Relative impacts of land-use, management intensity and fertilization upon soil microbial community structure in agricultural systems

Soil microbial communities under three agricultural management systems (conventionally tilled cropland, hayed pasture, and grazed pasture) and two fertilizer systems (inorganic fertilizer and poultry litter) were compared to that of a ∼150-y-old forest near Watkinsville, Georgia. Both 16S rRNA gene clone libraries and phospholipid fatty acid (PLFA) analyses indicated that the structure and composition of bacterial communities in the forest soil were significantly different than in the agricultural soils. Within the agricultural soils, the effect of fertilizer amendment on bacterial communities was more dramatic than either land use or season. Fertilizer amendment altered the abundance of more bacterial groups throughout the agricultural soils. In addition, the changes in the composition of bacterial groups were more pronounced in cropland than in pastures. There was much less seasonal variation between the soil libraries. Community-level differences were associated with differences in soil pH, mineralizable carbon and nitrogen, and extractable nutrients. Bacterial community diversity exhibited a complex relationship with the land use intensity in these agro-ecosystems. The pastures had the highest bacterial diversity and could be characterized as having an intermediate degree of intervention compared to low intervention in forest and high intervention in cropland. Changes in bacterial diversity could be attributed to the abundance of a few operational taxonomic units (OTUs). The microdiversity of abundant OTUs in both forest and cropland was consistent with an increase in abundance of many phenotypically similar species rather than a single species for each OTU. Soil microbial communities were significantly altered by long-term agricultural management systems, especially fertilizer amendment, and these results provide a basis for promoting conservation agricultural systems.