Northern Arizona University Flagstaff, AZ, United States
Temperatures in northern latitudes are disproportionately affected by climate change, and warming Arctic tundra ecosystems are further affecting carbon balance to the atmosphere. Warming increases permafrost thaw and soil microbial decomposition rates that act as a carbon source, the magnitude of which is partially offset by accelerated plant productivity, shrub encroachment, and altered litter inputs into soil. Understanding how individual microbial taxa will respond to warming and resultant ecosystem changes can provide insight into the underlying mechanisms that will determine net soil carbon flux to the atmosphere. Here, we investigate how thirty years of experimental warming in a moist acidic tundra affected bacterial and fungal growth rates. To uncouple the direct effects of warming on bacterial and fungal growth rates from effects mediated by other ecosystem responses to warming, we conducted a parallel, short-term warming experiment (3 months). Intact, active layer soil was assayed in the field for 30 days with 18O-labeled water. Rates of 18O incorporation into DNA were estimated for each taxon as a proxy for growth, and microbial interactions were evaluated. The ecology of these microbial growth responses was then investigated via network analysis.
Mean relative growth rates of fungi across treatments were greater than those of bacteria. Short-term warming increased relative growth rates of bacteria by 36%, and long-term warming increased relative growth rates of bacteria by 251% and fungi by 31%. Long-term warming and the control shared a core microbial network, the majority of which were fungi, but the short-term warming network was dominated by bacteria that did not grow in other treatments. In all networks, relative growth rates of bacteria were negatively correlated with metrics of node betweenness and positively correlated with node transitivity. This suggests that interconnectedness within a bacterial taxon’s local cluster may increase growth, while connectivity to the network at-large may decrease growth. Metrics of network stability (i.e. modularity and negative:positive interactions) decreased in short-term warming and increased in long-term warming. Overall, our results indicate that bacterial and fungal communities demonstrate resilience in response to warming. Warming in the short-term resulted in an increase of relative growth rates of a transient bacterial cohort and a perturbation of community structure and stability. However, in subsequent decades, core network structure returned, bacterial and fungal growth continued to increase, and microbial networks exhibited qualities of increased stability.