Soil Bacterial Communities Respond to Mowing and Nutrient Addition in a Steppe Ecosystem

<div><p>In many grassland ecosystems, nitrogen (N) and phosphorus (P) are added to improve plant productivity, and the aboveground plant biomass is mowed and stored as hay for the bullamacow. Nutrient addition and mowing affect the biodiversity and ecosystem functioning, and most of the previous studies have primarily focused on their effects on macro-organisms, neglecting the responses of soil microbial communities. In this study, we examined the changes in three community attributes (abundance, richness, and composition) of the entire bacterial kingdom and 16 dominant bacterial phyla/classes in response to mowing, N addition, P addition, and their combinations, by conducting a 5-year experiment in a steppe ecosystem in Inner Mongolia, China. Overall, N addition had a greater effect than mowing and P addition on most of these bacterial groups, as indicated by changes in the abundance, richness and composition in response to these treatments. N addition affected these soil bacterial groups primarily through reducing soil pH and increasing available N content. Meanwhile, the 16 bacterial phyla/classes responded differentially to these experimental treatments, with Acidobacteria, Acidimicrobidae, Deltaproteobacteria, and Gammaproteobacteria being the most sensitive. The changes in the abundance, richness, and composition of various bacterial groups could imply some potential shift in their ecosystem functions. Furthermore, the important role of decreased soil pH caused by N addition in affecting soil bacterial communities suggests the importance of restoring acidified soil to maintain soil bacterial diversity.</p></div

The effects of mowing (M), nitrogen addition (N), phosphorus addition (P) and their combination on the composition of the 16 dominant bacterial phyla/classes revealed by PERMANOVA.

<p>The composition of each bacterial phylum/class means the relative abundance of each OTU within this phylum/class. See the effect of these treatments on the composition of the entire bacterial kingdom in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0084210#pone-0084210-g001" target="_blank">Fig. 1c</a>. * denotes <i>P</i><0.05.</p

Effects of experimental treatments on the relative abundances of 16 dominant bacterial phyla/classes.

<p>Three-way ANOVA was used to test the effect of experimental treatments. For clarity, only the significant statistical results (<i>P</i><0.05) are shown in the figure. The bars represent one standard error (n = 4). The black and gray columns represent the treatments without and with mowing, respectively.</p

Variables responsible for the changes in abundance, richness, and composition of various bacterial groups.

<p>Effective factors represent the factors with significant effects on soil bacterial communities. See the details in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0084210#pone-0084210-g001" target="_blank">Figs 1</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0084210#pone-0084210-g003" target="_blank">3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0084210#pone-0084210-t001" target="_blank">Table 1</a>. In this part of effective factors, M, N, and P represent mowing, N addition, and P addition, respectively. ‘×’ represents the interaction among different treatments. In the result part, pH, N, P, water, Nav, C/N, C/P, N/P represent soil pH, N content, P content, water content, available N content, C/N ratio, C/P ratio and N/P ratio, respectively. There were 32 samples for the regressions.</p

Fabricating Upconversion Fluorescent Probes for Rapidly Sensing Foodborne Pathogens

Rare earth-doped upconversion nanoparticles (UCNPs) have promising potential in the field of food safety because of their unique frequency upconverting capability and high detection sensitivity. Here, we report a rapid and sensitive UCNP-based bacterium-sensing strategy using <i>Escherichia coli</i>. Highly fluorescent and water-soluble UCNPs were fabricated and conjugated with antibodies against <i>E. coli</i> for use as fluorescent probes. The <i>E. coli</i> were successively captured by the fluorescent probes. After the captured cell samples were pelleted, the differences in the fluorescence intensities between sample supernatants and the control were observed to increase linearly with <i>E. coli</i> concentration from 42 to 42 × 10⁶ colony-forming units (cfu)/mL (<i>R</i>² = 0.9802), resulting in a relatively low limit of detection of 10 cfu/mL. Furthermore, the ability of the bioassay to detect <i>E. coli</i> was also confirmed in adulterated meat and milk samples

Effects of experimental treatments on the abundance, richness, and composition of the entire bacterial community.

<p>For clarity, only the significant statistical results (<i>P</i><0.05) are shown in the figure. In (a) and (b), the bars represent one standard error (n = 4).</p

Effects of experimental treatments on the OTU richness of 16 dominant bacterial phyla/classes.

<p>Three-way ANOVA was used to test the effect of experimental treatments. For clarity, only the significant statistical results (<i>P</i><0.05) are shown in the figure. The bars represent one standard error (n = 4). The black and gray columns represent the treatments without and with mowing, respectively. The number in the brackets following the phylum/class name (e.g., 232 in Acidobacteria(232) in Fig. 3a) represents the sampled sequence number from which OTU richness was calculated.</p

Effects of treatments on the abundance, richness, and composition of the entire soil bacterial community.

<p>The effects of block on abundance and richness were non-significant (<i>P</i>>0.05), and were not shown in the figure. The compositional variation is represented with the measure of weighted UniFrac, with the r² values between ordination distance and distance in the original space being 0.52 and 0.10 for axis 1 and axis 2, respectively (c). W and T represent watering and warming, respectively. The bars represent one standard error.</p

Effects of watering, warming, and their combination on the relative abundance of the 16 bacterial phyla/classes.

<p>Two-way ANOVA for a blocked split-plot design was used to test the effects of experimental treatments. The effects of block were non-significant (<i>P</i>>0.05), and were not shown in the figure. W and T represent watering and warming, respectively. The bars represent one standard error.</p

Changes in the relative biomass (%) of plant functional groups in four grasslands.

<p>The plant functional groups were classified as PB (perennial bunchgrasses), PR (perennial rhizome grass), and others. SG–F, fenced <i>S. grandis</i> grassland; SG–G, grazed <i>S. grandis</i> grassland; LC–F, fenced <i>L. chinensis</i> grassland; LC–G, grazed <i>L. chinensis</i> grassland.</p