Bacterial diversity of the broadbalk 'classical' winter wheat experiment in relation to long-term fertilizer inputs

With more than 160 years of contrasting fertilizer regimes, the Broadbalk winter wheat experiment represents a unique experimental resource for studying the effects of long-term fertilizer application on microbial population diversity. Using DGGE and clone library analysis, we report here on eubacterial species diversity (16S rRNA gene) and diversity within two sets of gene products associated with microbial N acquisition: NifH (nitrogen fixation) and AmtB (ammonium transport). Comparisons were made within and between soils treated with mineral N fertilizer, farmyard manure or receiving no fertilizer. Analysis of 16S rRNA gene DGGE profiles showed no clear patterns to qualitatively distinguish bacterial community structure between the three different treatments (P > 0.05), with all samples containing a range of eubacterial taxa similar to those that are characteristic of soil bacteria reported elsewhere. Intra-plot heterogeneity was high and of a similar magnitude to that between treatments. This lack of qualitative between plot differences was echoed in the representative sequences of 16S rRNA, nifH, and amtB genes in the various samples. Taken together, both phylogenetic and functional gene analyses showed bacterial communities in the Broadbalk-trial soil were very stable and relatively non-responsive to long-term management of balanced fertilizer inputs.