Calcium-amended soils exhibit altered bacterial community structure at Hubbard brook experimental forest (HBEF), USA

Soil Ca depletion due to acidic deposition related soil chemistry changes has led to the decline of forest productivity and carbon sequestration in the northeastern USA. In 1999, acidic watershed (WS) 1 at the Hubbard Brook Experimental Forest (HBEF), NH, USA was amended with Ca-silicate to restore soil Ca pools. In 2006, soil samples were collected from the Ca-amended (WS1) and reference watershed (WS3) for comparison of bacterial community composition between the two watersheds. The sites were about 125 meters apart and were known to have similar stream chemistry and tree populations before Ca-amendment. Ca-amended soil had higher Ca and P, and lower Al and acidity as compared to the reference soils. Analysis of bacterial populations by PhyloChip revealed that the bacterial community structure in the Ca-amended and the reference soils was significantly different, and that the differences were more pronounced in the mineral soils. Overall, the relative abundance of 300 taxa was significantly affected. Numbers of detectable taxa in families such as Acidobacteriaceae, Comamonadaceae and Pseudomonadaceae were lower in the Ca-amended soils, while Flavobacteriaceae and Geobacteraceae were higher. The other functionally important groups, e.g. ammonia-oxidizing Nitrosomonadaceae, had lower numbers of taxa in the Ca-amended organic soil but higher in the mineral soil. The NMDS ordination of PhyloChip data revealed that both the organic and the mineral soil bacterial communities diverged from their respective reference soils following Ca addition. The data show that both Ca treatment and soil horizon explain a significant amount of variance in the bacterial community composition, with Ca addition explaining more. Further analysis showed that among all the soil chemistry variables tested, only Ca, P, Al, and pH were positively correlated to bacterial community structure. Overall the study detected 1756 bacterial taxa spanning 42 phyla, 53 classes, 127 orders, and 154 families from the two watersheds.