Co-occurrence of in-stream nitrogen fixation and denitrification across a nitrogen gradient in a western U.S. watershed
It is frequently assumed that nitrogen (N2) fixation and denitrification do not co-occur in streams because each process should be favored under different concentrations of dissolved inorganic nitrogen (DIN), and therefore these processes are rarely quantified together. We asked if these processes could co-exist by conducting a spatial survey of N2 fixation using acetylene reduction and denitrification using acetylene block [with and without amendments of carbon (C) as glucose and nitrogen (N) as nitrate]. Rates were measured on rocks and sediment in 8 southeastern Idaho streams encompassing a DIN gradient of 26–615 µg L−1. Sampling at each site was repeated in summer 2015 and 2016. We found that both denitrification and N2 fixation occurred across the gradient of DIN concentrations, with N2 fixation occurring primarily on rocks and denitrification occurring in sediment. N2 fixation rates on rocks significantly decreased 100× across the DIN gradient in 1 year of the study, and amended (with N and C) denitrification rates increased 10× across the DIN gradient in both years. Multiple linear regression and partial least squares models with environmental characteristics measured at the scale of entire stream reaches showed that C and phosphorus were positive predictors of amended and unamended denitrification rates, but no significant model could explain N2 fixation rates across all streams and years. This, coupled with the observation that detectable rates of N2 fixation occurred primarily on rocks and denitrification occurred primarily on sediment, suggests that microhabitat scale factors may better predict the co-occurrence of these processes within stream reaches. Overlooking the potential co-occurrence of N2 fixation and denitrification in stream ecosystems will impede understanding by oversimplifying the contribution of each process to the N cycle.
KeywordsNitrogen fixation Denitrification Dissolved inorganic nitrogen Streams
We thank C. Allison, A. Copley, J. Cornell, E. Coscarelli, A. Eckersell, H. Harris, M. Kelly, D. Larson, K. Nevorski, J. Ortiz, J. Paris, and R. Van Goethem for field and laboratory assistance. We also thank J. Bump, S. Techtmann, and three anonymous reviewers for providing comments that helped to improve this manuscript. This research was funded by the National Science Foundation award DEB 14-51919 to A.M.M. and NSF-EPSCoR (IIA-1301792) support to C.V.B. This is contribution no. 48 of the Great Lakes Research Center at Michigan Tech.
- American Public Health Administration (2005) Standard Methods for examination of water and wastewater. APHA, Washington DCGoogle Scholar
- Barton GJ (2004) Surface and ground-water relations on the Portneuf River, and temporal changes in ground-water levels in the Portneuf Valley, Caribou and Bannock Counties, Idaho, 2001–2002. USGS Scientific Investigations Report 2004-5170Google Scholar
- Brandes J, Devol A, Yoshinari T et al (1998) Isotopic composition of nitrate in the central Arabian Sea and eastern tropical North Pacific: a tracer for mixing and nitrogen cycles. Limnol Oceanogr 44:106–115Google Scholar
- Burnham KP, Anderson DR (2002) Model selection and multimodel inference: a practical information-theoretic approach, 2nd edn. Springer, New YorkGoogle Scholar
- Capone DG (1993) Determination of nitrogenase activity in aquatic samples using the acetylene reduction procedure. In: Kemp PF, Sherr BF, Sherr EB, Cole JJ (eds) Handbook of methods in aquatic microbial ecology. Lewis Publishers, Boca RatonGoogle Scholar
- Dodds WK, Castenholz RW (1988) The nitrogen budget of an oligotrophic cold water pond. Arch Hydrobiol 79(Suppl):343–362Google Scholar
- Dodds WK, Burgin AJ, Marcarelli AM et al (2017) Nitrogen transformations. In: Lamberti GA, Hauer FR (eds) Methods in stream ecology, vol 2. ecosystem function. Elsevier/Academic Press, Amsterdam/New York, pp 143–196Google Scholar
- Grace JB, Anderson TM, Olff H et al (2010) On the specification of structural equation models for ecological systems. Ecol Monogr 56:1001–1014Google Scholar
- Knowles R (1982) Denitrification. Microbiological Rev 46:43Google Scholar
- Lemmon PE (1956) A spherical densitometer for estimating forest overstory density. For Sci 2:314–320Google Scholar
- Scott JT, Marcarelli AM (2012) Cyanobacteria in freshwater benthic environments. In: Whitton BA (ed) Ecology of Cyanobacteria II: their diversity in space and time. Springer, DordrechtGoogle Scholar
- Seitzinger SP (1988) Denitrification in freshwater and coastal marine ecosystems: ecological and geochemical significance. Limnol Oceanogr 33:702–724Google Scholar