Long term changes in organic matter content drive microbial community structure in soils

Background/Question/Methods

            Soil organic matter content can change in response to land management and such changes are often associated with alteration of microbial community structure and diversity. Confounding environmental factors and the low resolution of molecular fingerprinting techniques have made it difficult to disentangle the influence of land-use and SOM change on microbial community structure. Our goal was to examine how management induced changes in SOM over several decades altered microbial community composition and spatial structure. We hypothesized that similar SOM would converge community compositions, and increasing SOM would cause more heterogeneity in community spatial structure.

            We examined 4 sites with similar edaphic characteristics representing a gradient of SOM quantity and quality. The sites included 3 fields with similar long-term history of cultivation including one field conventionally-managed continuous corn (0.9% C), and two fields organically managed with diversified crop rotations and green manures for 10 years (1.3% C) or 20 years (1.7% C). In addition, we examined a field in pasture for more than 50 years (2.1% C). We sampled soils along spatially explicit transects with distances ranging from 30cm to 5.6km. Soil DNA was extracted and PCR amplified using multiplexed primers targeting the 16S rRNA gene, and sequenced using 454 pyrosequencing.

Results/Conclusions

            Microbial beta-diversity was strongly correlated to SOM when controlling for spatial distance (Mantel’s r=0.86, p<0.01). Greater SOM differences produced more phylogenetically dissimilar communities (weighted UniFrac metric). Community dissimilarity also increased with spatial distance, but the distance-decay relationship was much weaker than that of SOM (Mantel’s r=0.14, p<0.01). Spatial autocorrelation was observed within each site with greater spatial variation present in 20 year organic and pasture sites than in conventional and 10 year organic sites.

            Our results suggest management-induced SOM change strongly impacted soil microbial community structure at the landscape scale. Organic management rebuilt SOM content over several decades. The correlation between beta-diversity and SOM suggests SOM accretion was associated with changes in microbial structure over time. Management practices were identical between 10 year and 20 year organic fields but microbial community composition was significantly different indicating management alone did not drive patterns of community structure. Organic management also increased microbial spatial heterogeneity in the 20 year organic sites versus conventional and 10 year organic sites, and this change is likely associated with changes in community structure over time. Our results imply that restoration of SOM pools through long-term management can potentially reconstruct soil microbial community beta-diversity.