Bacterial communities of an agricultural soil amended with solid pig and dairy manures, and urea fertilizer

• Bacterial diversity based on pyrosequencing increased with manure treatments.
• Diversity also increased with progression of the growing season.
• Communities of manure and urea treated soils converged with progression of growing season.
• Diversity increase was not from taxa added but likely from nutritional resources of the manures.

Agricultural management practices impact the bacterial diversity of soil but it is unclear how bacterial communities respond to different nutrient sources. This study examined the impacts of manure and granular urea N additions on the diversity and composition of soil bacterial communities. Bacterial communities in an annual cropping system were examined in the short-term (within season) and medium-term (after three successive annual additions) following manure and urea N applications. Soil samples were collected from an experimental field site in fall 2007 prior to imposition of treatments, and post-planting, mid-season, and post-harvest in 2010 following three successive annual applications. Treatments included: solid pig manure (SPM), solid dairy manure (SDM), granular urea N-fertilizer, and unamended control. Pyrosequencing was used to characterize bacterial communities in soil and the manure added. Psychrobacter was the most abundant genus in both SPM and SDM, however it was not detected in soil. Solid pig manure treatments had greater diversity than urea and control treatments and diversity was greatest at post-harvest in fall than post-planting in spring. In 2010, the relative abundances of many bacterial taxa were affected by treatment and sample season but not their interaction. Where Actinobacteria, Firmicutes, Gemmatimonadetes and Bacteroidetes were significantly different for the urea and manure treatments, Proteobacteria declined in relative abundance over the growing season. Communities of manure and urea treated soils converged with progression of growing season. Redundancy analysis showed SO4−2, NO3− and NH4+ concentrations were significant, explaining 44% of the variation observed in bacterial communities across treatments and sample seasons. In conclusion, bacterial diversity increased with manure treatment and with progression of the growing season, the former being not as a result of introduction of taxa from the manure but likely from nutritional resources provided in the organic amendments.