High-frequency fire alters C : N : P stoichiometry in forest litter

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.

Effect of plant-soil system on the restoration of community stability after wildfire in the northeast margin of Qinghai-Tibet plateau

Wildfires, as an environmental filter, are pivotal ecological disturbances that reshape plant communities and soil dynamics, playing a crucial role in regulating biogeographic patterns and ecosystem services. In this study, we aim to explore the effects of wildfires on forest ecosystems, specifically focusing on the plant-soil feedback mechanisms within the northeastern margin of the Qinghai-Tibet Plateau (QTP). Utilizing Partial Least Squares Path Modeling (PLS-PM), we investigated the interrelationships among soil physicochemical properties, enzyme activities, species diversity, and community stability at varying post-fire recovery stages (5, 15, and 23 years). Results indicated that in the early recovery stages, rapid changes in soil properties such as decreased pH (p < 0.001) and increased nutrient availability facilitate the emergence of early successional species with high resource utilization traits. As the ecosystem evolved toward a climax community, the soil and vegetation exhibit increased stability. Furthermore, soil enzyme activities displayed dynamic patterns that corresponded with changes in soil nutrient content, directly influencing the regeneration and diversity of plant communities. Importantly, our study documented a transition in the influence of soil properties on community stability from direct positive effects in initial recovery phases to negative impacts in later stages, while indirect benefits accrue through increased species diversity and enzyme activity. Vegetation composition and structure changed dynamically with recovery time during community succession. Plant nutrient absorption and accumulation affected nutrient dynamics in the soil, influencing plant regeneration, distribution, and diversity. Our results underscore the complex interactions between soil and vegetation that drive the recovery dynamics post-wildfire, highlighting the resilience of forest ecosystems to fire disturbances. This study contributes to the understanding of post-fire recovery processes and offers valuable insights for the management and restoration of fire-affected forest ecosystems.

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.

Fire shapes fungal guild diversity and composition through direct and indirect pathways

Fire has shaped global ecosystems for millennia by directly killing organisms and indirectly altering habitats and resources. All terrestrial ecosystems, including fire-prone ecosystems, rely on soil-inhabiting fungi, where they play vital roles in ecological processes. Yet our understanding of how fire regimes influence soil fungi remains limited and our knowledge of these interactions in semiarid landscapes is virtually absent. We collected soil samples and vegetation measurements from sites across a gradient in time-since-fire ages (0-75 years-since-fire) and fire frequency (burnt 0-5 times during the recent 29-year period) in a semiarid heathland of south-eastern Australia. We characterized fungal communities using ITS amplicon-sequencing and assigned fungi taxonomically to trophic guilds. We used structural equation models to examine direct, indirect and total effects of time-since-fire and fire frequency on total fungal, ectomycorrhizal, saprotrophic and pathogenic richness. We used multivariate analyses to investigate how total fungal, ectomycorrhizal, saprotrophic and pathogenic species composition differed between post-fire successional stages and fire frequency classes. Time-since-fire was an important driver of saprotrophic richness; directly, saprotrophic richness increased with time-since-fire, and indirectly, saprotrophic richness declined with time-since-fire (resulting in a positive total effect), mediated through the impact of fire on substrates. Frequently burnt sites had lower numbers of saprotrophic and pathogenic species. Post-fire successional stages and fire frequency classes were characterized by distinct fungal communities, with large differences in ectomycorrhizal species composition. Understanding the complex responses of fungal communities to fire can be improved by exploring how the effects of fire flow through ecosystems. Diverse fire histories may be important for maintaining the functional diversity of fungi in semiarid regions.

Fire does not transform shrublands of Echinospartum horridum (Vahl) Rothm. into grasslands in the Pyrenees: Development of community structure and nutritive value after single prescribed burns

Prescribed fire has been extensively used in recent years to control woody encroachment into mountain and other grassland-dominated landscapes. In the Aragon Pyrenees, prescribed burns have been mainly used to remove the native thorny shrub Echinospartum horridum (Vahl) Rothm., whose populations are spreading to the detriment of grasslands. To study the effectiveness of the burning of E. horridum to preserve grasslands for livestock grazing, the vegetation of six sites burned 0.5, 2, 3, 6, 15 and 35 years ago was sampled and compared with that of nearby unburned shrubland (control) and grassland (reference). In addition, the nutritional quality of E. horridum was examined and compared to that of the reference grassland to evaluate to what extent shrub growth can be controlled by herbivores after burning. Initial shrub cover recovered as early as 15 years after fire, with E. horridum being dominant. Plant diversity was greatest at intermediate number of years after fire. Initial floristic composition and life-form spectrum were restored 15-35 years after burning. Echinospartum horridum exhibited early lignification that restricts its availability as a palatable forage for the first two years after burning and makes it unlikely to be consumed thereafter, highlighting the difficulty in controlling the expansion of this species by livestock herbivory. The analysis of the nutrient levels suggested an increased shortage of limiting elements, such as phosphorus or sulfur, in the mid-term after burning due to substantial nutrient losses and exports during and after the burn. Our results question the suitability and sustainability of a single prescribed burn as management tool alone to control the expansion of E. horridum and call for caution in its application for fighting shrub encroachment in the Central Pyrenees.

The Effects of Fire Disturbance on Litter Decomposition and C:N:P Stoichiometry in a Larix gmelinii Forest Ecosystem of Boreal China

Fire disturbance can affect the function of the boreal forest ecosystem through litter decomposition and nutrient element return. In this study, we selected the Larix gmelinii forest, a typical forest ecosystem in boreal China, to explore the effect of different years (3 years, 9 years, 28 years) after high burn severity fire disturbance on the decomposition rate (k) of leaf litter and the Carbon:Nitrogen:Phosphorus (C:N:P) stoichiometry characteristics. Our results indicated that compared with the unburned control stands, the k increased by 91–109% within 9 years after fire disturbance, but 28 years after fire disturbance the decomposition rate of the upper litter decreased by 45% compared with the unburned control stands. After fire disturbance, litter decomposition in boreal forests can be promoted in the short term (e.g., 9 years after a fire) and inhibited in the long term (e.g., 28 years after a fire). Changes in litter nutrient elements caused by the effect of fire disturbance on litter decomposition and on the C, N, and C:N of litter were the main litter stoichiometry factors for litter decomposition 28 years after fire disturbance. The findings of this research characterize the long-term dynamic change of litter decomposition in the boreal forest ecosystem, providing data and theoretical support for further exploring the relationship between fire and litter decomposition.

Remarkable Resilience of Forest Structure and Biodiversity Following Fire in the Peri-Urban Bushland of Sydney, Australia

In rapidly urbanizing areas, natural vegetation becomes fragmented, making conservation planning challenging, particularly as climate change accelerates fire risk. We studied urban forest fragments in two threatened eucalypt-dominated (scribbly gum woodland, SGW, and ironbark forest, IF) communities across ~2000 ha near Sydney, Australia, to evaluate effects of fire frequency (0–4 in last 25 years) and time since fire (0.5 to >25 years) on canopy structure, habitat quality and biodiversity (e.g., species richness). Airborne lidar was used to assess canopy height and density, and ground-based surveys of 148 (400 m2) plots measured leaf area index (LAI), plant species composition and habitat metrics such as litter cover and hollow-bearing trees. LAI, canopy density, litter, and microbiotic soil crust increased with time since fire in both communities, while tree and mistletoe cover increased in IF. Unexpectedly, plant species richness increased with fire frequency, owing to increased shrub richness which offset decreased tree richness in both communities. These findings indicate biodiversity and canopy structure are generally resilient to a range of times since fire and fire frequencies across this study area. Nevertheless, reduced arboreal habitat quality and subtle shifts in community composition of resprouters and obligate seeders signal early concern for a scenario of increasing fire frequency under climate change. Ongoing assessment of fire responses is needed to ensure that biodiversity, canopy structure and ecosystem function are maintained in the remaining fragments of urban forests under future climate change which will likely drive hotter and more frequent fires.

Allocation Strategies of Carbon, Nitrogen, and Phosphorus at Species and Community Levels With Recovery After Wildfire

Plant stoichiometry and nutrient allocation can reflect a plant's adaptation to environmental nutrient changes. However, the allocation strategies of carbon (C), nitrogen (N), and phosphorus (P) between leaf and fine root in response to wildfire have been poorly studied. Our primary objective was to elucidate the trade-off of elemental allocation between above- and belowground parts in response to the soil nutrient changes after a wildfire. We explored the allocation sloping exponents of C, N, and P between leaf and fine root at the species and community levels at four recovery periods (year 2, 10, 20, and 30) after moderately severe wildfire and one unburned treatment in boreal forests in Great Xing'an Mountains, northeast China. Compared with the unburned treatment, leaf C concentration decreased and fine root C increased at year 2 after recovery. The leaf N concentration at year 10 after recovery was higher than that of unburned treatment. Plant growth tended to be limited by P concentration at year 10 after recovery. Nutrient allocation between leaf and fine root differed between species and community levels, especially in the early recovery periods (i.e., 2 and 10 years). At the community level, the nutrient concentrations of the leaf changed more as compared to that of the fine root at year 2 after recovery when the fine root nutrients changed more than those of the leaf. The different C, N, and P allocation strategies advanced the understanding of plant adaptation to soil nutrient changes during the postfire ecosystem restoration.

Post-fire dynamics of ectomycorrhizal fungal communities in a Scots pine (Pinus sylvestris L.) forest of Poland

Background

Global warming and drying have markedly enhanced in most forests the risk of fires across the world, which can affect the taxonomic and functional composition of key tree-associated organisms such as ectomycorrhizal (ECM) fungi. The present study was conducted to characterise the alterations in the extent of root ECM colonisation, the ECM fungal communities, and their exploration types (i.e., indicator of ECM soil foraging strategies) in regenerated pines within a burned site as compared with an unburned site (five years after the fire event) in the Forest District Myszyniec, Poland.

Methods

To assess the ECM fungal communities of burned and control sites, soil soil-root monoliths were collected from the study sites in September 2019. A total of 96 soil subsamples were collected for soil analysis and mycorrhizal assessment (6 trees × 2 sites × 4 study plots × 2 microsites (north and south) = 96 subsamples).

Results

The percentage of root ECM colonisation was significantly lower in the burned site in comparison with the unburned (control) site. However, the ECM species richness did not differ between the control and burned sites. The identified ECM species in both sites were Imleria badia, Thelephora terrestris, Russula paludosa, R. badia, R. turci, R. vesca, Lactarius plumbeus, Phialocephala fortinii, and Hyaloscypha variabilis. The most frequent species in the burned and control sites were I. badia and T. terrestris, respectively. The relative abundances of contact, medium-distance smooth and long-distance exploration types in the burned site were significantly different from the control site, dominated by the medium-distance exploration type in both sites. The abundance of the long-distance exploration type in the burned site was markedly greater (27%) than that of the control site (14%), suggesting that the fire event had favoured this ECM foraging strategy. The results demonstrated that the fire led to reduced ECM colonisation of Scots pine trees in the burned site whereas the species richness was not affected, which can be attributed to degrees of fire-resistance in the ECM species, survival of ECM propagules in deeper soil layers, and/or continuous entry of spores/propagules of the ECM fungi from the adjacent forests via wind, water run-off or animals.

Litter removal through fire - A key process for wetland vegetation and ecosystem dynamics

Fire is a major driver of global vegetation patterns. It strongly reduces litter and thus alters physical and chemical properties of the environment. Studies investigating the interplay of fire and litter are scarce, and wetland ecosystems are strongly under-represented in research focusing on litter dynamics. We present data on short-term effects of fires in floodplain wetlands along the Amur River in the Russian Far East, an area with a high fire recurrence rate. We analysed vegetation and plant growth patterns as well as soil temperature and nutrient concentrations on recently burnt and unburnt control plots. Directly after fire, litter was reduced by more than 50% on burnt plots. This effect was no longer visible 15 months after fire, probably due to the high productivity of the floodplain ecosystem. Litter was found to act as a key determinant in the net of direct and indirect fire effects, by influencing early plant growth patterns of herbs and grasses. Furthermore, litter removal through fire significantly increased plant species diversity and soil temperature. Contrary, N and P concentrations in living plant biomass of grasses and herbs decreased with decreasing litter cover. Combustion during burning seems to be responsible for the negative direct fire impacts on nutrient concentrations, which were found for N and Mg. Litter removal through fire can strongly affect diversity patterns, dominance structures, and nutrient cycling in wetlands. With increasing fire frequency in the course of global change, significant structural and compositional changes in herbaceous wetland vegetation must be anticipated and the studied ecosystem may shift to reinforced N-limitation.

High fire frequency reduces soil fertility underneath woody plant canopies of Mediterranean ecosystems

Spatial heterogeneity of soil properties plays a major role in regulating ecosystem structure and functioning. In general, soil resources accumulate beneath woody plant-covered patches more than in the open interspace, making them function as fertility islands. Whilst wildfire is a common disturbance, little information is available on the role of particular plant species in maintaining soil fertility underneath in areas that are subjected to recurrent fires. This is an important issue given that land abandonment, together with a warmer and drier climate, is increasing fire danger in regions such as the Mediterranean. We determined whether increasing fire frequency, producing changes from a Quercus ilex L., woodland to a shrubland, modifies the effect of woody plant canopy on soil fertility. Additionally, the effect of fire history on species-specific leaf and litter nutrient concentration was assessed. Areas affected by none, one, two or three fires were selected. Within each area, soil fertility was measured underneath Cistus ladanifer L., Retama sphaerocarpa L., Phillyrea angustifolia L. and Quercus ilex canopies and in open interspace. Unburned soils located underneath P. angustifolia and Q. ilex canopies were significantly more fertile than in open interspaces. The microsite effect on soil fertility was fire frequency dependent. As fire frequency increased, the plant canopy microsite effect decreased for soil organic matter (SOM), cation exchange capacity (CEC), total C, P, Ca, K and Mg, labile phosphate, arylsulfatase and acid phosphatase activities. Total N, ammonium, nitrate and β-glucosidase activity decreased with increasing fire frequency, but their spatial variability was maintained along all fire frequency scenarios. Fire frequency decreased foliar N concentration but increased P concentration in some species, leading to a decrease in their N:P ratio. Our findings suggest that soil fertility heterogeneity will be reduced with increasing fire frequency. This could compromise the recovery of soil and ecosystem functioning.

Frequent fire slows microbial decomposition of newly deposited fine fuels in a pyrophilic ecosystem

Frequent fires maintain nearly 50% of terrestrial ecosystems, and drive ecosystem changes that govern future fires. Since fires are dependent on available plant or fine fuels, ecosystem processes that alter fine fuel loads like microbial decomposition are particularly important and could modify future fires. We hypothesized that variation in short-term fire history would influence fuel dynamics in such ecosystems. We predicted that frequent fires within a short-time period would slow microbial decomposition of new fine fuels. We expected that fire effects would differ based on dominant substrates and that fire history would also alter soil nutrient availability, indirectly slowing decomposition. We measured decomposition of newly deposited fine fuels in a Longleaf pine savanna, comparing plots that burned 0, 1, 2, or 3 times between 2014 and 2016, and which were located in either close proximity to or away from overstory pines (Longleaf pine, Pinus palustris). Microbial decomposition was slower in plots near longleaf pines and, as the numbers of fires increased, decomposition slowed. We then used structural equation modeling to assess pathways for these effects (number of fires, 2016 fuel/fire characteristics, and soil chemistry). Increased fire frequency was directly associated with decreased microbial decomposition. While increased fires decreased nutrient availability, changes in nutrients were not associated with decomposition. Our findings indicate that increasing numbers of fires over short-time intervals can slow microbial decomposition of newly deposited fine fuels. This could favor fine fuel accumulation and drive positive feedbacks on future fires.

Distinct fungal successional trajectories following wildfire between soil horizons in a cold-temperate forest

Soil fungi represent a major component of below-ground biodiversity that determines the succession and recovery of forests after disturbance. However, their successional trajectories and driving mechanisms following wildfire remain unclear. We examined fungal biomass, richness, composition and enzymes across three soil horizons (Oe, A1 and A2) along a near-complete fire chronosequence (1, 2, 8, 14, 30, 49 and c. 260 yr) in cold-temperate forests of the Great Khingan Mountains, China. The importance of soil properties, spatial distance and tree composition were also tested. Ectomycorrhizal fungal richness and β-glucosidase activity were strongly reduced by burning and significantly increased with 'time since fire' in the Oe horizon but not in the mineral horizons. Time since fire and soil C : N ratio were the primary drivers of fungal composition in the Oe and A1/A2 horizons, respectively. Ectomycorrhizal fungal composition was remarkably sensitive to fire history in the Oe horizon, while saprotroph community was strongly affected by time since fire in the deeper soil horizon and this effect emerged 18 years after fire in the A2 horizon. Our study demonstrates pronounced horizon-dependent successional trajectories following wildfire and indicates interactive effects of time since fire, soil stoichiometry and spatial distance in the reassembly of below-ground fungal communities in a cold and fire-prone region.

Temporal variations in litterfall biomass input and nutrient return under long-term prescribed burning in a wet sclerophyll forest, Queensland, Australia

Litterfall helps maintaining nutrient return in forest ecosystems. However, the influence of long-term prescribed burning on the dynamics of litterfall biomass and carbon (C) and nitrogen (N) cycling is poorly understood. A 39-year old prescribed burning field trial in a wet sclerophyll forest, southeast Queensland, Australia, was used to investigate the interactive effects of prescribed fire regimes and temporal variation on the quantity and quality of litterfall and C and N return. Treatments included no burning (NB) since 1969, 2 yearly burning (2yrB; burned 19 times) and 4 yearly burning (4yrB; burned 9 times) since 1972. Litterfall was collected monthly on 32 occasions between 2011 and 2013. Significant temporal variation was observed in monthly and annual litterfall biomass. Both burning treatments had lower monthly inputs of total litterfall and leaf litter, mean annual cumulative litter biomass, litter C concentrations and C return via leaf litter, compared with the NB treatment. Most significant reductions in litter N concentrations and N return via litter were associated with 2yrB treatment. The 4yrB and the NB treatments did not differ significantly in terms of twig biomass, litterfall C:N ratios and N return via leaf litter. Despite both long-term prescribed burning treatments negatively impacting C return to the soil by reducing the quantity and quality of litter inputs, previous studies at the site suggest no difference in 0-10 cm soil organic carbon levels between the 4yrB treatment and the unburnt treatment. Hence a longer period of prescribed burning at the 4yrB frequency is likely required before lower C return translates to differences in ecosystem productivity in this wet sclerophyll forest ecosystem. The 2yrB can potentially alter forest C and N cycling and net primary productivity, but these alterations are unlikely to be detected through short-term studies.

Responses of labile soil organic carbon and nitrogen pools to long-term prescribed burning regimes in a wet sclerophyll forest of southeast Queensland, Australia

Soil labile organic carbon (C) and nitrogen (N) pools play a central role in nutrient cycling, while fire is a key driver of biogeochemical cycle, shaping ecosystem structure and functioning. However, how soil labile organic C and N responds to the long-term repeated prescribed fire is largely unknown. In this study, a prescribed fire field experiment in a wet sclerophyll forest established in 1972 in southeast Queensland was used to evaluate the long-term impacts of different fire frequency regimes on labile organic C and N measured by different extraction methods. The fire frequency regimes included long unburnt (NB), burnt every two years (2yrB) and burnt every four years (4yrB). Results revealed that the 2yrB treatment had significantly lower C and N concentrations in hot water and K₂SO₄ extracts and in density fractions (LF_D<2.3 and HF_D>1.6) compared with the NB treatment. Concentrations of carbohydrate-C in hot water extracts and acid soluble and insoluble organic matter-C in cold-water extracts followed a similar trend. The maximum reduction was observed for carbohydrate‑C (72%) and the hot water extractable N (54%) in the 2yrB treatment compared with the NB treatment, showing these parameters are most sensitive indicators. However, there was no significant difference in most of the above parameters between the 4yrB and the NB treatments, indicating that less frequent fire (4yrB) allows the ecosystem to have sufficient time to recover from fire disturbance and may be a sustainable practice for fire management in this wet sclerophyll forest ecosystem.

Fire severity effects on soil carbon and nutrients and microbial processes in a Siberian larch forest

Fire frequency and severity are increasing in tundra and boreal regions as climate warms, which can directly affect climate feedbacks by increasing carbon (C) emissions from combustion of the large soil C pool and indirectly via changes in vegetation, permafrost thaw, hydrology, and nutrient availability. To better understand the direct and indirect effects of changing fire regimes in northern ecosystems, we examined how differences in soil burn severity (i.e., extent of soil organic matter combustion) affect soil C, nitrogen (N), and phosphorus (P) availability and microbial processes over time. We created experimental burns of three fire severities (low, moderate, and high) in a larch forest in the northeastern Siberian Arctic and analyzed soils at 1, 8 days, and 1 year postfire. Labile dissolved C and N increased with increasing soil burn severity immediately (1 day) postfire by up to an order of magnitude, but declined significantly 1 week later; both variables were comparable or lower than unburned soils by 1 year postfire. Soil burn severity had no effect on P in the organic layer, but P increased with increasing severity in mineral soil horizons. Most extracellular enzyme activities decreased by up to 70% with increasing soil burn severity. Increasing soil burn severity reduced soil respiration 1 year postfire by 50%. However, increasing soil burn severity increased net N mineralization rates 1 year postfire, which were 10-fold higher in the highest burn severity. While fires of high severity consumed approximately five times more soil C than those of low severity, soil C pools will also be driven by indirect effects of fire on soil processes. Our data suggest that despite an initial increase in labile C and nutrients with soil burn severity, soil respiration and extracellular activities related to the turnover of organic matter were greatly reduced, which may mitigate future C losses following fire.

Long-term effects of wildfire on available soil nutrient composition and stoichiometry in a Chinese boreal forest

Wildfire disturbance is a major driver of biogeochemical processes in Eurasian boreal forests, yet little is known about the response of soil nutrient stoichiometry to wildfire in this ecosystem. To fill this gap, we measured the composition of available soil nutrients and their stoichiometric ratios in a Chinese boreal forest along a gradient of fire history. In the 1-year-post-fire site, wildfire increased the relative abundances of element nitrogen (N), phosphorus (P), Sulphur (S), iron (Fe), and aluminum (Al), and reduced the abundances of element calcium (Ca), magnesium (Mg), and potassium (K). Available soil N:K, N:S, P:K, P:S, and S:K ratios were 240%, 70%, 440%, 160%, and 150% higher than the control, but N:P ratio was not significantly different. In the 11-year-post-fire site, the soil nutrient composition recovered to the pre-fire levels. Although most of the soil nutrient stoichiometry returned to pre-fire levels, soil N:P ratios became significantly higher. These results showed immediate wildfire effects on soil nutrient availability and composition were strongly related to fire severity, but such effects could be subdued by soil environment and topographical variations over time. Although wildfire effects on soil nutrients are mostly short-term, it could produce relatively long-term effects on balance between N and P.

Flexible Carbon-Use Efficiency across Litter Types and during Decomposition Partly Compensates Nutrient Imbalances-Results from Analytical Stoichiometric Models

Mathematical models involving explicit representations of microbial processes have been developed to infer microbial community properties from laboratory and field measurements. While this approach has been used to estimate the kinetic constants related to microbial activity, it has not been fully exploited for inference of stoichiometric traits, such as carbon-use efficiency (CUE). Here, a hierarchy of analytically-solvable mass-balance models of litter carbon (C) and nitrogen (N) dynamics is developed, to infer decomposer CUE from measured C and N contents during litter decomposition. The models are solved in the phase space—expressing litter remaining N as a function of remaining C—rather than in time, thus focusing on the stoichiometric relations during decomposition rather than the kinetics of degradation. This approach leads to explicit formulas that depend on CUE and other microbial properties, which can then be treated as model parameters and retrieved via nonlinear regression. CUE is either assumed time-invariant or as a function of the fraction of remaining litter C as a substitute for time. In all models, CUE tends to increase with increasing litter N availability across a range of litter types. When temporal trends in CUE are considered, CUE increases during decomposition of N-poor litter cohorts, in which decomposers are initially N-limited, but decreases in N-rich litter possibly due to C-limitation. These patterns of flexible CUE that partly compensate stoichiometric imbalances are robust to moderate shifts in decomposer C:N ratio and hold across wide climatic gradients.

Long term repeated fire disturbance alters soil bacterial diversity but not the abundance in an Australian wet sclerophyll forest

Effects of fire on biogeochemical cycling in terrestrial ecosystem are widely acknowledged, while few studies have focused on the bacterial community under the disturbance of long-term frequent prescribed fire. In this study, three treatments (burning every two years (B2), burning every four years (B4) and no burning (B0)) were applied for 38 years in an Australian wet sclerophyll forest. Results showed that bacterial alpha diversity (i.e. bacterial OTU) in the top soil (0-10 cm) was significantly higher in the B2 treatment compared with the B0 and B4 treatments. Non-metric multidimensional analysis (NMDS) of bacterial community showed clear separation of the soil bacterial community structure among different fire frequency regimes and between the depths. Different frequency fire did not have a substantial effect on bacterial composition at phylum level or bacterial 16S rRNA gene abundance. Soil pH and C:N ratio were the major drivers for bacterial community structure in the most frequent fire treatment (B2), while other factors (EC, DOC, DON, MBC, NH4(+), TC and TN) were significant in the less frequent burning and no burning treatments (B4 and B0). This study suggested that burning had a dramatic impact on bacterial diversity but not abundance with more frequent fire.