TY - JOUR T1 - Partitioning assimilatory nitrogen uptake in streams: an analysis of stable isotope tracer additions across continents JF - Ecological Monographs Y1 - 2018 A1 - Tank, J.L. A1 - Marti, E. A1 - Riis, T. A1 - von Schiller, D. A1 - Reisinger, A.J. A1 - W. K. Dodds A1 - M.R. Whiles A1 - L.R. Ashkenas A1 - W.B. Bowden A1 - S. M. Collins A1 - Crenshaw, C.L. A1 - Crowl, T.A. A1 - Griffiths, N.A. A1 - Grimm, N.B. A1 - Hamilton, S.K. A1 - Johnson, S.L. A1 - McDowell, W.H. A1 - Norman, B.M. A1 - Rosi, E.J. A1 - Simon, K.S. A1 - Thomas, S.A. A1 - Webster, J.R. AB -
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.
VL - 88 UR - https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1280 IS - 1 ER -