Effects of Dissolved Nutrient Ratios and Concentrations on Litter-associated Microbial Activity in Streamside Channels

Heterotrophic consumers, such as microorganisms and invertebrates, play a fundamental role in the flow of carbon and energy in streams. The effects of dissolved nutrient concentrations, and especially ratios, on litter—associated microorganisms and decomposition rates of detritus are poorly understood. This study addressed the responses of heterotrophic microbes to a wide range of concentrations and ratios of dissolved inorganic nitrogen (N) and phosphorus (P) in streamside channels simulating headwater streams. Two main questions were: (1) do microbial parameters and litter decomposition rates peak at a dissolved N:P ratio that approaches the nutrient ratio of microbial biomass, and (2) does microbial activity stabilize the nutrient content of decomposing litter possessing different initial nutrient content and carbon quality (i.e., leaf litter vs. wood)? The study was conducted using an array of 30 streamside channels that replicated the width, depth and flow rate of headwater streams at Coweeta Long Term Ecological Research Site situated in the southern Appalachian Mountains (Macon County, NC). Dissolved nutrients were added in low, medium and high concentrations at 2:1, 16:1 and 128:1 molar N:P ratios. Microbial parameters (respiration, fungal biomass, fungal and bacterial production) and litter decomposition rates peaked at a dissolved nutrient ratio of 16:1, a value that approaches the stoichiometry of fungal biomass (ca. 10:1). Multiple microbial parameters correlated with dissolved inorganic N and/or P concentrations but rarely with dissolved N:P ratios. Decreases in detrital C:N and C:P ratios strongly correlated with fungal biomass accrual due to fungal immobilization of dissolved nutrients. Microbial colonization stabilized detrital stoichiometry via more drastic changes in C:nutrient ratios of resources with high initial C:nutrient ratios (e.g., wood) compared to high quality (low initial C:nutrient ratios) leaf litter. Nutrient ratios and concentrations control microbial activity, plant litter decomposition and detrital stoichiometry, altering the flow of carbon, nutrients and energy in headwater streams