Non-monotonic response of net nitrification in soils experiencing chronically-elevated nitrogen inputs.
Gilliam, Frank*,1, Willson, David1, May, Jeffrey1, Adams, Mary2, 1 Department of Biological Sciences, Huntington, WV, USA2 Timber and Watershed Laboratory, Parsons, WV, USA
ABSTRACT- In 1989, WS3 of the Fernow Experimental Forest (FEF), West Virginia, began receiving aerial applications of nitrogen (N) as ammonium sulfate at 35 kg N/ha/yr. Because WS3 exhibited symptoms of N saturation prior to initiation of treatment, these additions simulate ecosystem responses at later stages of N saturation. Published studies have documented N saturation also occurring in an adjacent untreated watershed (WS4). Previous work has shown that net nitrification, based on in situ incubations carried out from 1993 to 1995, increased by 60% during this period on the treated WS3, from 117 to nearly 187 kg N/ha/yr; increases also occurred on untreated WS4, but to a lesser extent (40%), from 108 to 151 kg N/ha/yr. The objectives of this study were (1) to re-examine in situ net nitrification on these watersheds for an assessment of greater temporal scale, and (2) determine nitrification potentials using laboratory incubations under a variety of controlled temperatures. In situ incubations were carried out from May to August 2002 using methods identical to those of previous work. Briefly, mineral soil was taken to a 5 cm depth from each of seven plots per watershed, placed in polyethylene bags, and buried for 28 d. Soil was extracted with 1 N KCl and analyzed for ammonium and nitrate prior to and following incubation. Nitrification potentials were determined by incubating soils taken from these same plots at 4, 15, 25, and 35 C for 28 d, followed by extraction and analysis. Net nitrification decreased by 51% from 1995 to 2002 (92 kg N/ha/yr) on WS3, suggesting a non-monotonic response of net nitrification to the N treatments. However, declines also occurred on WS4 to 68 kg/ha/yr (55%). Declines in net nitrification on both watersheds are consistent with significant decreases in stream nitrate concentrations since 1999 of 0.14 and 0.05 mg N/L/yr for WS3 and WS4, respectively. Nitrification potential averaged higher on untreated WS4 than on treated WS3, 2.6 versus 1.5 ug N/g soil/d, respectively. Results suggest that chronic N inputs have an inhibitory effect on nitrification and nitrate mobility in the later stages of N saturation.
Key words: nitrogen saturation, nitrogen cycling, nitrification
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