Spatial turnover of soil microbial communities in six forests across latitude gradients

Background/Question/Methods

The species-area (S-A) relationship has been studied intensively in animals and plants but little is known about this relationship in forest soil microbes. The parameter, z value of power model(S=cA^z), as an important indicator of beta diversity, needs to be determined on forest soil microbes. To examine the spatial turnover rate (z-value) of forest soil microbes and determine whether it varies in a systematic fashion with latitudes, we implemented a nested sampling design to survey the background pools of regional taxonomic diversity at six forest sites across America with latitude gradients. At each site, we located a central subplot first and then laid out 1-m² subplots in four directions with distances of 1m, 10m, 50m, 100m and 200m from the central subplot. In each m²plot, 9 soil cores were collected and pooled to form a soil sample. By this sampling method, 21 samples were collected for each site with a total of 126 samples. Microbial community DNA was extracted from 5 g of well-mixed soil for each sample and 16S rRNA gene amplicons from the V4 region were sequenced by Illumina MiSeq. 

Results/Conclusions

The numbers of species identified as OTUs within subplots were summarized together for the nested design regions with different areas in each site. We performed regression analysis of species-area relationship by power model. The slope (z value) of the power model falls between 0.075 and 0.088, lower than the frequently observed values (0.2-0.4) for plants and animals. The results also show that the site, Niwot, with minimum annual temperature (-4⁰C) among six sites has the lowest z value, 0.075, while the site, BCI, with maximum annual temperature (27⁰C) has the highest z value, 0.088. This result indicates that spatial turnover rate (z-value) shows a temperature gradient pattern with z systematically increasing toward tropical latitudes with a high annual temperature.