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

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.

Effects of wildfire on soil microbial communities in karst forest ecosystems of southern Guizhou Province, China

Wildfires are unpredictable disturbances with profound effects on soil properties and microbial communities within forest ecosystems. However, knowledge of post-fire microbial communities in karst forests remains limited. In this study, microbial amplicon sequencing techniques were employed to investigate the impact of wildfires on the composition, diversity, function, and co-occurrence network of soil microbial communities in karst forest landscapes and to identify the key soil physicochemical factors affecting the post-fire microbial communities. The wildfire affected the fungal community to a greater extent than the bacterial community, with the former shifting from a dominance of Basidiomycota to Ascomycota at the phylum level, while the relative abundance of Actinobacteria increased significantly in the bacterial community. Moreover, the wildfire increased the α-diversity of the microbial community and changed the β-diversity. Network analysis indicated significant reductions in the complexity of microbial community networks and the hub microbiome in burned soils compared to those of unburned soils. Functional predictions indicated an increase in the highly abundant functional taxa of chemoheterotrophic and aerobic chemoheterotrophic bacteria, along with a significant rise in saprotrophic functional fungal taxa following the fire. In addition, soil organic matter, total nitrogen, total phosphorus, and soil water content emerged as key soil physicochemical factors affecting post-fire soil microbial communities in the karst forest. Overall, this study revealed the structural and functional characteristics of soil microbial communities and their key influencing factors after a fire in a karst forest, which will provide a valuable theoretical basis for ecosystem restoration after a wildfire.IMPORTANCEDespite the significant impacts of wildfires on forest ecosystems, most existing studies have largely focused on boreal and Mediterranean coniferous forest types, with limited research on the impacts of coniferous and broadleaf forest types in subtropical karst regions. This study reveals the effects of wildfires on soil microbial communities of coniferous and broadleaf forest types in a karst forest. The results of this study not only improve the understanding of the effects of wildfires on the composition, diversity, function, and network of soil microbial communities but also provide a meaningful theoretical basis for post-fire ecosystem restoration in the karst forest.

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 fire and post-fire management on soil microbial communities in a lower subtropical forest ecosystem after a mountain fire

Wildfires and post-fire management exert profound effects on soil properties and microbial communities in forest ecosystems. Understanding microbial community recovery from fire and what the best post-fire management should be is very important but needs to be sufficiently studied. In light of these gaps in our understanding, this study aimed to assess the short-term effects of wildfire and post-fire management on both bacteria and fungi community composition, diversity, structure, and co-occurrence networks, and to identify the principal determinants of soil processes influencing the restoration of these communities. Specifically, we investigated soil bacterial and fungal community composition, diversity, structure, and co-occurrence networks in lower subtropical forests during a short-term (<3 years) post-fire recovery period at four main sites in Guangdong Province, southern China. Our results revealed significant effects of wildfires on fungal community composition, diversity, and co-occurrence patterns. Network analysis indicated reduced bacterial network complexity and connectivity post-fire, while the same features were enhanced in fungal networks. However, post-fire management effects on microbial communities were negligible. Bacterial diversity correlated positively with soil microbial biomass nitrogen, soil organic carbon, and soil total nitrogen. Conversely, based on the best random forest model, fungal community dynamics were negatively linked to nitrate-nitrogen and soil water content. Spearman's correlation analysis suggested positive associations between bacterial networks and soil factors, whereas fungal networks exhibited predominantly negative associations. This study elucidates the pivotal role of post-fire management in shaping ecological outcomes. Additionally, it accentuates the discernible distinctions between bacterial and fungal responses to fire throughout a short-term recovery period. These findings contribute novel insights that bear significance in evaluating the efficacy of environmental management strategies.

Formation characteristics of bacteria and fungi in sewers: In terms of signal molecule generation

Bio-metabolism of diverse communities is the main reason of water quality variation in sewers, and the signal molecule generation of communities is dementated to be the key regulation procedure for community metabolism. To reveal the mechanism of pollutant biotransformation in complex sewer environment, this study explored the formation of bacteria and fungi and the signal molecule transduction characteristics in a pilot sewer. In this study, several kinds of signal molecules that produced by bacteria and fungi (C4-HSL, C6-HSL, C8-HSL, farnesol and tyrosol) were detected along the formation process of sewer biofilms. The results showed that, in the early stage, bacterial AHLs signaling molecules are beneficial to the synthesis of EPS, providing a good material basis for the growth of bacterial flora. In addition, tyrosol stimulates the formation of embryonic tubes in yeast cells, further promoting the growth of hyphae. At the later stage, AHLs signaling molecules and tyrosol jointly promoted the growth of biofilms. In conclusion, it is precisely because of the coexistence of bacteria and fungi in the sewer system that the generated signal molecules can jointly promote the synthesis and growth of biofilms through different pathways, and have positive feedback on the biodegradation of various pollutants. Based on the exploration, the ecological patterns of bacterial-fungal communities in urban sewer system were proposed and it could improve the understanding on the pollutant transformation behaviors in sewers.

Prescribed fire selects for a pyrophilous soil sub-community in a northern California mixed conifer forest

Prescribed fire is a critical strategy for mitigating the effects of catastrophic wildfires. While the above-ground response to fire has been well-documented, fewer studies have addressed the effect of prescribed fire on soil microorganisms. To understand how soil microbial communities respond to prescribed fire, we sampled four plots at a high temporal resolution (two burned, two controls), for 17 months, in a mixed conifer forest in northern California, USA. Using amplicon sequencing, we found that prescribed fire significantly altered both fungal and bacterial community structure. We found that most differentially abundant fungal taxa had a positive fold-change, while differentially abundant bacterial taxa generally had a negative fold-change. We tested the null hypothesis that these communities assembled due to neutral processes (i.e., drift and/or dispersal), finding that >90% of taxa fit this neutral prediction. However, a dynamic sub-community composed of burn-associated indicator taxa that were positively differentially abundant was enriched for non-neutral amplicon sequence variants, suggesting assembly via deterministic processes. In synthesizing these results, we identified 15 pyrophilous taxa with a significant and positive response to prescribed burns. Together, these results lay the foundation for building a process-driven understanding of microbial community assembly in the context of the classical disturbance regime of fire.

Effects of wildfire disturbance on forest soil microbes and colonization of ericoid mycorrhizal fungi in northern China

Wildfires affect forest succession and restoration by changing the community structure of soil microorganisms. Mycorrhizal formation is essential for plant growth and development. However, the driving mechanism of their natural succession after wildfire is still unclear. In this study, we examined the community structure of soil bacteria and fungi along a time series of natural recovery after wildfires in the Greater Khingan Range of China (2020 fires, 2017 fires, 2012 fires, 2004 fires, 1991 fires, and unburned). By exploring the effects of wildfire on plant traits, fruit nutrition, colonization of mycorrhizal fungi and its influencing mechanism. The results show that natural succession after wildfires significantly changed the community composition of bacteria and fungi, with β diversity having a greater impact but less impact on the α diversity of microorganisms. Wildfires significantly changed plant traits and fruit nutrient content. The changes in colonization rate and customization intensity of mycorrhizal fungi were caused by increased MDA content and soluble sugar content and increased MADS-box gene and DREB1 gene expression in lingonberry (Vaccinium vitis-idaea L.). Our results showed that the soil bacterial and fungal communities in the boreal forest ecosystem changed significantly during wildfire recovery and changed the colonization rate of lingonberry mycorrhizal fungi. This study provides a theoretical basis for the restoration of forest ecosystems after wildfires.

Deciphering the Synergies of Reductive Soil Disinfestation Combined with Biochar and Antagonistic Microbial Inoculation in Cucumber Fusarium Wilt Suppression Through Rhizosphere Microbiota Structure

Application of reductive soil disinfestation (RSD), biochar, and antagonistic microbes have become increasingly popular strategies in a microbiome-based approach to control soil-borne diseases. The combined effect of these remediation methods on the suppression of cucumber Fusarium wilt associated with microbiota reconstruction, however, is still unknown. In this study, we applied RSD treatment together with biochar and microbial application of Trichoderma and Bacillus spp. in Fusarium-diseased cucumbers to investigate their effects on wilt suppression, soil chemical changes, microbial abundances, and the rhizosphere communities. The results showed that initial RSD treatment followed by biochar amendment (RSD-BC) and combined applications of microbial inoculation and biochar (RSD-SQR-T37-BC) decreased nitrate concentration and raised soil pH, soil organic carbon (SOC), and ammonium in the treated soils. Under RSD, the applications of Bacillus (RSD-SQR), Trichoderma (RSD-T37), and biochar (RSD-BC) suppressed wilt incidence by 26.8%, 37.5%, and 32.5%, respectively, compared to non-RSD treatments. Moreover, RSD-SQR-T37-BC and RSD-T37 caused greater suppressiveness of Fusarium wilt and F. oxysporum by 57.0 and 33.5%, respectively. Rhizosphere beta diversity and alpha diversity revealed a difference between RSD-treated and non-RSD microbial groups. The significant increase in the abundance, richness, and diversity of bacteria, and the decrease in the abundance and diversity of fungi under RSD-induced treatments attributed to the general suppression. Identified bacterial (Bacillus, Pseudoxanthomonas, Flavobacterium, Flavisolibacter, and Arthrobacter) and fungal (Trichoderma, Chaetomium, Cladosporium, Psathyrella, and Westerdykella) genera were likely the potential antagonists of specific disease suppression for their significant increase of abundances under RSD-treated soils and high relative importance in linear models. This study infers that the RSD treatment induces potential synergies with biochar amendment and microbial applications, resulting in enhanced general-to-specific suppression mechanisms by changing the microbial community composition in the cucumber rhizosphere.

Fire and Rhizosphere Effects on Bacterial Co-Occurrence Patterns

Fires are common in Mediterranean soils and constitute an important driver of their evolution. Although fire effects on vegetation dynamics are widely studied, their influence on the assembly rules of soil prokaryotes in a small-scale environment has attracted limited attention. In the present study, we reanalyzed the data from Aponte et al. (2022) to test whether the direct and/or indirect effects of fire are reflected in the network of relationships among soil prokaryotes in a Chilean sclerophyllous ecosystem. We focused on bacterial (genus and species level) co-occurrence patterns in the rhizospheres and bulk soils in burned and unburned plots. Four soils were considered: bulk-burnt (BB), bulk-unburnt (BU), rhizosphere-burnt (RB), and rhizosphere-unburnt (RU). The largest differences in network parameters were recorded between RU and BB soils, while RB and BU networks exhibited similar values. The network in the BB soil was the most compact and centralized, while the RU network was the least connected, with no central nodes. The robustness of bacterial communities was enhanced in burnt soils, but this was more pronounced in BB soil. The mechanisms mainly responsible for bacterial community structure were stochastic in all soils, whether burnt or unburnt; however, communities in RB were much more stochastic than in RU.

Decrease in Soil Functionalities and Herbs' Diversity, but Not That of Arbuscular Mycorrhizal Fungi, Linked to Short Fire Interval in Semi-Arid Oak Forest Ecosystem, West Iran

The semi-arid forest ecosystems of western Iran dominated by Quercus brantii are often disturbed by wildfires. Here, we assessed the effects of short fire intervals on the soil properties and community diversity of herbaceous plants and arbuscular mycorrhizal fungi (AMF), as well as the interactions between these ecosystem features. Plots burned once or twice within 10 years were compared to unburned plots over a long time period (control sites). Soil physical properties were not affected by the short fire interval, except bulk density, which increased. Soil geochemical and biological properties were affected by the fires. Soil organic matter and nitrogen concentrations were depleted by two fires. Short intervals impaired microbial respiration, microbial biomass carbon, substrate-induced respiration, and urease enzyme activity. The successive fires affected the AMF's Shannon diversity. The diversity of the herb community increased after one fire and dropped after two, indicating that the whole community structure was altered. Two fires had greater direct than indirect effects on plant and fungal diversity, as well as soil properties. Short-interval fires depleted soil functional properties and reduced herb diversity. With short-interval fires probably fostered by anthropogenic climate change, the functionalities of this semi-arid oak forest could collapse, necessitating fire mitigation.

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.