PNNL

Abstract

Soil microbial communities affect the formation of micro-scale organo-mineral associations where complex processes, including aggregate formation, microbial mineral weathering and soil organic matter stabilization occur in a narrow zone of biogeochemical gradients. We designed a field study to examine these processes using in-growth mesh bags containing biotite that were placed in a ponderosa pine root zone for 6 months. The mesh-bags minerals were colonized by a microbial community that produced organic matter in situ, and we compared the content of the mesh-bags to the surrounding bulk soil. The goals of the study were to determine the composition of the microbial community, to analyze the direct interactions between microbes and biotite and to identify the nature of the newly formed organo-mineral associations within the mesh-bags. The 16S rRNA gene sequencing and ITS2 region characterization revealed a phylogenetic similarity between the mesh bag and bulk soil archaea/bacteria and fungi microbiomes, with significant differences in alpha- and beta-diversity and species abundances. Organic carbon pools of the mesh bags, analyzed by Fourier transform ion cyclotron resonance mass spectrometry, contained lipid-like and unsaturated hydrocarbons while the bulk soil was comprised of lignin-like and carboxyl-rich alicyclic molecules. These results support the in-situ formation of organic compounds by microbes in the mesh bags and indicate less stable organic carbon pools than found in the bulk soil. However, organic material that strongly adhered to the mineral surfaces facilitated aggregation and was stabilized by organo-mineral assemblies as "nanocrusts" on the biotite surfaces, as documented by high-resolution electron microscopy. Furthermore, elemental depletion was observed at the microbe-mineral interface by nano-scale chemical analysis that indicated mineral cations biogenically released from the biotite. Overall, this study elucidated the direct and indirect involvement of rhizospheric microbial communities in the formation of organo-mineral associations, soil organic carbon stabilization, and mineral weathering at micro- and nano-scale.