Temperature-dependent latitudinal gradients in forest soil microbial diversity

Background/Question/Methods

Although the latitudinal of biodiversity pattern is well-documented and intensively studied in plant and animal ecology, it is not clear whether microbes also exhibit similar latitudinal diversity gradients. Due to their small size, extreme abundance, high dispersal capabilities, and low extinction rates, some have hypothesized that unicellular organisms should exhibit little or no latitudinal diversity patterns. To determine whether soil microbes exhibit latitudinal diversity gradients and whether such patterns are closely and mechanistically related temperature as predicted by metabolic theory of ecology (MTE), we surveyed the diversity of microbes in 126 soil samples from six forest sites in North America. These sites provide variation in ecosystem type from boreal to tropical forest, in average annual temperature from -4 to 27°C, and a rough gradient of latitude from 9-40^oN, and in other abiotic and biotic environmental variables such as soil pH, moisture and plant diversity. High-throughput sequencing technology has been implemented on 16S rRNA genes for bacteria, on internal transcribed spacers (ITS) for fungi, and on nifH for nitrogen fixing bacteria. 

Results/Conclusions

The microbes in the soil at our forest sites exhibited strong correlations with latitude and environmental temperature. Taxon richness and taxonomic diversity measured by the Shannon index decreased significantly with the increasing latitude in all three assays: for bacteria, fungi, and N-fixers. As predicted by MTE, the numbers of microbial taxa based on all three genes increased exponentially with environmental temperature (R²=0.26-0.92). However, the turnover rates between sites in taxon richness was 2-5  times smaller than for coexisting tree species, suggesting that the methods of estimating biodiversity for microbes and plants are not comparable, and/or the two groups of organisms respond very different to environmental variation in climate and related variables. To our knowledge, this is the first time that diversity of soils microbes has been shown to be associated with environmental temperature and latitude.  Elucidating such diversity patterns and the underlying mechanisms is important for understanding the complex ecology of forest soils and for assessing the effects of human-caused changes in climate, land use, and other factors.