Differential responses of soil microbiomes to ureolytic biostimulation across depths in Aridisols

Abstract. Soil microbiomes are key regulators of biogeochemical cycles and possess essential roles in ecosystem functions, particularly in arid environments. One beneficial function of various edaphic microbes is the ability to participate in Microbial Induced Calcite Precipitation (MICP). MICP is a biomineralization process extensively investigated as a soil improvement technique for various purposes, including mitigation of drought-related soil degradation and erosion control. One aspect rarely addressed in MICP studies is the microbial heterogeneity of the ecosystem in which it is applied and its post-treatment consequences. In this study, we examined MICP biostimulation rates in an Aridisol, considering the microbial heterogeneity across different soil depths that are relevant to surface reinforcement applications (from the topsoil to 1 meter below the surface). Biostimulation was achieved by inducing ureolysis, one of the most studied metabolic pathways to stimulate MICP. We characterized the native microbial communities and their response to biostimulation across the depths under consideration using 16S sequencing. We found that ureolysis rates were affected by soil depth, with higher rates detected at the topsoil. Before biostimulation, the native soils were dominated by Actinobacteria and contained diverse communities. The microbial communities of the deeper soil layers were richer in Firmicutes, and the deepest layer was less diverse than the topsoil. Following biostimulation, alpha-diversity and microbial richness were drastically reduced at all depths, resulting in homogenized communities dominated by Firmicutes, although microbial DNA concentrations increased. A notable decrease was detected in autotrophs (e.g., Cyanobacteria, Chloroflexi), which are important for the formation and function of biocrusts and, hence, to the entire ecosystem. We also found that biostimulation induced a shift in the composition of the Firmicutes, where specific members of the Planococcaceae family became the most prevalent Firmicutes, instead of Paenibacillaceae and Bacillaceae, following stimulation. Our findings demonstrate that environmental heterogeneity across soil depth is an influential variable affecting ureolytic biostimulation. In turn, biostimulation affects microbial diversity consistently, regardless of preexisting differences resulting from spatial heterogeneity. Our findings show that although feasible, implementing biostimulated MICP in arid environments induces a strong selective pressure with negative consequences for the native edaphic microbiomes.