Seasonal variation in fungal:bacterial dominance and its relationship to environmental factors and ecosystem processes

Background/Question/Methods

The fungal:bacterial dominance of a soil microbial community is often used as an indicator of that community’s expected relationship to environmental factors and its influence on ecosystem processes. These two expectations are driven by the hypothesis that fungi and bacteria each represent a distinct functional group within the microbial community. Specifically, fungi are expected to be more akin to K-selected organisms whereas bacteria are more akin to r-selected organisms. Under this dichotomous view, what is good for fungi is often considered bad for bacteria and vice versa. However, much of the influence a specific environmental factor has on fungal:bacterial dominance is considered without regard to other, often co-varying, factors. By employing structural equation modeling (SEM), we examine how a suite of environmental factors (i.e. land management regimes, soil carbon and nutrient pools, and soil physical-chemical properties) are seasonally related to variation in fungal:bacterial dominance (determined via quantitative PCR [qPCR]) across a land-use gradient in the Southeastern United States. We also examine how, after accounting for variation in environmental factors, fungal:bacterial dominance influences the elemental composition and activity of the microbial community and in turn how this relates to ecosystem processes.   

Results/Conclusions

We note marked seasonal variation in fungal:bacterial dominance with the highest fungal-to-bacterial dominance in the fall and winter and the lowest fungal:bacterial dominance in the summer. Furthermore, preliminary results from SEM modeling indicate that across all seasons fungal:bacterial dominance is significantly affected by dissolved organic nitrogen (DON), nitrate (NO₃^-), soil pH, and bio-available carbon (C). Specifically, as bio-available C and NO₃^- increase, and pH and DON decrease an increase in fungal:bacterial dominance is observed. On one hand these results support other observed relationships between pH and fungal:bacterial dominance. On the other hand, the positive influence of labile C and N on fungal:bacterial dominance may indicate that the general expectation that fungi dominate in low resource environments may not hold true. In conclusion via SEM, we show how fungal:bacterial dominance is influenced, in a sometimes unexpected manner, by interacting environmental factors and we plan to further examine these results by determining how fungal:bacterial dominance influences both the microbial community and ecosystem processes.