Fungi mediate the cycling of soil carbon (C) and nitrogen (N)—two of the most important elements needed to sustain biological functions in ecosystems. Since both C and N utilization are energetically expensive, fungi may preferentially invest in acquiring one resource at the expense of another. This hypothesis emerged as a result of ecosystem-scale observations showing soil C accumulation as a result of elevated soil N enrichment. Among other mechanisms (e.g., reductions in fungal biomass and shifts in fungal community composition), evolutionary adaptation may cause fungi to specialize on acquiring N at the expense of losing decomposition traits. Over thousands of generations, fungi may lose the ability to decompose certain types of organic substrates and may become more efficient at taking up N. Genome analysis of over 1000 fungi provided evidence to support this idea of an ecological tradeoff between decomposition and N uptake; fungal functional groups with high frequency of decomposition genes have a lower frequency of N uptake genes, and vice versa. Is this ecological tradeoff shaping fungal communities exposed to elevated N? Are fungi exposed to elevated N losing their ability to decompose certain types of organic substrates? Here, we aimed to answer these questions by testing patterns of organic substrate use for 25 different fungal species isolated from soils from three long-term simulated N deposition field studies on the northeast of the USA. We hypothesized that fungi isolated from N enriched plots would use organic substrates at lower rates in comparison to the same species isolated from control plots. We predicted that this observation would be true even in the absence of N.
Results/Conclusions
We found support for our hypothesis because fungi isolated from N enriched plots used certain organic substrates at lower rates compared to the same species from control plots, regardless of N availability. Our results show that fungi exposed to elevated N have lost their ability to decompose organic substrates as efficiently as fungi from control plots. Since this observation is consistent even in the absence of N, we concluded that the availability of N is not a factor affecting gene expression profiles of decomposition or N uptake genes. Instead, the absence of N, suggests that the lower organic substrate use rate of fungi from N enriched plots is the results of evolutionary adaptation. In other words, exposure to elevated soil N concentrations, acted as a selection force, favoring fungi with lower capacity to decompose.