Inorganic Fertilizer and Poultry-Litter Manure Amendments Alter the Soil Microbial Communities in Agricultural Systems.

To better understand the effects of agricultural land management practices on soil bacterial communities, their composition and diversity in soils under different land uses (conventionally tilled cropland, hayed pasture, and grazed pasture), fertilizer amendments (inorganic fertilizer or IF and poultry litter manure or PL) and seasons (summer and winter) near Watkinsville, Georgia were studied using 16S rRNA gene clone libraries and phospholipid fatty acid (PLFA) analyses. A >150-year-old control forest in a nearby area was sampled for comparison. Both, rRNA libraries and PLFA analyses indicated that the difference in structure and composition of bacterial communities was largest when comparing the forest to agricultural soils. Within the agricultural soils, the effect of fertilizer amendment on bacterial communities was more dramatic than either land use or season. While fertilizer amendments changed the abundance of more bacterial groups throughout the agricultural soils, the changes in the composition of bacterial groups were more profound in cropland than the two pastures. There was very less seasonal variation between the soil libraries. The community-level differences correlated to differences in soil pH, mineralizable C and N, and extractable nutrients. The bacterial community diversity exhibited a complex relationship with the intensity of human intervention in agro-ecosystems. The pastures received an intermediate degree of intervention when compared to cropland and forest, and they possessed the highest bacterial diversity. Changes in bacterial diversity reflected to a large degree the abundance of a few operational taxonomic units (OTUs). The microdiversity of the abundant OTUs found in both the forest and cropland were consistent with increases in the abundance of many phenotypically similar species rather than a single species for each OTU.