Diversity and activity of free-living diazotrophic and total bacterial communities in organic and conventionally managed soil

Agricultural soils are heterogeneous environments in which conditions affecting microbial growth and diversity fluctuate widely in space and time. In this study, the molecular ecology of the total bacterial and free-living nitrogen fixing communities in soils from the Nafferton factorial systems comparison (NFSC) study in North East England were examined. The field experiment is factorial in design with organic versus conventional crop rotation, crop protection and fertility management factors. Soils were sampled on three dates (March, June and September) in 2007. Total RNA was extracted from all soil samples and reverse transcribed. DGGE and qPCR were used to analyse nifH and 16S rRNA genes in order to study free-living diazotrophs and the total bacterial community respectively. Crop rotation was shown to have a significant effect on total bacterial diversity (and that of free-living N fixers) (P=<0.001). On all three dates nifH activity was higher in the conventional crop rotation. In contrast, qPCR analysis of free-living N fixers indicated significantly higher levels of activity in conventionally fertilised plots in June (P=0.0324) and in plots with organic crop protection in September (P=0.0143). To our knowledge the effects of organic and conventional farming systems on free-living diazotrophs have never been studied. An increased understanding of the impacts of management practices on free-living N fixers could allow modifications in soil management practices to optimize the activity of these organisms.