorrelation range of RS decreased pre-burning (2.72m), then increased post-burning (1 d: 2.44m; 1 yr: 40.14m; 2 yr: 9.8m), indicating a more homogeneous distribution of patch reduction. Pyrogenic carbon (PyC) in the soil gradually decreased in the short term after burning with reductions of 19%, 52%, and 49% (1d., 1 yr And 2 yr After the fire, respectively). However, PyC and RS exhibited a strong spatial positive correlation from 1 d.- 1 yr post-burning. The spatial regression model of dissolved organic carbon (DOC) on RS demonstrated significant positive spatial correlation in all measurements (pre- and post-burning). Microbial carbon to soil nitrogen ratio (MCN) notably influenced RS pre-burning and 1-2 yr post-burning. RS also showed significant spatial correlation in cross-variance with NH4+-N and NO3--N post-burning. The renewal of the PyC positively influenced RS, subsequently affecting its spatial distribution in 1d.- 1yr. Introducing PyC into RS studies helps enhances understanding of prescribed fire effects on forest soil carbon (C) pools, and provides valuable information regarding regional or ecosystem C cycling, facilitating a more accurate prediction of post-burning changes in forest soil C pools.

Stimulated photosynthesis of regrowth after fire in coastal scrub vegetation: increased water or nutrient availability?

Fire-prone landscapes experience frequent fires, disrupting above-ground biomass and altering below-ground soil nutrient availability. Augmentation of leaf nutrients or leaf water balance can both reduce limitations to photosynthesis and facilitate post-fire recovery in plants. These modes of fire responses are often studied separately and hence are rarely compared. We hypothesized that under severe burning, woody plants of a coastal scrub ecosystem would have higher rates of photosynthesis (Anet) than in unburned areas due to a transient release from leaf nutrient and water limitations, facilitating biomass recovery post-burn. To compare these fire recovery mechanisms in regrowing plants, we measured leaf gas exchange, leaf and soil N and P concentrations, and plant stomatal limitations in Australian native coastal scrub species across a burn sequence of sites at 1 year after severe fire, 7 years following a light controlled fire, and decades after any fire at North Head, Sydney, Australia. Recent burning stimulated increases in Anet by 20% over unburned trees and across three tree species. These species showed increases in total leaf N and P as a result of burning of 28% and 50% for these macronutrients, respectively, across the three species. The boost in leaf nutrients and stimulated leaf biochemical capacity for photosynthesis, alongside species-specific stomatal conductance (gs) increases, together contributed to increased photosynthetic rates after burning compared with the long-unburned area. Photosynthetic stimulation after burning occurred due to increases in nutrient concentrations in leaves, particularly N, as well as stomatal opening for some species. The findings suggest that changes in species photosynthesis and growth with increased future fire intensity or frequency may be facilitated by changes in leaf physiology after burning. On this basis, species dominance during regrowth depends on nutrient and water availability during post-fire recovery.

Global net climate effects of anthropogenic reactive nitrogen

Anthropogenic activities have substantially enhanced the loadings of reactive nitrogen (Nr) in the Earth system since pre-industrial times1,2, contributing to widespread eutrophication and air pollution3-6. Increased Nr can also influence global climate through a variety of effects on atmospheric and land processes but the cumulative net climate effect is yet to be unravelled. Here we show that anthropogenic Nr causes a net negative direct radiative forcing of -0.34 [-0.20, -0.50] W m-2 in the year 2019 relative to the year 1850. This net cooling effect is the result of increased aerosol loading, reduced methane lifetime and increased terrestrial carbon sequestration associated with increases in anthropogenic Nr, which are not offset by the warming effects of enhanced atmospheric nitrous oxide and ozone. Future predictions using three representative scenarios show that this cooling effect may be weakened primarily as a result of reduced aerosol loading and increased lifetime of methane, whereas in particular N2O-induced warming will probably continue to increase under all scenarios. Our results indicate that future reductions in anthropogenic Nr to achieve environmental protection goals need to be accompanied by enhanced efforts to reduce anthropogenic greenhouse gas emissions to achieve climate change mitigation in line with the Paris Agreement.

Fire effects on soil CH4 and N2O fluxes across terrestrial ecosystems

Fire, as a natural disturbance, significantly shapes and influences the functions and services of terrestrial ecosystems via biotic and abiotic processes. Comprehending the influence of fire on soil greenhouse gas dynamics is crucial for understanding the feedback mechanisms between fire disturbances and climate change. Despite work on CO2 fluxes, there is a large uncertainty as to whether and how soil CH4 and N2O fluxes change in response to fire disturbance in terrestrial ecosystems. To narrow this knowledge gap, we performed a meta-analysis synthesizing 3615 paired observations from 116 global studies. Our findings revealed that fire increased global soil CH4 uptake in uplands by 23.2 %, soil CH4 emissions from peatlands by 74.7 %, and soil N2O emissions in terrestrial ecosystems (including upland and peatland) by 18.8 %. Fire increased soil CH4 uptake in boreal, temperate, and subtropical forests by 20.1 %, 38.8 %, and 30.2 %, respectively, and soil CH4 emissions in tropical forests by 193.3 %. Additionally, fire negatively affected soil total carbon (TC; -10.3 %), soil organic carbon (SOC; -15.6 %), microbial biomass carbon (MBC; -44.8 %), dissolved organic carbon (DOC; -27 %), microbial biomass nitrogen (MBN; -24.7 %), soil water content (SWC; -9.2 %), and water table depth (WTD; -68.2 %). Conversely, the fire increased soil bulk density (BD; +10.8 %), ammonium nitrogen (NH4+-N; +46 %), nitrate nitrogen (NO3--N; +54 %), pH (+4.4 %), and soil temperature (+15.4 %). Our meta-regression analysis showed that the positive effects of fire on soil CH4 and N2O emissions were significantly positively correlated with mean annual temperature (MAT) and mean annual precipitation (MAP), indicating that climate warming will amplify the positive effects of fire disturbance on soil CH4 and N2O emissions. Taken together, since higher future temperatures are likely to prolong the fire season and increase the potential of fires, this could lead to positive feedback between warming, fire events, CH4 and N2O emissions, and future climate change.

Impact of wildfire on soil characteristics and arbuscular mycorrhizal fungi

This study explored whether wildfire alters the soil properties and arbuscular mycorrhizal fungi (AMF) community composition when compared with burnt rangeland, non-burnt rangeland and adjacent tilled in mesothermal ecosystems. The study was carried out in August 2020, 1 year later after wildfire. The results of this study showed that the wildfire played a key role in altering soil characteristics and AMF community composition in Bartin Province located in the Western Black Sea Region. Soil samples were made according to standard methods. AMF spores were isolated according to the wet sieving method, and the spores of AMF were identified according to their morphological characteristics. Analysis of variance was performed to determine the differences between the parameters, and correlation analysis was performed to determine the relationships between the parameters. The highest values of soil organic carbon (2.20%), total nitrogen (0.18%), K2O (74.68 kg/da), root colonization (87.5%) and the frequency of occurrence of Funneliformis geosporum (20%), Claroideoglomus claroideum (16%) and Claroideoglomus etunicatum (11%) were found in burnt rangeland. Sporulation of Acaulospora dilatata, Acaulospora morrowiae, Acaulospora tuberculata, Scutellospora castanea, Scutellospora coralloidea, Scutellospora scutata, Glomus coremioides and Glomus multicaule was either decreased or completely inhibited in the burnt rangeland. While species diversity of AMF (12) decreased, the number of AMF spores (325.6 (number/50 gr soil)) increased in burnt areas. In conclusion, the number of spores and root colonization of AMF increased but species diversity of AMF reduced after the wildfire. In ecosystems with high fire risk where AMF transfer is planned, it is suggested that it would be more appropriate to select species with an increase in spore number after fire.

Postfire Phosphorus Enrichment Mitigates Nitrogen Loss in Boreal Forests

Net nitrogen mineralization (Nmin) and nitrification regulate soil N availability and loss after severe wildfires in boreal forests experiencing slow vegetation recovery. Yet, how microorganisms respond to postfire phosphorus (P) enrichment to alter soil N transformations remains unclear in N-limited boreal forests. Here, we investigated postfire N-P interactions using an intensive regional-scale sampling of 17 boreal forests in the Greater Khingan Mountains (Inner Mongolia-China), a laboratory P-addition incubation, and a continental-scale meta-analysis. We found that postfire soils had an increased risk of N loss by accelerated Nmin and nitrification along with low plant N demand, especially during the early vegetation recovery period. The postfire N/P imbalance created by P enrichment acts as a "N retention" strategy by inhibiting Nmin but not nitrification in boreal forests. This strategy is attributed to enhanced microbial N-use efficiency and N immobilization. Importantly, our meta-analysis found that there was a greater risk of N loss in boreal forest soils after fires than in other climatic zones, which was consistent with our results from the 17 soils in the Greater Khingan Mountains. These findings demonstrate that postfire N-P interactions play an essential role in mitigating N limitation and maintaining nutrient balance in boreal forests.

Contrasting immediate impact of prescribed fires and experimental summer fires on soil organic matter quality and microbial properties in the forest floor and mineral soil in Mediterranean black pine forest

Prescribed fire (PB) is used to achieve ecological objectives and to reduce fuel hazard thus limiting detrimental impacts of wildfire and appropriate selection of prescription window is critical for these goals. Operational use of PB in the Mediterranean forest is scarce and information about its effects on soil remains incomplete. This study for the first time i) compared the immediate impact of spring and autumn PB and experimental summer fire on key properties of forest floor and mineral topsoil in Mediterranean black pine forest, and ii) assessed the capacity of PB to reduce fuel, with limited immediate impacts on soil. PB significantly reduced the 32.5 % of pre-fire forest floor depth, while summer fire consumed 88.5 % and exposed about 30 % of the mineral soil surface. Mean maximum temperature during fire at the mineral soil surface was 23 °C in PB, in contrast to 128 °C in summer fire, while soil heating at 2 cm depth was negligible in both cases. PB did not cause immediate changes in OM quality parameters, and chemical (C and N concentrations, C/N and pH) and microbiological properties (Cmic, Cmic/C, and β-glucosidase, acid phosphatase and alkaline phosphatase activities) in forest floor or mineral topsoil (0-2 cm). By contrast, summer fire greatly increased OM recalcitrance and reduced Cmic, Cmic/C and enzyme activities in forest floor immediately after fire. In the mineral topsoil, only microbial properties were significantly reduced. The maximum temperature reached during fire in forest floor and topsoil was associated with most of the overall changes in properties in both layers. The findings suggest that prescribed fire can significantly reduce fuel with limited initial impacts on soil. Although these findings are encouraging for operational use of prescribed burning in the ecosystem under study, long-term monitoring of repeated application of the technique on soil properties and other ecosystem components is necessary.

Wildfire impact on soil microbiome life history traits and roles in ecosystem carbon cycling

Wildfires, which are increasing in frequency and severity with climate change, reduce soil microbial biomass and alter microbial community composition and function. The soil microbiome plays a vital role in carbon (C) and nitrogen (N) cycling, but its complexity makes it challenging to predict post-wildfire soil microbial dynamics and resulting impacts on ecosystem biogeochemistry. The application of biogeochemically relevant conceptual trait-based frameworks to the soil microbiome can distill this complexity, enabling enhanced predictability of soil microbiome recovery following wildfire and subsequent impacts to biogeochemical cycles. Conceptual frameworks that have direct links to soil C and N cycling have been developed for the soil microbiome; the Y-A-S framework overviews soil microbiome life history strategies that have tradeoffs with one another and others have proposed frameworks specific to wildfire. Here, we aimed to delineate post-wildfire changes of bacterial traits in western US coniferous forests to inform how severe wildfire influences soil microbiome recovery and resultant biogeochemical cycling. We utilized a comprehensive metagenome-assembled genome catalog from post-wildfire soils representing 1 to 11 years following low- and high-severity burning to identify traits that enable the persistence of microbial taxa in burned soils and influence ecosystem C and N cycling. We found that high-severity wildfire initially selects for fast growers and, up to a decade post-fire, taxa that invest in genes for acquiring diverse resources from the external environment, which in combination could increase soil C losses. This work begins to disentangle how climate change-induced shifts in wildfire behavior might alter microbially mediated soil biogeochemical cycling.

A global synthesis and conceptualization of the magnitude and duration of soil carbon losses in response to forest disturbances

Aim

Forest disturbances are increasing around the globe due to changes in climate and management, deteriorating forests' carbon sink strength. Estimates of global forest carbon budgets account for losses of plant biomass but often neglect the effects of disturbances on soil organic carbon (SOC). Here, we aimed to quantify and conceptualize SOC losses in response to different disturbance agents on a global scale.

Location

Global.

Time Period

1983-2022.

Major Taxa Studied

Forest soils.

Methods

We conducted a comprehensive global analysis of the effects of harvesting, wildfires, windstorms and insect infestations on forest SOC stocks in the surface organic layer and top mineral soil, synthesizing 927 paired observations from 151 existing field studies worldwide. We further used global mapping to assess potential SOC losses upon disturbance.

Results

We found that both natural and anthropogenic forest disturbances can cause large SOC losses up to 60 Mg ha-1. On average, the largest SOC losses were found after wildfires, followed by disturbances from windstorms, harvests and insects. However, initial carbon stock size, rather than disturbance agent, had the strongest influence on the magnitude of SOC losses. SOC losses were greatest in cold-climate forests (boreal and mountainous regions) with large accumulations of organic matter on or near the soil surface. Negative effects are present for at least four decades post-disturbance. In contrast, forests with small initial SOC stocks experienced quantitatively lower carbon losses, and their stocks returned to pre-disturbance levels more quickly.

Main Conclusions

Our results indicate that the more carbon is stored in the forest's organic layers and top mineral soils, the more carbon will be lost after disturbance. Robust estimates of forest carbon budgets must therefore consider disturbance-induced SOC losses, which strongly depend on site-specific stocks. Particularly in cold-climate forests, these disturbance-related losses may challenge forest management efforts to sequester CO2.

Fire effects on soil carbon cycling pools in forest ecosystems: A global meta-analysis

Changes in soil carbon (C) pools driven by fire in forest ecosystems remain equivocal, especially at a global scale. In this study we analyzed data from 232 studies consisting of 1702 observations to investigate whether ecosystem type, climate zone, stand age, soil depth, slope, elevation, and the time since fire in influence of forest soil carbon pools to fire regime (fire type, fire season, fire intensity). Additionally, we explored the potential mechanisms of the relationships between multiple response variables to the fire using linear regression and random forest models. On aggregate, fires significantly increased the mean effect sizes of several key soil carbon cycling components-including microbial biomass carbon (MBC), dissolved organic carbon (DOC), total carbon (TC), pyrogenic carbon (PyC), soil organic matter (SOM), soil organic carbon (SOC) by 0.77, 0.89, 0.87, 1.22, 0.97 and 0.93, respectively, compared to unburned forests ecosystems. However, the fire effects on soil C pools vary widely between environmental factors and duration, and are mediated by factors such as tree species, fire type, and soil layer. A correlation analysis displayed the effects of fire on MBC and DOC were significantly and negatively correlated with elevation. Fire effects on the forest floor and mineral soil indicated significantly increased PyC. SOC and TC in coniferous tree species are the most sensitive to fires, thereby altering important feedback relationships with the fire-vegetatale-climate system. Interestingly, latitude has a stronger influence on SOC than mean annual precipitation or elevation, indicating that variations in latitude play a significant role in regulating the amount of SOC in forest ecosystems. Overall, the results illustrated geographic variation in fire effects on soil C cycling underscores the need for region-specific fire management plans, and help us understand the responses of soil C cycling to fire in forest ecosystems, and facilitate decision-making to forest fire management.

Response and driving factors of soil enzyme activity related to acid rain: a meta-analysis

As a global pollution, acid rain can significantly alter soil physicochemical and biochemical processes, but our knowledge of how acid rain affects soil enzyme activity is still limited. To quantify the overall magnitude and direction of the response of soil enzyme activity to acid rain, we conducted a linear mixed model-based meta-analysis of 40 articles. Our analysis revealed that acid rain decreased enzyme activity by an average of 4.87%. Soil dehydrogenase and protease activities were particularly sensitive to acid rain, with significant inhibitions observed. The effect of acid rain was moderated by acid rain intensity (i.e., H+ addition rate, total H+ added, and acid rain pH) and soil fraction (i.e., rhizosphere and bulk soil). Structural equation modelling further revealed that acid rain suppressed soil microbial biomass by acidifying the soil and that the reduction in microbial biomass directly led to the inhibition of enzyme activity in bulk soil. However, the enzyme activity in the rhizosphere soil was not affected by acid rain due to the rhizosphere effect, which was also not impacted by the decreased soil pH induced by acid rain in rhizosphere. Our study gives an insight into how bulk soil enzyme activity is impacted by acid rain and highlights the need to incorporate rhizosphere processes into acid rain-terrestrial ecosystem models.

Post-fire forest floor succession in a Central European temperate forest depends on organic matter input from recovering vegetation rather than on pyrogenic carbon input from fire

The predicted global increase in the frequency, severity, and intensity of forest fires includes Central Europe, which is not currently considered as a wildfire hotspot. Because of this, a detailed knowledge of long-term post-fire forest floor succession is essential for understanding the role of wildfires in Central European temperate forests. In this study, we used a space-for-time substitution approach and exploited a unique opportunity to observe successional changes in the physical, chemical, and microbial properties of the forest floor in coniferous forest stands on a chronosequence up to 110 years after fire. In addition, we assessed whether the depletion of organic matter (OM) and input of pyrogenic carbon (pyC) have significant effects on the post-fire forest floor succession. The bulk density (+174 %), pH (+4 %), and dissolved phosphorus content (+500 %) increased, whereas the water holding capacity (-51 %), content of total organic carbon and total nitrogen (-50 %), total phosphorus (-40 %), dissolved organic carbon (-23 %), microbial respiration and biomass (-60 %), and the abundance of fungi (-65 %) and bacteria (-45 %) decreased shortly after the fire event and then gradually decreased or increased, respectively, relative to the pre-disturbance state. The post-fire forest floor succession was largely dependent on changes in the OM content rather than the pyC content, and thus was dependent on vegetation recovery. The time needed to recover to the pre-disturbance state was <110 years for physical and chemical properties and < 45 years for microbial properties. These times closely correspond to previous studies focusing on the recovery of forest floor properties in different climate zones, suggesting that the times needed for forest vegetation and forest floor properties to recover to the pre-disturbance state are similar across climate zones.

Mapping Fire Severity in Southwest China Using the Combination of Sentinel 2 and GF Series Satellite Images

Fire severity mapping can capture heterogeneous fire severity patterns over large spatial extents. Although numerous remote sensing approaches have been established, regional-scale fire severity mapping at fine spatial scales (<5 m) from high-resolution satellite images is challenging. The fire severity of a vast forest fire that occurred in Southwest China was mapped at 2 m spatial resolution by random forest models using Sentinel 2 and GF series remote sensing images. This study demonstrated that using the combination of Sentinel 2 and GF series satellite images showed some improvement (from 85% to 91%) in global classification accuracy compared to using only Sentinel 2 images. The classification accuracy of unburnt, moderate, and high severity classes was significantly higher (>85%) than the accuracy of low severity classes in both cases. Adding high-resolution GF series images to the training dataset reduced the probability of low severity being under-predicted and improved the accuracy of the low severity class from 54.55% to 72.73%. RdNBR was the most important feature, and the red edge bands of Sentinel 2 images had relatively high importance. Additional studies are needed to explore the sensitivity of different spatial scales satellite images for mapping fire severity at fine spatial scales across various ecosystems.

Soil biogeochemistry and microbial community dynamics in Pinus pinaster Ait. forests subjected to increased fire frequency

Fire frequency might increase in many fire-dominated ecosystems of the world due to the combined effects of global warming, land-use change and increased human pressures. Understanding how changes in fire frequency can affect the main soil biogeochemical dynamics, as well as the microbial community, in the long term is utmost important. Here we determined the effect of changes in fire frequency and other fire history characteristics on soil C and N dynamics and the main microbial groups (using soil fatty acid profiles), in Pinus pinaster forests from central Spain. Stands were chosen to differ in the number of fires (1 to 3) occurred between 1976 and 2018, in the time elapsed since the last fire and the interval undergone between the last two consecutive fires. We found that, in general, most of the studied biogeochemical and microbial variables showed clear differences between unburned and burned stands. The time elapsed since the last fire was the most important fire history covariable and governed the main soil nutrient dynamics and microbial groups. Recovery to pre-fire values took 30-40 years. Increased wildfire frequency only modified total C and nitrification rate, but results were not consistent between stands burned twice and thrice. The time interval (years) between the last two fires was not a significant covariable. The fact that some stands burnt up to thrice in a period of 43 years supports the strong capacity of this ecosystem to recover, even under an increased fire frequency.

Long-term effects of forest fires on fungal community and soil properties along a hemiboreal Scots pine forest fire chronosequence

We studied long-term effects of forest fires on the dynamics of soil fungal community along a post-fire chronosequence in hemiboreal Scots pine stands in north-western Estonia. Effects of fire on soil and fungi were studied on six sites that differed in time since fire (10, 21, 36, 67, 78 and 181 years ago), without further management interventions. Soil fungal communities along the chronosequence were dominated by soil saprotrophs and ectomycorrhizal (EcM) fungi. Across the chronosequence, the most dominant phylum was Ascomycota. The most abundant OTUs were identified as Umbelopsis sp., Hyaloscyphaceae sp. and Pezoloma ericae with relative abundances of 9.5, 8.9 and 6.8 %, respectively. Fungal species richness was similar among sample areas except in the area where fire occurred 36 years ago, where it was significantly lower. There were considerable differences in EcM fungal species composition along the chronosequence. The most recently burned site had Piloderma sphaerosporum, Pseudotomentella sp. and Clavulinaceae sp. as most abundant EcM OTUs while in three oldest burned areas Clavulinaceae sp. and Cortinarius sp. were abundant. Soil C and N stocks were lower in the most recently burned area but differences with other areas were not statistically significant. Soil pH had a significant effect on fungal species composition. Older areas had substantially lower pH compared to more recently burned areas.