Abstract
The cultivation of grasslands can modify both bacterial community structure and impact on nutrient cycling as well as the productivity and diversity of plant communities. In this study, two pristine New Zealand grassland sites dominated by indigenous tall tussocks (Chionochloa pallens or C. teretifolia) were examined to investigate the extent and predictability of variation of the bacterial community. The contribution of free-living bacteria to biological nitrogen fixation is predicted to be ecologically significant in these soils; therefore, the diazotrophic community was also examined. The C. teretifolia site had N-poor and poorly-drained peaty soils, and the C. pallens had N-rich and well-drained fertile soils. These soils also differ in the proportion of organic carbon (C), Olsen phosphorus (P) and soil pH. The nutrient-rich soils showed increased relative abundances of some copiotrophic bacterial taxa (including members of the Proteobacteria, Bacteroidetes and Firmicutes phyla). Other copiotrophs, Actinobacteria and the oliogotrophic Acidobacteria showed increased relative abundance in nutrient-poor soils. Greater diversity based on 16S rRNA gene sequences and the Tax4Fun prediction of enhanced spore formation associated with nutrient-rich soils could indicate increased resilience of the bacterial community. The two sites had distinct diazotrophic communities with higher diversity in C. teretifolia soils that had less available nitrate and ammonium, potentially indicating increased resilience of the diazotroph community at this site. The C. teretifolia soils had more 16S rRNA gene and nifH copies per g soil than the nutrient rich site. However, the proportion of the bacterial community that was diazotrophic was similar in the two soils. We suggest that edaphic and vegetation factors are contributing to major differences in the composition and diversity of total bacterial and diazotrophic communities at these sites. We predict the differences in the communities at the two sites will result in different responses to environmental change.
Highlights
Widespread agricultural development of grasslands has made them among the most threatened ecosystems worldwide [1,2,3]
For the 16S rRNA gene sequences the number of OTUs (6667 ± 453 [mean ± SD] versus 4852 ± 444; ANOVA, P < 0.001) and the Shannon diversity index (9.9 ± 0.2 versus 9.1 ± 0.3; ANOVA, P < 0.001) were higher in C. pallens soils compared to C. teretifolia soils (Fig 1a)
It was unexpected that the higher diversity and richness of the diazotrophic community was at the C. teretifolia site where there was lower diversity and richness of the bacterial community based on 16S rRNA gene sequences
Summary
Widespread agricultural development of grasslands has made them among the most threatened ecosystems worldwide [1,2,3]. The diversity of soil microbial communities can impact on a range of ecosystem processes; for example, fertilizer addition, elevated temperature and/or CO2 have been linked to altered bacterial community composition and community level activity (reviewed in [12]) [13,14]. For a variety of soil types, differences in N availability, drainage and the proportion of organic C have previously been shown to alter both the total bacterial community [15] and the diazotrophic community [16]. Consistencies in the response of grassland bacterial community composition to nutrient addition have been identified across a broad geographic range; local variation in bacterial community structure demonstrates the need for ecosystem and soil-type specific bacterial community analyses [3,17,19]
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