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 NO3- 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 NO3- concentrations after a 2-wk incubation. After disturbance by sampling, bulked Oa horizons showed rapid increases in NO3- concentration, with change detectable within I h. Initial net nitrification rates were as high as 74 mumol 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 NH4+ continued to increase. Intact cores also showed increases in NO3- 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 NH4+ to intact cores caused an increase in net nitrate after 20 h. These results suggest that sampling stimulates, nitrification rapidly through an increase in NH4+. availability. The NH4+ increase may be because of a stimulation of ammonification caused by mixing, a decrease in spatial heterogeneity, or a disturbance in other types of NH4+ 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 NH4+ supply.