Dissolved nutrient ratios and concentrations affect fungal reproduction and community structure associated with submerged leaf litter and wood

Litter-associated fungi are important intermediaries in carbon and energy flow in streams. They can obtain nitrogen (N) and phosphorus (P) from both the substrate and the water column. I tested the effects of dissolved nutrients (dissolved inorganic nitrogen (DIN) 40-975 microg/L, soluble reactive phosphorus (SRP) 2-135 microg/L, N:P ratios 2:1, 16:1, 128:1) on fungi associated with leaf litter (maple and rhododendron) and wood in streamside channels at Coweeta Long Term Ecological Research site, North Carolina. This study addressed two main questions: (1) will fungal reproduction (sporulation rate of aquatic hyphomycetes) peak at a dissolved N:P ratio similar to nutrient stoichiometry of fungal biomass, and (2) what are the effects of dissolved nutrients on fungal community structure and sporulation associated with plant litter that differs in initial N and P content (e.g., leaves vs. wood). The highest fungal sporulation rates were found at a dissolved N:P ratio of 16:1, which is comparable to the stoichiometry of fungal biomass (~10:1). Fungal sporulation rates and cumulative spore production per unit of plant litter by the end of the experiment generally showed statistically significant relationships with DIN rather than SRP. The greatest effects of dissolved nutrients were observed on wood compared to leaf litter. Very strong relationships between fungal production and sporulation rates and between cumulative spore production and plant litter decomposition rates were found for all substrates, suggesting that fungal activity is an important driver of plant litter decomposition. Finally, dissolved nutrients affected the relative abundances of dominant species in litter-associated fungal communities. Thus, dissolved inorganic nutrients affect reproductive output and community structure of aquatic fungi, which may have important consequences for plant litter decomposition and the flow of carbon, nutrients and energy in streams.