Spatial and temporal variability of nitrogen dynamics in an upland stream before and after a drought

La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research Centre

MDFRC item.

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.