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DIVISION S-7—FOREST & RANGE SOILS

Sampling-Induced Increases in Net Nitrification in the Brush Brook (Vermont) Watershed

Department of Plant and Soil Science, University of Vermont Burlington, VT 05405

* Corresponding author (donald.ross{at}uvm.edu)

Accurate measurement of nitrification rates in disturbance-sensitive soils can be problematic. A series of experiments illustrated the extent of net nitrification increases and explored possible causes. In highly disturbed soils, weekly leachates of columns containing homogenized Oi/Oe material had NO^-₃ concentrations over 5 mmol L^-1 after 20 wk and over 1 mmol L^-1 after 2 yr. Small-scale impact disturbance in the field (footprints) resulted in a tripling of NO^-₃ concentrations after a 2-wk incubation. After disturbance by sampling, bulked Oa horizons showed rapid increases in NO^-₃ concentration, with change detectable within 1 h. Initial net nitrification rates were as high as 74 µmol kg^-1 h^-1 (300 mg N m^-2 d^-1). Ammonium concentrations increased over the first 10 h but decreased afterwards. Incubation with acetylene stopped nitrate accumulation but NH⁺₄ continued to increase. Intact cores also showed increases in NO^-₃ after 1 to 3 d but at a much lower rate than bulked samples. Gross rates of nitrification in intact cores were much lower than net rates in bulk samples. The addition of NH⁺₄ to intact cores caused an increase in net nitrate after 20 h. These results suggest that sampling stimulates nitrification rapidly through an increase in NH⁺₄ availability. The NH⁺₄ increase may be because of a stimulation of ammonification caused by mixing, a decrease in spatial heterogeneity, or a disturbance in other types of NH⁺₄ consumption. In soils that exhibit these disturbance effects, nitrification rate measurements will not be accurate but still may represent the potential for nitrification with increased NH⁺₄ supply.

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