Shifts in the phylogenetic structure and functional capacity of soil microbial communities follow alteration of native tussock grassland ecosystems

Globally, tussock-based grasslands are being modified to increase productive capacity. The impacts of cultivation and over-sowing with exotic grass and legumes on soil microbiology were assessed at four sites in New Zealand which differed in soil type, climate and vegetation. Primary alteration of the soil physicochemical status occurred with land use change. This was driven by addition of mineral fertiliser and alteration of pH. Genes associated with several biogeochemical cycles (GeoChip data) were impacted by land-use but not sampling location. A number of functional gene families associated with biogeochemical cycling of C, N and S were present in greater relative abundance in the undisturbed soils. Similarly, soil bacterial (PhyloChip) and fungal (TRFLP) communities were strongly influenced by land-use change, but unaffected by sampling location. Alteration of land-use increased the relative abundance of Firmicutes, Actinobacteria and OD1 phyla, but many of the less-common phyla, such as Verrucomicrobia and Dictyoglomi decreased in abundance; these phyla may be important in internal soil nutrient cycling processes. This work provides evidence that tussock grassland soils are strongly dependent on microbially-mediated nutrient cycling, and these processes are highly-sensitive to exogenous nutrient inputs and/or alteration of pH. De-coupling of processes following addition of fertilisers or removal of organic matter (grazing) may make these improved grassland systems more susceptible to nutrient leakage. This has important implications for environmental quality.