Soil bacterial communities are resistant to rising mean annual temperature in Hawaiian tropical montane wet forests

Background/Question/Methods

Soil microorganisms play key roles in regulating Earth’s biogeochemical cycles, including soil carbon dynamics. If climate warming increases rates of carbon flux between the biosphere and the atmosphere as expected, soil bacterial communities will likely mediate a large portion of this increased flux. However, the response of soil bacterial community structure to warming remains poorly understood, with implications for feedbacks to climate change. We used 454 pyrosequencing of bacterial 16S rRNA genes to assess the diversity and composition of soil bacteria in nine permanent plots across a 5.2° C mean annual temperature (MAT) gradient in tropical montane wet forests on the Island of Hawaii. This gradient is highly constrained, with constant vegetation, disturbance history, soil type, geology, and soil water balance. Previous research revealed a substantial increase in rates of carbon cycling across this MAT gradient, leading us to hypothesize that the temperature-induced increase in labile carbon supply would alter the diversity and community composition of soil bacteria.

Results/Conclusions

Across the MAT gradient we found that (i) Proteobacteria and Acidobacteria were the dominant phyla, (ii) estimated richness of soil bacteria averaged ~ 1100 OTUs and (iii) Faith’s phylogenetic diversity averaged ~ 45; all of these results are consistent with other low pH wet tropical forest soils. In contrast to our hypothesis, we found that neither OTU richness nor phylogenetic diversity of soil bacteria varied significantly as a function of MAT (P > 0.4) and that MAT explained only a small proportion of the variance in bacterial community composition (R² = 0.03, P < 0.10). These results suggest that the structure of soil bacterial communities in these tropical montane wet forests is largely resistant to rising MAT and temperature-induced increases in labile carbon supply, likely because the chemical composition of labile carbon inputs remained similar across the MAT gradient due to constant vegetation. We conclude that over long time periods, rising temperature and large climate mediated increases in carbon supply will not alter the structure of soil bacterial communities in the absence of changes in soil pH, water balance, plant species composition, or disturbance regimes.