Altered bacterial community structure of Chronic N-amended soils of hardwood stand at Harvard Forest, MA, USA

At the Harvard Forest, Petersham, MA, the impact of twenty years of annual ammonium nitrate application on the forest soil bacterial communities was studied using pyrosequencing of 16S rRNA genes. Amplification of 16S rRNA genes was done using DNA extracted from 30 soil samples (three treatments x two horizons x five subplots) collected from untreated (control; Con), low N-amended (LN; 50 kg ha^-1 yr^-1) and high N-amended (HN; 150 kg ha^-1 yr^-1) plots. A total of 1.3 million sequences were processed using QIIME program. Although Acidobacteria represented the most abundant phylum based on the number of sequences, Proteobacteria were the most diverse in terms of operational taxonomic units (OTUs). UniFrac analyses revealed that the bacterial communities differed among soil horizons and treatments. It also depicted microsite variability among five subplots. Non-metric Multidimensional Scaling (NMS) ordination of the OTU data followed by permutational MANOVA further confirmed these observations. Richness indicators revealed higher bacterial diversity associated with N amendment. Only 28 to 35% of the taxa were common to all three treatments; the rest were specific to one treatment or common to two. N-amended samples exhibited a greater richness of bacterial taxa compared to control samples. Shifts in the investigated bacterial communities were evident at the family and genus levels. Indicator species analysis revealed 27, 81 and 78 unique taxa were present in Con-Org, LN-Org and HN-Org soils; while 21,40 and 59 taxa were present in Con-Min, LN-Min and HN-Min soils, respectively (p < 0.05). Thus chronic N treatment affected the diversity and the composition of bacterial communities within organic and mineral soil horizons. The changes observed in the bacterial community structure in response to chronic N treatments may be a cumulative outcome of N-driven soil base cation changes, net changes in aboveground plant and belowground productivity. Future studies must examine the metabolic functions of these bacterial populations following changes in response to chronic N amendment and changes that occur after short-term N amendment.