Aggregate Stratification Assessment of Soil Bacterial Communities and Organic Matter Composition: Coupling Pyrosequencing and Mid-Infrared Spectroscopy Techniques.

This study integrated physical, chemical, and molecular techniques to assess relationships between soil bacterial communities and the quantity and quality of soil organic carbon (SOC) at the soil microenvironment scale (e.g., within different aggregate size-fractions).  To accomplish this goal soil was separated into macroaggregates, microaggregates, and silt+clay fractions and analyzed for (1) bacterial composition via pyrosequencing of the 16s rDNA gene and (2) SOC quantity and quality using combustion and mid-infrared diffuse reflectance spectroscopy (mid-IR) methods, respectively. Soil samples (0-5cm) were collected from the Texas High Plains region under five different cropping systems. Results from pyrosequencing showed that each soil microenvironment supported a distinct bacterial community.  Similarly, mid-IR data revealed distinct spectral features indicating that these fractions were also distinguished by organic and mineral composition. Macroaggregates showed relatively high abundance of  Actinobacteria (excluding order Rubrobacteriales) and α-Proteobacteria and contained the most SOC. Microaggregates showed high relative abundance of Rubrobacteriales and the least amount of SOC. Predominance within soil microenvironment and correlations along the mid-IR spectra were different between members of the order Rubrobacteriales compared to all other members of the Actinobacteria phyla, suggesting they have different ecological niches.  Mid-IR results revealed microaggregates had greater absorbance in the 1370-1450 cm^-1 region for phenolic alkyl groups, possibly recalcitrant C. Silt+clay fractions were distinguished by Gemmatimonadetes and OP10 phyla, which positively correlate with spectral absorption in the1250-1150 cm^-1 range (indicating both degradable and recalcitrant C forms). In contrast to general diversity index measurements, the distributions of the more rare bacterial phyla (phyla representing < 6% of the identified population) were more important for differentiating between communities in soil microenvironments. To our knowledge, this is the first study to investigate soil bacterial communities among soil aggregates using pyrosequenging and to associate these communities to specific soil C chemistries as indicated by mid-IR absorbance.