r- and K-life history strategies drive microbial distribution in distinct soil pore size classes

Life history strategy-based microbial distribution at the soil pore scale was examined using the high-throughput sequencing of bacterial 16S rDNA and fungal ITS region, with functional predictions for bacteria using the bioinformatic tool PICRUSt2. Soil cores (Herndon silt loam with a barley straw) were packed at two bulk densities (1.16 g cm −3 and 0.78 g cm −3 ) to create contrasting pore size distributions. Following saturation, water was selectively removed from specific pore size classes using the pressure plate desorption method at 4, 10, or 100 kPa. Additional treatments for pore water manipulation included unsaturated soil control without water manipulation and wet soil control, i.e., soil subjected to free drainage under open air after saturation. These pore size and moisture manipulations created different pore size classes with varying moistures. Both microbial diversity and community composition were associated with pore size distributions (i.e., bulk density), pore water manipulation, and sampling time. By taking advantage of both soil moisture change and microbial succession during barley straw decomposition, we used the ratio of microbial taxa or predicted functions between day 38 and day 10 of the incubation to infer microbial preferences for specific pore size classes. If microbial distribution had no pore size preference, the ratios for abundance change on day 38 to day 10 should be similar among soils subjected to different pore size and moisture manipulations. The fact that considerable differences occurred among these soil treatments supported microbial pore size preference. Bacteroidetes and Firmicutes, often considered as r-strategists/copiotrophs were more abundant in large pores, while Actinobacteria, K-strategists/oligotrophs were relatively enriched in relatively small pores. However, this life history strategy-based pattern was likely modified by pore environmental conditions. Acidobacteria, commonly considered K-strategic/oligotrophic, exhibited the stronger temporal change in large pores, likely due to decomposition-induced pH changes and subsequent shifts in members of the phylum. Predicted metabolic functions also differed in large vs small pores, and separations were partially related to the ecological needs of microbes with narrow functions. These results emphasize the importance of pore size and water distribution in structuring microbial communities and shaping their functional roles.