Soil bacteria play a fundamental role in pedogenesis. However, knowledge about both the impact of climate and slope aspects on microbial communities and the consequences of these items in pedogenesis is lacking. Therefore, soil-bacterial communities from four sites and two different aspects along the climate gradient of the Chilean Coastal Cordillera were investigated. Using a combination of microbiological and physicochemical methods, soils that developed in arid, semi-arid, mediterranean, and humid climates were analyzed. Proteobacteria, Acidobacteria, Chloroflexi, Verrucomicrobia, and Planctomycetes were found to increase in abundance from arid to humid climates, while Actinobacteria and Gemmatimonadetes decreased along the transect. Bacterial-community structure varied with climate and aspect and was influenced by pH, bulk density, plant-available phosphorus, clay, and total organic-matter content. Higher bacterial specialization was found in arid and humid climates and on the south-facing slope and was likely promoted by stable microclimatic conditions. The presence of specialists was associated with ecosystem-functional traits, which shifted from pioneers that accumulated organic matter in arid climates to organic decomposers in humid climates. These findings provide new perspectives on how climate and slope aspects influence the composition and functional capabilities of bacteria, with most of these capabilities being involved in pedogenetic processes.

The microbiota is attributed to be important for initial soil formation under extreme climate conditions, but experimental evidence for its relevance is scarce. To fill this gap, we investigated the impact of in situ microbial communities and their interrelationship with biocrust and plants compared to abiotic controls on soil formation in initial arid and semiarid soils. Additionally, we assessed the response of bacterial communities to climate change. Topsoil and subsoil samples from arid and semiarid sites in the Chilean Coastal Cordillera were incubated for 16 weeks under diurnal temperature and moisture variations to simulate humid climate conditions as part of a climate change scenario. Our findings indicate that microorganism-plant interaction intensified aggregate formation and stabilized soil structure, facilitating initial soil formation. Interestingly, microorganisms alone or in conjunction with biocrust showed no discernible patterns compared to abiotic controls, potentially due to water-masking effects. Arid soils displayed reduced bacterial diversity and developed a new community structure dominated by Proteobacteria, Actinobacteriota, and Planctomycetota, while semiarid soils maintained a consistently dominant community of Acidobacteriota and Proteobacteria. This highlighted a sensitive and specialized bacterial community in arid soils, while semiarid soils exhibited a more complex and stable community. We conclude that microorganism-plant interaction has measurable impacts on initial soil formation in arid and semiarid regions on short time scales under climate change. Additionally, we propose that soil and climate legacies are decisive for the present soil microbial community structure and interactions, future soil development, and microbial responses.

Background: In arid and semiarid environments, microbial activity is restricted by low water availability and high evapotranspiration rates, and soil development is limited. Under humid conditions, such limitations can be overcome, accelerating pedogenesis by microbial processes. Our study aims to broaden our understanding of soil development under a climate change scenario toward humid conditions and to identify the microorganisms that help transform initial soils from arid and semiarid sites. We characterized pedogenetic microbial processes and how their gene expression differs between soils from arid and semiarid sites under a sixteen-week climate simulation experiment using metagenomic and metatranscriptomic approaches. Results: We found that an intense functional response is triggered under humid climate conditions in the arid site compared to the semiarid site, which showed greater resilience. The arid site undergoes higher transcription of genes involved in soil aggregate formation, phosphorus metabolism, and weathering, potentially adapting the development of arid sites to climate change. Additionally, a transcriptional reconfiguration linked to soil carbon and nitrogen dynamics suggests that soil microorganisms use available organic resources alongside autotrophy in response to increased moisture. Pseudomonadota and Actinomycetota dominated the overall transcriptional profile and specific functions associated with the early stages of soil development in both sites. Conclusions: Our findings highlight the rapid activation of pathways related to pedogenesis under humid conditions in arid sites, potentially driven by their metabolic requirements and environmental stressors, influencing soil development dynamics under global climate change.