%0 Journal Article %J Ecological Monographs %D 2018 %T Partitioning assimilatory nitrogen uptake in streams: an analysis of stable isotope tracer additions across continents %A Tank, J.L. %A Marti, E. %A Riis, T. %A von Schiller, D. %A Reisinger, A.J. %A W. K. Dodds %A M.R. Whiles %A L.R. Ashkenas %A W.B. Bowden %A S. M. Collins %A Crenshaw, C.L. %A Crowl, T.A. %A Griffiths, N.A. %A Grimm, N.B. %A Hamilton, S.K. %A Johnson, S.L. %A McDowell, W.H. %A Norman, B.M. %A Rosi, E.J. %A Simon, K.S. %A Thomas, S.A. %A Webster, J.R. %X
Headwater streams remove, transform, and store inorganic nitrogen (N) delivered from surrounding watersheds, but excessive N inputs from human activity can saturate removal capacity. Most research has focused on quantifying N removal from the water column over short periods and in individual reaches, and these ecosystem-scale measurements suggest that assimilatory N uptake accounts for most N removal. However, cross-system comparisons addressing the relative role of particular biota responsible for incorporating inorganic N into biomass are lacking. Here we assess the importance of different primary uptake compartments on reach-scale ammonium (NH4+-N) uptake and storage across a wide range of streams varying in abundance of biota and local environmental factors. We analyzed data from 17 15N-NH4+tracer addition experiments globally, and found that assimilatory N uptake by autotrophic compartments (i.e., epilithic biofilm, filamentous algae, bryophytes/macrophytes) was higher but more variable than for heterotrophic microorganisms colonizing detrital organic matter (i.e., leaves, small wood, and fine particles). Autotrophic compartments played a disproportionate role in N uptake relative to their biomass, although uptake rates were similar when we rescaled heterotrophic assimilatory N uptake associated only with live microbial biomass. Assimilatory NH4+-N uptake, either estimated as removal from the water column or from the sum uptake of all individual compartments, was four times higher in open- than in closed-canopy streams. Using Bayesian Model Averaging, we found that canopy cover and gross primary production (GPP) controlled autotrophic assimilatory N uptake while ecosystem respiration (ER) was more important for the heterotrophic contribution. The ratio of autotrophic to heterotrophic N storage was positively correlated with metabolism (GPP: ER), which was also higher in open- than in closed-canopy streams. Our analysis shows riparian canopy cover influences the relative abundance of different biotic uptake compartments and thus GPP:ER. As such, the simple categorical variable of canopy cover explained differences in assimilatory N uptake among streams at the reach scale, as well as the relative roles of autotrophs and heterotrophs in N storage. Finally, this synthesis links cumulative N uptake by stream biota to reach-scale N demand and provides a mechanistic and predictive framework for estimating and modeling N cycling in other streams.
%B Ecological Monographs %V 88 %P 138 %G eng %U https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1280 %N 1 %M KNZ001822 %& 120 %R 10.1002/ecm.1280