The hydrothermal conditions in the Jinsha River valley exhibit significant variations at different elevations. The lower elevation areas exhibit a persistently arid and high-temperature climate, characterized by minimal fluctuations in both moisture and temperature throughout the dry and wet seasons. This stands in stark contrast to the high elevation forests and the transition zones between them. Numerous studies have highlighted the crucial role of soil microorganisms in the ecological restoration of the dry-hot valley. We propose that soil microorganisms in the low elevation DHVs have developed tolerance mechanisms to effectively cope with seasonal dry-wet alternation and even the extreme dry and hot environments they encounter during long-term adaptation processes [4, 6, 38, 39]. As expected, the bacterial communities within DHVs exhibited strong stability and adaptability to seasonal dry and wet alternation (Fig. 2). Our results indicated that the core bacterial taxa, which accounted for over 80% of the community, remained consistent. These taxa were prominently represented by Proteobacteria, Acidobacteria, Actinobacteria, and Chlorobacteria (Fig. 2a, b). However, these findings diverge from observations in other regions of the DHVs in southwest China [13, 23–25, 40]. It may suggest that although the arid and hot environment exerts a selective pressure on the soil microbial community [41, 42], the composition of the core microbial community is intricately linked to soil properties, vegetation diversity, and other environmental factors [43, 44]. Within DHVs and the transition zones, the abundance of Actinobacteria and Chloroflexi was increased during the dry season, in contrast to Acidobacteria, which displayed an opposite trend (Fig. 2f, g). These microbial dynamics are posited to critically influence the microbiome's adaptation to seasonal moisture fluctuations [45]. Notably, Actinobacteria exhibit robustness to environmental stress, including drought, and are instrumental in soil functionality under challenging conditions [10, 46, 47]. Their symbiotic relationships with plant roots further bolster drought tolerance by facilitating phytohormone production and enhanced nutrient acquisition [10, 42, 48]. In addition, Chloroflexi, a phylum adapted to oligotrophic conditions, demonstrates the capacity to prosper in resource-limited environments [49–51]. Our findings align with the results [45] in suggesting that Chloroflexi may interact with other soil microorganisms, thereby shaping community structure and function. Taken together, these results underscore the crucial role of Actinobacteria and Chloroflexi in promoting soil resilience and ecosystem stability under the extreme conditions found in DHVs [12].
Seasonal fluctuations in soil temperature, moisture, and nutrient levels can significantly influence belowground microbial communities. However, these seasonal variations may not be the primary driver of microbiota diversity, as it is influenced more by taxon-specific gene expression [8]. Our research revealed that soil bacterial communities at various elevations maintained consistent richness, diversity, and coverage throughout both dry and wet seasons, except within the transition zones (see Fig. 2c). This observation suggests that the regulation of season-specific microbial clades may contribute to the stability of microbial community structure. For example, our study identified an increase in the activity of clades associated with Acidobacteria and Patescibacteria within DHVs and the transition zones during the wet season, which subsequently decreased following a prolonged dry period. In contrast, specific bacterial phyla displayed adaptability by increasing the number of clades (Fig. 3a-c). Interestingly, some clades exhibited complex regulatory patterns, such as those related to Proteobacteria.
Within the lower elevation DHVs, clades associated with Actinobacteria were more abundant during dry seasons compared to wet seasons. Previous studies have highlighted the pivotal role of Actinobacteria in responding to drought stress, including their ability to maintain activity and enter a dormant state under dry conditions. Santos-Medellín et al. [52] proposed that prolonged drought leads to a significant enrichment of Actinobacteria in the rice rhizosphere microbiome, with their abundance declining upon recovery. Therefore, we hypothesize that Actinobacteria and its related clades play a regulatory role in response to changes in water availability, enhancing the adaptability and functionality of soil bacterial communities during seasonal dry-wet cycles. Acidobacteria, a widely distributed phylum in diverse ecosystems [53, 54], demonstrates a remarkable regulatory capacity and adaptive mechanisms to thrive in complex environmental conditions. Kalam et al. [55] conducted a comprehensive review of current research on Acidobacteria in soil, highlighting its significant ecological importance. Our findings revealed a notable decline in the abundance of Acidobacteria-related microbial clades in DHVs and the transition zones during dry seasons, with a resurgence of activity observed during wet seasons when water availability increased (Fig. 3a, b). Studies suggest that Acidobacteria may possess specific genes facilitating survival and competitive colonization in the rhizosphere, fostering beneficial interactions with plants [56, 57]. Consequently, it is proposed that Acidobacteria-related microbial clades play a pivotal role in the ecological restoration of DHVs. In contrast, fluctuations in the abundance of Proteobacteria-associated clades across different sampling areas did not exhibit consistent patterns during the transition from wet to dry seasons (Fig. 3a-c), indicating diverse regulatory strategies employed by Proteobacteria-related microbial clades in response to seasonal variations [9, 47, 58]. Notably, seasonal fluctuations in the alpine soil microbial community, while evident, are relatively minor compared to DHVs and the transition zones, possibly due to the limited impact of moisture and temperature changes in alpine regions. This suggests that soil bacterial communities in alpine zones maintain stability under the mild seasonal fluctuations of wet and dry conditions. Thus, our results reveal that soil microbial communities undergo rapid evolution in response to selective pressures imposed by seasonal dry-wet alternations. Adaptive evolution may result in the emergence of novel traits or genetic variants that confer fitness advantages under specific moisture regimes [59–61]. Genetic adaptation to fluctuating environmental conditions contributes to the resilience and stability of soil microbial aggregates over time.
Recent advancements in microbial community assembly research have illuminated our understanding of these intricate ecological dynamics. However, the impact of seasonal dry and wet alternation on community assembly within unique savanna-like dry-hot valley ecosystems remains poorly characterized across spatial and temporal scales. This study investigates the influence of environmental factors on microbial communities in these distinct ecosystems. Our results indicate a minimal effect of seasonal dry and wet alternation in valley regions, but varying degrees of alteration in transition and alpine zones. This observation led us to hypothesize that soil microbial community assembly in dry-hot valley ecosystems is predominantly driven by stochastic processes, in contrast to the deterministic processes observed in the transition and alpine zones. Employing the neutral community model (NCM), our analysis emphasizes the dominance of stochastic mechanisms in microbial community assembly within valley ecosystems, particularly in savanna-like dry-hot valleys (Fig. 6). This finding partially supports our hypothesis and suggests that stochastic processes play a significant role in shaping microbial communities in these ecosystems. However, the influence of stochastic processes extends to the transition and alpine zones as well. Our findings challenge previous assumptions about the primary influence of environmental factors, such as seasonal dry and wet alternation, on shaping microbial communities across all ecosystem types. Soil pH, a crucial abiotic factor affecting soil microbial communities, could mediate the balance between stochastic and deterministic assembly of bacteria [62]. In our study, soil pH in DHVs and the transition zones did not vary significantly between dry and wet seasons, except in the alpine zones (Additional file 1: Supplementary Table S1). Nevertheless, the potential role of deterministic processes in dry and hot valley regions cannot be discounted. It is well-established that deterministic and stochastic processes coexist in regulating ecological community assembly [20, 21, 63]. Moreover, Thompson et al. [64] proposed that narrow abiotic niche curves lead to strong species responses to environmental variation (deterministic processes), while broad or flat curves result in weak or absent responses (stochastic processes). Our study suggests that the dry-hot valleys may exhibit characteristics that favor stochastic assembly, potentially influenced by reduced environmental variability or increased dispersal rates [65].