The microbial community composition in aggregate sized fractions down the soil profile in Irish soils. Aggregates

The influence of horizon depth and aggregate-size on bacterial and fungal community composition was determined. From nine sites, soils were collected from the top three horizons. Physical fractionation separated samples into large macroaggregate (LM), macroaggregate (MAC), microaggregate (MIC), and silt and clay (SC) fractions. In all samples, the structure of the bacterial and fungal community composition was assessed via restriction fragment length polymorphism (T-RFLP), and for the four aggregate sizes from the top two horizons in-depth analysis of the bacterial community was conducted using next generation sequencing (NGS). Bacterial and fungal communities both differed between aggregate-sizes. Changes in the composition of the bacterial and fungal communities also occurred among soil horizons, with a significant interaction between aggregate size and horizon depth evident for the bacterial community. Using NGS, it was shown that aggregate-size had a significant effect on the bacterial community in both horizons at the family level, and at the phyla level in horizon 2 (H2). MAC and MIC significantly differed in the abundance of bacterial groups, potentially indicating differing predation pressures. These results indicate that both horizon depth and aggregate-size support distinct microbial communities. Understanding these parameters is critical in our comprehension of the patterns of microbial diversity in soil.