Linking the effects of nitrogen and phosphorus enrichment to controls of detrital carbon loss rates from streams
Manning, David William Pierce
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Human activities such as agriculture and urbanization result in mobilization of nitrogen (N) and phosphorus (P) to aquatic ecosystems. Despite increased availability of both N and P, little is known about the relative importance of N vs. P on detrital carbon (C) loss rates, or the combined effects of N, P and increased temperature or dissolved organic C (DOC) due to land use or climate change. Here, we focused on how N and P controls detrital C loss rates mediated by microbial decomposers and/or detritivores, and the interactive effects of elevated nutrients, temperature and DOC. To test N vs. P effects on detrital C, five streams were experimentally enriched with crossed N and P concentration gradients and ratios of N:P. I examined naturally occurring detritus (leaf litter, wood, and fine particles), and deployed detrital resources (four leaf species and wood veneers) across seasonal temperature gradients, and determined how increased N and P altered microbial and detritivore biomass, resource stoichiometry (C:nutrient content), respiration and breakdown rates. I used nutrient and DOC additions to stream mesocosms to determine their effects on detrital C loss. Breakdown and respiration rates of coarse detrital substrates increased with elevated nutrients and temperature; the largest response to nutrients was for breakdown rates (~2.8× higher with nutrients), followed by respiration (1.5× higher with nutrients, or seasonal temperature). DOC had negligible effects on respiration or litter decomposition. Nutrient enrichment increased nutrient content (reduced C:N, C:P) of all detritus types; nutrient-poor detritus tended to decrease the most, such that detrital stoichiometry converged during decay. Nutrient effects on detrital C:nutrient stoichiometry were critical predictors of increased detrital C loss rates, and detritivore biomass. These data suggest that N and P enrichment predictably increases detrital C loss rates, and that nutrient-altered detrital stoichiometry is a critical mechanism for predicting the occurrence of increased detrital C loss from streams. Mitigating excessive nutrient pollution is a key management goal for streams, and these studies imply that detrital stoichiometry could be used as an integrative measure of nutrient pollution and its effects on a key ecosystem function that is currently overlooked in nutrient management policies.