Nutrient input to terrestrial landscapes has increased substantially since the Industrial Revolution, and much of this nutrient load is transported to coastal areas by stream networks. However, streams do not act simply as solute conduits but are sites of significant nutrient processing. The transformation of nutrients in streams is conceptualized by the nutrient spiraling concept where nutrients travel a certain distance downstream in dissolved form (uptake length) and a certain distance in particulate form (turnover length). Although this model describes the transport of both dissolved and particulate nutrients, the majority of spiraling studies only quantify uptake of nutrients and do not address regenerative processes such as mineralization. Factors controlling microbial mineralization may significantly influence stream nutrient transport and retention. The rate and stoichiometry of microbial mineralization should 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 also change with nutrient availability and over the course of leaf decomposition. We explored spatial and temporal patterns of microbial mineralization by placing packs of red maple leaves in five Appalachian streams spanning a range of nitrogen and phosphorus availability. Packs were collected four times from each stream. Leaf disks from these packs were incubated in microcosms. Uptake rates and steady state concentrations of ammonium and phosphate were used to calculate mineralization rates.
Results/Conclusions
Neither uptake nor mineralization of ammonium or phosphate was related to stream water chemistry. However, ammonium uptake increased with stream water N:P. Uptake and mineralization across streams changed over time with the greatest fluxes of both nutrients occurring during the middle stages of decomposition. Ammonium and phosphate processing appeared to be temporally decoupled with peak ammonium uptake occurring after 55 days and peak phosphate uptake occurring after 60-65 days. Net mineralization of ammonium and phosphate was observed in all streams but did not depend on time or nutrient availability. Our results suggest that the relative availability of nitrogen and phosphorus is an important driver of microbial nutrient processing and that microbial mineralization within heterotrophic biofilms in streams is a flux equal to or greater than that of uptake even during periods of high nutrient demand.
