4.1 The response of microbial community richness and composition to high temperature stress
This study investigated the effects of extreme high temperature stress on soil microbial communities from different terrestrial ecosystems and the factors affecting the stability of microbial communities. We found that high temperature stress mainly caused a reduction in the richness of bacterial and fungal communities. During stress stage, the abundance of complex ecosystems is reduced to a greater extent than that of simple systems (Fig. 1). Complex systems inherently have higher richness (Fig. S4), resulting in more species crowding into the same ecological niche amplitude, and thus, stress is more likely to erase greater numbers of species (Pianka, 1981; Pinsky, 2019). Most bacterial communities show a further reduction in richness during recovery stage, which is also potentially related to the secondary extinctions caused by the disappearance of cooperative members in complex inter-collaborative networks (May and MacDonald, 1978; Damore and Gore, 2012). In contrast, most fungal communities didn’t show such secondary extinctions during the recovery stage, which was potentially related to the fact that fungi are more adapted to terrestrial habitats (de Vries and Ashley, 2013). And the fungal networks were also reported more robust than bacterial networks under climatic stress (de Vries et al., 2018), which reduced the possibility of further extinction induced by change of interaction (May and MacDonald, 1978; Damore and Gore, 2012).
The composition of bacterial and fungal communities in different ecosystems show distinctive trajectories of change in response to extreme high temperature stress (Figs. 2 and 3). The α-Proteobacteria in simple ecosystems increased RA during stress stage while decreased RA during the recovery stage (Fig. 2). This was possibly due to simple ecosystems were relative scarcity of carbon and nitrogen nutrients (Fig. S3) and were unfavorable for r-strategic microorganisms such as α-Proteobacteria (de Vries and Ashley, 2013). While high temperatures promote substrate use by microorganisms and increase the dominance of r-strategic microorganisms (de Vries and Ashley, 2013). In contrast, α-Proteobacteria RAs in complex ecosystem were reduced during both the stress and recovery stages and were largely replaced by phylum such as Firmicute (Fig. 2). Complex ecosystems had relatively higher carbon and nitrogen resources (Fig.S3), and higher lignin content (Li and Xiong, 1995; Jiang et al., 2018), which were may create more relatively anaerobic microsites and enrich lignin-intimate microbes at high temperatures (Wu and He, 2013; Kato et al., 2015). The changes in fungal communities under stress are more specific than those in bacterial communities (Figs. 2 and 3). Fungal communities are more dependent on vegetation type and fungi are dispersal limited than bacteria, leading to greater differences in the original species composition of fungal communities in different ecosystems and, in turn, to differential cultural succession trajectories (de Vries and Ashley, 2013; de Vries et al., 2018).
This research showed that extreme high temperature stress significantly influenced the richness and community structure of bacterial and fungal communities, but it was difficult to derive a consistent pattern for compositional change. To further elucidate the rules for soil microbial response to climate extremes, we analyzed the microbial community stability and the factors influence the stability. We found a rough trend that forests had higher resilience, but lower resistance to simulated extreme high-temperature stress, than shrub, grassland, and bare soil (Fig. 4A), which was opposite to the trend displayed by plant communities where complex systems were likely to have higher resistance and lower resilience (Isbell et al., 2015). We suggest that such differences arise from the reproductive rates, physiological resistance, and differences in richness between plants and microbes (Curtis, 2006; Konopka, 2006). Compared to microbes, plants have much lower reproductive rates and richness (Curtis, 2006; Konopka, 2006). When faced with short term high temperature stress, on the order of only several days, the dominant forest plant keystone population was likely to remain unchanged unless large scale regional death occurred, and thus appearing to have high resistance (Curtis, 2006; Konopka, 2006). However, microbes have extremely high richness as compared to plants in terrestrial ecosystems, and are able to reproduce in a timespan as short as hours or even minutes (Curtis, 2006; Konopka, 2006). These indicate that competitive functional groups with different temperature range tolerances could potentially replace the original dominant groups and thus led to lower resistance of the microbial community (Kai et al., 2017; Pinsky, 2019). However, with keystone species dominated by organisms with high stress tolerance, such as the communities from bare soil (Fig. 4A) that are exposed to high day-night temperature variation, dry-wet alteration, and high ultra-violate, could exhibit high resistance because those keystone species were not likely to change (Remias et al., 2012; Harrison and LaForgia, 2019).
4.2 The resistance and resilience of microbial community to high temperature stress
We found a strong negative linear relationship between microbial community resistance and resilience, which indicated a trade-off between resistance and resilience of microbial communities in the ecosystems studied here (Fig. 4B and Fig. 4C). This is plausible and inevitable from both basic logic and an evolutionary perspective. The essence of resistance and resilience is the ability of altering the relative abundance of species as conditions change (de Vries and Ashley, 2013). Thus, a microbial community that is more readily prone to change simultaneously has less resistance and higher resilience, and vice versa (Miller and Chesson, 2009; Griffiths and Philippot, 2013). From an evolutionary perspective, communities need to coordinate the functions of different components to ensure the continuation of key ecological processes for survival of the community under variable environmental conditions. This can be realized by assigning key functions to a few stress tolerant functional species (such as the high resistance community in bare soil) (Craine et al., 2013), or to alternative functional groups composed of different stress tolerant members that are capable to quickly reproduce under certain environmental conditions (like the low resistance community in forests) (Whitham et al., 2006; Walworth et al., 2020), that can be treated as the trade-off between K-strategy members and r-strategy members of community evolution or succession. However, to simultaneously possess K- and r-strategy wastes energy and is an evolutionary dead-end (Whitham et al., 2006; Li et al., 2020; Walworth et al., 2020). Thus, a microbial community prone to have a higher resistance or a higher resilience rather than both.
4.3 The factors influencing microbial community resistance and resilience
Our results found that richness had higher total effects on resistance rather than on resilience while environmental factors had higher total effects on resilience rather than on resistance, which indicated that richness and environmental factors had biases for influencing different stability components (Fig. 5). To better understand these biases, we need to elucidate the relationship between richness and resistance, and the relationship between nutrients and resilience.
The results showed that high richness was unfavorable for high resistance as it exerted negative total effects on resistance, and this was supported by several field and theoretical studies (Fig. 5). Certain stressors may cause the simultaneous extinction of more species in higher richness community if they belonged to the same niche (Kalmykov and Kalmykov, 2012). A recent study reported that stress decreased not only the dominance of Tricholoma matsutake but also the dominance of its competitors (Zhou et al., 2021). Also, higher richness offered higher possibility for the trade-off of relative abundances among functionally similar groups when facing stressors (Louca et al., 2018; Pinsky, 2019). Both situations are unfavorable for maintaining an unchanged community composition under stress (Kalmykov and Kalmykov, 2012; Louca et al., 2018; Pinsky, 2019). In our study, richness had higher total effects on inhibiting resistance than on accelerating resilience (Fig. 5), however we still lack reasonable ecological theory offering plausible explain. We believe this might be linked to the relatively isolated incubation strategy utilized here. A complete recovery requires the original species occupying their original functions. However, if several species became extinct due to stress and were unable to be reintroduced into the community during the recovery state, the community would need to use other species to fill those functional roles (Louca et al., 2018). Though higher richness would indicate the availability of more potential replacement species to accelerate recovery (a kind of positive effect) (Louca et al., 2018), it would also indicated a more or less incomplete recovery (a kind of negative effect) (Hillebrand et al., 2018). Thus, the total effect from richness on resilience was the balance of those opposing effects (we should note these two opposing effects might not be the opposite direct and indirect effect in SEM), which potentially explains why richness had lower total effects on accelerating resilience than on inhibiting resistance.
Our results found that nutrient sufficient condition was unfavorable for resistance but favorable for resilience (Fig. 5). As discussed above, in a microbial community which has highly richness, functional redundancy and fast individual reproduction rate, stress (like temperature change in this study) may cause significant change to population reproduction rate and vary the relative abundances of several species within few days (Curtis, 2006; Prosser et al., 2007). Also, a nutrient sufficient condition would accelerate the compositional change under stress (lower resistance) as well as the compositional change after removing the stress (higher resilience). Interestingly, nutrients offered less inhibiting effect on resistance but higher accelerating effect on resilience (Fig. 5). We suggested this was caused by ecological memory shaped in the 17°C field site (sampling sites) and the reactivation stage temperature (17°C), such that the microbial community had acclimated, compositionally and functionally, to environmental conditions under 17°C (Veen et al., 2015; Canarini et al., 2021). Though stresses forced the community to change to a state which operates more efficiently under 25°C, several environmental factors were unlikely to be changed by only a few days of stress exposure, such as the chemical composition of litters (Veen et al., 2015), leading the community to still more suitable work under the original state. Thus, nutrients had higher accelerating effect on returning from a stressed state to original state rather than accelerating from the original state to a stressed state. Above all, due to the high richness and functional redundancy of the microbial community and the ecological memory under the original state, richness had a greater contribution to resistance than resilience, and environmental factors had a greater contribution to resilience than resistance.
Since different microbial communities rely on high resistance or resilience to maintain their stability, and resistance and resilience are respectively influenced by richness and nutrients mainly, we are then capable to estimate the stability of communities in the face of climate extremes according to richness and nutrients information (Fig. 6). The relatively high richness communities in eutrophic ecosystem (such as forest) possess considerable stability to extreme climate stresses due to high resilience. And the relatively low richness communities in oligotrophic ecosystem (such as bare land) also possess considerable stability due to high resistance. The relatively low richness communities in eutrophic ecosystem were seldom reported in natural ecosystems and theoretically hard to exist. The relatively high richness communities in oligotrophic ecosystem were potentially most sensitive to extreme climate change and the region may be the hotspots for ecosystem degradation, such as grassland and shrub (Fig. 4), which was supported by alpine grassland related researches (Borer et al., 2017).