Organic Matter Stoichiometry Influences Nitrogen and Phosphorus Uptake in a Headwater Stream

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ammonium, phosphorus, nutrient uptake, organic matter, stoichiometry, season


The concept of nutrient spiraling combines the geological, hydrological, and biological processes that influence nutrient cycling in streams. Spiraling studies have demonstrated connections between metabolic and nutrient cycles, organic matter (OM) dynamics, and hydrologic controls. Most spiraling studies have addressed the dynamics of a single element. However, nutrients do not move through ecosystems in isolation. Recent models have used ecological stoichiometric theory to couple N and P cycles, but empirical data to support these conceptual frameworks are generally lacking. We investigated the relationship between N and P uptake and the extent to which OM stoichiometry was related to the relative uptake of N and P in a headwater stream across 2 seasons. In addition, we explored whether our results were consistent with theoretical predictions derived from ecological stoichiometry and consumer–resource imbalances. We found that higher respiration led to higher NH4+ and P uptake rates. NH4+ and P uptake were strongly correlated, but the nature of this relationship shifted with a seasonal change in the dominant OM to fresh leaf litter in autumn. OM stoichiometry was a strong predictor of relative nutrient uptake (NH4∶SRP uptake ratios). Seasonal input of low N∶P leaf litter led to relatively higher NH4+ uptake from the water column, which caused a shift in relative nutrient uptake but did not alter the strength of the coupling. Our results indicate that stoichiometric imbalances between nutrient consumers and resources have a strong influence on nutrient uptake in streams. Moreover, stoichiometric models of consumer–resource imbalances between microbes and dominant OM substrates accurately captured N and P uptake dynamics in our study system. Integrating stoichiometry with metabolic controls provides insights into nutrient dynamics and acts as a framework to link N and P cycles.

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Freshwater Science