Effects of nutrient enrichment on yellow poplar leaf decomposition and fungal activity in streams

We examined the effect of nutrient addition on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and rates of fungal sporulation associated with yellow poplar (Liriodendron tulipifera L.) leaf disks in 3 streams that differed in water chemistry. We carried out these studies in flow-through channels that received additions of KH2PO4, NaNO3, both nutrients, or controls with no additions. When limiting nutrients were added to the water in all 3 streams, leaf-decaying fungi responded and decomposition rates increased. Two streams, Walker Branch and Payne Creek, contained low concentrations of both inorganic N (<40 μg/L) and P (<16 μg/L). In these streams, rates of leaf decomposition, concentrations of fungal biomass, and rates of sporulation were stimulated only when N and P were added together, indicating that these nutrients potentially colimited fungal activity. The other stream, Little Schultz Creek, contained low concentrations of P (<5 μg/L), but higher concentrations of N (65–200 μg/L) than Walker Branch and Payne Creek. Rates of leaf decomposition, fungal biomass, and sporulation were stimulated by P addition and when both nutrients were added together, indicating potential limition of fungal activity by P in this stream. Results from all 3 streams provide direct experimental evidence that leaf-decaying fungi can use nutrients dissolved in stream water and that, in some streams, rates of leaf decomposition are stimulated by the addition of these nutrients.