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Nitrogen Pathways in an Ancient Forest Streams and Beyond: LINX1 and LINX2 at the H.J. Andrews

Nitrogen Pathways in an Ancient Forest Streams and Beyond: LINX1 and LINX2 at the H.J. Andrews

May 2005

L.R. Ashkenas, S.L. Johnson, S.V. Gregory, D.J. Sobota

The LINX study (Lotic Intersite Nitrogen eXperiment) is a collaborative effort aimed at understanding the dynamics of nitrogen in stream ecosystems. Begun as a workshop at Coweeta in 1995, it is currently in the second phase of parallel investigations in streams ranging from the Arctic to the tropics. For additional information on both LINX1 and LINX2, please see the webpages at http://www.biol.vt.edu/faculty/webster/linx/

The first LINX study focused on the uptake, retention and cycling of nitrogen in 10 relatively undisturbed stream ecosystems, ranging from the Arctic to the tropics. Mack Creek was the last of the 10 streams investigated, with sampling completed in October 1998. We measured several key hydrodynamic, chemical and metabolic parameters using the same techniques employed at the other nine LINX sites. These measurements, in addition to the stable isotope work described below, have been used in the LINX group integration efforts, which will combine data from all sites to try to identify broad patterns in nitrogen dynamics in stream ecosystems. These cross-site syntheses also include an examination of factors controlling stream metabolism (Mulholland et al 2001), microbial biomass variability (Findlay et al 2002) as well as methods development (Mullholland et al 2001). The role of biophysical factors in controlling the uptake and export of nitrogen across biomes was quantified and a model developed based on the LINX cross-site analyses (Peterson et al 2001).

After quantifying background parameters, each LINX site added the stable isotope of nitrogen, 15N in the form of 15N labeled ammonium for approximately 6 weeks. The amounts added were sufficient to track uptake and assimilation, but not enough to achieve a fertilization effect. At Mack Creek, we sampled various components of the food web weekly along a 225 m downstream gradient. Each of the LINX sites tried to sample approximately similar portions of the food web. For example, epilithon (organic matter and organisms such as algae and bacteria growing on rocks) and grazing invertebrates were sampled at all sites; wherever possible, we all tried to sample similar taxa. At Mack Creek, we also attempted to sample as broad a representation of the food web as possible, such as riparian plants (leaves, roots, stems), small mammals that forage in the riparian zone (in cooperation with the Oregon Cooperative Fisheries & Wildlife Unit of the NBS/USGS), and aquatic birds (in cooperation with USFWS).

The results from this study were recently published (Ashkenas et al., 2004) and focus on the dynamics of nitrogen in streams of old growth forests. This work builds on earlier studies on stream nitrogen budgets at the H.J. Andrews Forest, with an additional emphasis on the structure of stream food webs. The most striking results from the Mack Creek study were the significance of in-stream bryophytes, the previously-overlooked role of biofilms on large wood in uptake and retention of nitrogen, and the transfer of dissolved, in-stream nitrogen to components of the terrestrial ecosystem.

The second phase of the LINX study (LINXII), involving the same group of stream researchers, is focusing on the impacts of land use type (urban/suburban, agriculture and forest/reference) on nitrate dynamics in small streams. Recent research has suggested (Alexander et al.) that small streams may be key players in retention and processing of nitrogen. The field component of this project began in 2003, and uses a 24-hour input of 15N-labelled nitrate. In addition to the parameters measured with LINX1, we are also attempting to track the conversion of nitrate to dissolved gases, such as N2 and N2O. Mack Creek at the H.J. Andrews was the site of such a tracer release in August 2004, and will serve as an important cross-control in the overall study. The isotopic field study results will be combined into a landscape level model, along with synoptic sampling, to allow us to assess the effects of human disturbance on nitrogen dynamics in stream networks.

Literature Cited:

Alexander, R. B., R. A. Smith and G. E. Schwartz. 2000. Effect of stream channel size on the delivery of nitrogen to the Gulf of Mexico. Nature 403:758-861.

Ashkenas, L. R., S. L. Johnson, S. V., Gregory, J. L. Tank, and W. M. Wollheim. 2004. A stable isotope tracer study of nitrogen uptake and transformation in an old-growth forest stream. Ecology 85(6):1725-1739.

Findlay, S., J. Tank, S. Dye, H.M. Valett, P. Mulholland, W.H. McDowell, S. Johnson, S.K. Hamilton, J. Edmonds, W.K. Dodds, and W.B. Bowden. 2002. Bacterial and fungal biomass in detritus-based microhabitats of headwater streams. Microbial Ecology 43:55-66. Mulholland, P.J., J.L. Tank, D.M. Sanzone, W. Wollheim, B.J. Peterson, J.R. Webster, and J.L. Meyer. 2000. Food resources of stream macroinvertebrates determined by natural-abundance stable C and N isotopes and a 15N addition. Journal of the North American Benthological Society 19:145-157.

Mulholland, P.J., J.L. Tank, D.M. Sanzone, B.J. Peterson, W. Wollheim, J.R. Webster, and J.L. Meyer. 2001. Ammonium uptake length in a small forested stream determined by 15N tracer and ammonium enrichment experiments. Verh. Internat. Verein. Limnol. 27:1320-1325.

Mulholland, P.J., C.S. Fellows, J.L. Tank, N.B. Grimm, J.R. Webster, S.K. Hamilton, E. Marti, L. Ashkenas, W.B. Bowden, W.K. Dodds, W.H. McDowell, J.L. Meyer, and B.J. Peterson. 2001. Controls on stream metabolism examined in an inter-biome comparison. Freshwater Biology 46:1503-1517.

Peterson, B.J., W. Wollheim, P.J. Mulholland, J.R. Webster, J.L. Meyer, J.L. Tank, Marti, E., W.B. Bowden, H.M. Vallet, A.E. Hershey, W.H. McDowell, W.K. Dodds, S.K. Hamilton, S.V. Gregory, and D.J. D'Angelo. Control of nitrogen exports by small headwater streams. Science 292:86-90.