Immobilization and mineralization of N and P by heterotrophic microbes during leaf decomposition

According to theory, the rate and stoichiometry of microbial mineralization depend, in part, on nutrient availability. For microbes associated with leaves in streams, nutrients are available from both the water column and the leaf. Therefore, microbial nutrient cycling may change with nutrient availability and during leaf decomposition. We explored spatial and temporal patterns of mineralization by heterotrophic microbes by placing packs of red maple leaves at sites in 5 Appalachian streams spanning a range of N and P availability. We collected packs 4 times from each site. Leaf disks from these packs were incubated in microcosms, and uptake rates and steady-state concentrations of NH4+ and soluble reactive P (SRP) were used to calculate mineralization rates. N uptake peaked between 50 and 60 d, whereas P uptake peaked ∼10 d later. Clear patterns were found for fungal biomass-specific uptake or mineralization fluxes of either nutrient over time or space, but the microbes grown in the site with the lowest nutrient availability had the highest fungal biomass-specific cycling. The ability of microbes to access nutrients from their substrate may prevent dissolved nutrient availability from being a strong driver of microbial nutrient cycling.