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

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.