You are here

Harvard Forest >

Harvard Forest Symposium Abstract 2012

  • Title: Chronic N-amended soils exhibit an altered bacterial community structure in Harvard Forest, MA
  • Primary Author: Swathi Turlapati (University of New Hampshire - Main Campus)
  • Additional Authors: Premsai Bhiravarasa (University of New Hampshire - Main Campus); Rakesh Minocha (USDA Forest Service); Subhash Minocha (University of New Hampshire); William Thomas (University of New Hampshire); Louis Tisa (University of New Hampshire)
  • Abstract:

    The impact of 20 years of annual ammonium nitrate (N) application on the soil bacterial communities at Harvard Forest was studied using pyrosequencing of rDNA isolated from the organic (Org) and the mineral (Min) horizons of soil samples from three sets of plots: a control (Con) plot, low N-treated (LN, 50 kg ha-1year-1), and high N-treated (HN, 150 kg ha-1year-1) plots. Primers with 30 different barcodes were designed to amplify a 433-bp fragment of the 16S rRNA gene from 30 soil DNA samples (3 treatments x 2 horizons x 5 replicates) collected in 2009. The 30 bar-coded samples were pooled in equimolar quantities for sequencing. The data (1.3 million sequences) were processed using the Quantitative Insights into Microbial Ecology (QIIME) pipeline and a variety of other standard software. An Operational Taxonomic Unit (OTU) is defined as a group of sequences identified in a taxonomic study without designation of their terminal taxonomic rank. The representative OTU sequences were aligned to the Greengenes database, and a taxonomic assignment was made for each representative sequence using the Ribosome Database Project (RDP).



    Whereas Acidobacteria represented the most abundant phylum based on sequence numbers, Proteobacteria were the most diverse in terms of OTUs. UniFrac and NMS analyses followed by permutational MANOVA) revealed that the bacterial communities differed significantly (P < 0.01) with the soil horizon and treatment (Fig. 1A). A Mantel test revealed a strong positive correlation between the soil chemistry and the ordination scores. The pH was observed to strongly influence the bacterial communities in the Org soil samples, as indicated by a strong positive correlation (Table 2). As many as one-third of the total OTUs were specific to each horizon, which demonstrates that the two soil horizons, which are separated vertically by only a few centimeters, harbor very different bacterial taxa. Of the total 8,100 OTUs unique to the 15 organic samples, 14, 19 and 22% were found only in the Con-Org, LN-Org, and HN-Org samples respectively; 13, 19 and 18% were found only in the Con-Org, LN-Org, and HN-Org samples respectively (Fig. 1B). Only 25-28% of these OTUs were common to all three treatments within each soil horizon, which suggests major rearrangements in the structure of the bacterial community in response to N addition. Shifts in the bacterial communities were obvious at the family and genus levels. The richness indicators also revealed higher bacterial diversity associated with N amendment. Whereas 2% of the OTUs were represented by >50% of the total sequences in each sample, ~80% of the OTUs accounted for only 10% of the total sequences. The changes in the bacterial community structure in response to chronic N treatment observed at Harvard Forest are perhaps the cumulative outcome of N-driven changes in the soil base cation content, foliar physiology, net aboveground forest productivity and fungal metabolism reported earlier for this study. This manuscript was submitted to PNAS Plus on Feb, 2012.

  • Research Category: Biodiversity Studies
    Ecological Informatics and Modelling
    Large Experiments and Permanent Plot Studies
    Physiological Ecology, Population Dynamics, and Species Interactions

  • Figures:
  • Harvard Forest Abstract-Turlapati et al.2012-Figure 1A and 1B.pdf
    Harvard Forest Abstract-Turlapati et al.2012-Figure 1C.pdf
    Table-1 -Turlapati et al.-2-08-12.pdf