Calcium addition alters the soil microbial diversity at Hubbard Brook Experimental Forest, NH, USA

Acidic deposition has depleted calcium (Ca) from soils in northeastern forests. Calcium was applied in the form of wollastonite (calcium silicate, CaSiO₃) at watershed 1 (WS1) of the Hubbard Brook Experimental Forest (HBEF), NH, USA in 1999 to address the feasibility of improving forest growth by restoration of Ca pools. Recent studies showed that Ca treatment caused a decline in microbial biomass N, potential net N mineralization and soil inorganic N levels. However, no detailed information is available on the effects of Ca-treatment on changes in the microbial community profiles (abundance and composition) or in specific microbial function/role(s) in these soils. In this study, we provide the first detailed characterization of bacterial diversity at the HBEF through analysis of small-subunit rRNA gene (16S rRNA). DNA was isolated from soil samples collected from reference and Ca-treated sites at the HBEF and amplified using universal 16S rRNA primers. The amplicons were cloned and nearly 400 clones from each site were partially sequenced. Preliminary results showed that the Acidobacteria phylotype was the most abundant and diverse group among the analyzed clones. Other phylotypes including Proteobacteria, Planctomycetes, Bacteroidetes and unclassified bacteria are also represented in the library, while divisions such as Verrucomicrobia, Genera_incertae_sedis_OP10 represented only 1% or less in the library of clones analyzed. An increase of about 12% (organic layer) and 5% (mineral layer) in Acidobacteria clones was found at the Ca-treated site as compared to reference site, suggesting that Ca-addition had an impact on members of this group. Nitrospira was found only at the reference site, while Firmicutes, Actinobacteria and Genera_incertae_sedis_TM7 were only seen at the Ca-treated site. Community profiling using DGGE revealed a qualitative difference between the organic and mineral layers in both the reference and Ca-treated sites.