Unraveling the response of pioneer soil-forming bacterial communities after simulating humid climate conditions in arid and semiarid soils

Pioneer microbial communities participate critically in soil formation processes in arid and semiarid environments. However, how its structure impacts soil formation and stabilization and how they will respond to changes in a climate still need to be better understood. Therefore, soil samples from A and B horizons of arid and semiarid climates were incubated in a four month-experiment simulating constant water availability and temperature regimes in humid climate conditions. We used 16S-rRNA sequencing to assess changes in abundance, diversity, and structure of pioneer bacterial communities and their relation to soil physicochemical properties at three-time points during the incubation. Humid conditions decreased bacterial diversity only in arid soils, while the structure changed in both soils but faster in the arid one (12 against 16 weeks). The bacterial phyla varied markedly in arid soils over time, reassembled into a community dominated by Proteobacteria, Actinobacteriota, and Gemmatimonadota, while semiarid one was led steadily by Acidobacteriota, Proteobacteria, and Planctomycetota. The bacterial community correlated with prominent soil formation indicators such as pH, electrical conductivity, carbon, nitrogen, and mean weight diameter of aggregates, suggesting that they are involved in soil formation processes. The co-occurrence network revealed a niche differentiation per soil type and a less developed soil with a bacterial community sensitive to environmental disturbances in the arid soil. Overall, we suggest that the pioneer bacterial community was sensitive to changes when exposed to climate conditions outside the range of their life history, which, in turn, influenced the stability (semiarid soil) or turnover (arid soil) of pioneer soil-forming bacteria.