Testing the relationship between fungal-to-bacterial dominance and carbon mineralization

Background/Question/Methods

Organic inputs (i.e. leaf litter and root exudates) are primarily consumed by bacteria and fungi in most terrestrial ecosystems and as such these organisms are central to the carbon (C) cycle. Because bacteria and fungi differ in a wide array of physiological and ecological attributes, shifts in the relative abundance of either of these groups is expected to directly impact this cycle. Fungal dominated systems are expected to cycle carbon internally leading to greater carbon storage and less loss of carbon as CO₂ per unit substrate while the opposite is expected for bacterial dominated systems. However this proposed dichotomy between fungal and bacterial dominated systems has only been implied and remains untested. Here, using a combination of stable isotope and molecular techniques, we set out to determine if fungal-to-bacterial dominance was related to the rate at which organic substrates are mineralized to CO₂. We tested this in situ, across a land-use gradient, in the Calhoun Experimental Forest (South Carolina, USA) where we added ¹³C-labeled glucose and subsequently tracked the production of ¹³CO₂. For these sites, we also determined the fungal-to-bacterial dominance, via qPCR, and the relative abundance of dominant fungal and bacterial taxa, via clone libraries, of the microbial community.

Results/Conclusions

Results from this experiment demonstrated that fungal-to-bacterial dominance was related to glucose mineralization (r²=0.45; P<0.05). With those sites dominated by fungi mineralizing less glucose than sites dominated by bacteria. However when we accounted for the actual composition of the microbial community (i.e. the relative abundance of fungal and bacterial taxa) our model explained ~40% more of the variation in glucose mineralization (r²=0.85; P<0.0001). One mechanism which may have accounted for the greater variation in glucose mineralization explained by community composition is the possibility that fungal and bacterial taxa overlap in a range of ecological attributes. The traditional idea concerning fungal-to-bacterial dominance typically treats all fungi as K-strategists and all bacteria as r-strategists. Our results suggest that this generalization may not entirely be the case. We suggest that taxa within either kingdom may exhibit attributes of either ecological strategy. Furthermore, if we are to accurately predict how microbial communities will process C-substrates we may need to account for the actual composition of microbial communities as well as the ecological strategies of specific taxa within these communities.