Spatial and temporal variability of nitrogen dynamics in an upland stream before and after a drought
D. S. Baldwin A B , G. N. Rees A , A. M. Mitchell A and G. Watson AA Murray–Darling Freshwater Research Centre and the CRC for Freshwater Ecology, PO Box 991, Wodonga, Vic. 3689, Australia.
B Corresponding author. Email: darren.baldwin@csiro.au
Marine and Freshwater Research 56(4) 457-464 https://doi.org/10.1071/MF04189
Submitted: 13 July 2004 Accepted: 1 April 2005 Published: 27 June 2005
Abstract
In the current study, we explore the spatial and temporal variability of ammonia, nitrate and urea dynamics in an upland stream before and after a major drying event, using short-term nitrogen additions to benthic chambers. The potential for an initial flush of mineral nitrogen from re-wetted sediments following a prolonged period of drying was also assessed. The distribution of dissolved nitrogen species at four sites spaced along a 1-km reach of the stream were quite variable over time but, in general, not between sites. Conversely, sediment nitrogen dynamics were spatially variable. For example, in one instance, sediments from the uppermost site were a net sink for ammonia, whereas the sediments immediately downstream (separated from the first site by a small sand bar) were a net source of ammonia; with measured sediment fluxes up to ~2 µg N m−2 s−1. In general, the short-term addition of nitrate, ammonia or urea did not substantially affect the sediment nitrogen dynamics. After ~3 months of in situ drying, upon re-wetting, the sediments from all sites immediately produced pulses of ammonia, nitrate and, to a substantially lesser extent, urea. The rates of release of nitrogen were spatially variable, with up to an order of magnitude difference in the rate of release of ammonia from re-wetted sediments from the same small pool. Some differences were observed between nitrogen dynamics before and after drying but a causal linkage could not be established.
Extra keywords: nitrogen cycling, sediments, spatial heterogeneity.
Acknowledgments
We would like to thank John Pengelly for the ammonia, nitrate and nitrite analyses; Janice Williams, Helen Gigney and Charlie Rankin for assistance with fieldwork; Leon and Bubbles Garry for access to their property; and the staff at the Royal Tara for their hospitality. This study was funded by the CRC for Freshwater Ecology project B220.
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