Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 8, 2007
Publication Date: June 11, 2007
Citation: Ibekwe, A.M., Kennedy, A.C., Halvorson, J.J., Yang, C. 2007. Characterization of developing microbial communities in Mount St. Helen's pyroclastic substrate. Soil Biology and Biochemistry. Vol 39:2496-2507 Interpretive Summary: The eruption of Mount St. Helens in 1980 and the subsequent deposit of volcanic substrates provided opportunities for scientists seeking to understand the mechanisms of early succession of ecosystems to study changes to above- and belowground species 20 years after the eruption. In this study, early legume colonists (lupines) of Mount St. Helens volcanic flows were important mediators of above and below-ground succession because they were the main sources of C and N that impacted soil genesis, establishment of other plant species, and development of soil microbial communities. The success of legumes as the early colonist of infertile volcanic substrates can be attributed to its ability to use carbon from photosynthesis to provide soil carbon and nitrogen through nitrogen fixation. The availability of these nutrients in the soil provided excellent opportunities for the development of localized areas for different species of microorganisms and plants to develop into new communities. Therefore, subsequent ecosystem development will likely be driven by changes in volcanic substrate quality as indicated by increasing C and N, microbial activities, and total microbial community composition.
Technical Abstract: The 1980 eruption of Mt. St. Helens created a unique opportunity to study microbial communities in a developing soil ecosystem containing little C or N. We collected surface samples (0-5cm) from a forest area near Mount St. Helens National Volcanic Monument and from bare soil with no plant development (BARE), soil under living (LIVE) and dead (DEAD) prairie lupine (Lupinus lepidus) in the pyroclastic plain near Spirit Lake. Phospholipids fatty acids (PLFAs) from pyroclastic materials were analyzed to determine changes in microbial composition among the four soils and compare shifts in principal component analysis of PLFA fingerprints of soil with and without plants. Total bacterial DNA was also extracted from the soils and DNA sequence analysis of cloned 16S rRNA gene fragments and denaturing gradient gel electrophoresis (DGGE) of 16S rRNA were used to determine the influence of plants on microbial development. Lupine plants influenced the PLFA fingerprint depending on the distance of the plant from the sample. Forest and bare soils were distinct from one another and more variable than those samples under live and dead plants. Shannon index of diversity calculated from the DGGE bands increased on the average from 1.03 in the bare soil to 1.19 in the forest soil and to 1.22 and 1.25 for dead and live soils, respectively. Phylogenetic analyses revealed that 62% of the 114 16S rRNA clones were classified as Proteobacteria, Actinobacteria, Acidobacterium, Verrucomicrobia, Cytophaga– Flexibacter–Bacteroides group, Cyanabacteria, Planctomyces, and candidate divisions TM7 and OP10. Thirty eight percent of the 16S clones could not be classified into known bacterial divisions. Members of Proteobacteria represented 29% of the clone library. Our data indicate the direct impact living and dead plant materials have on whole soil lipid profiles and microbial development in early successional ecosystems.