Many soils will undergo extremely rapid increases in net nitrification after a disturbance as simple as soil sampling, with nitrate concentrations often doubling within 10 hours. Using both net and gross rate measurements on samples from the Brush Brook watershed in central Vermont, we have shown that sampling disturbance causes an almost immediate shift in the consumption of ammonium--from heterotrophic assimilation to autotrophic nitrification. Actual (gross) rates of ammonium production do not appear to change. After disturbance, net and gross nitrification do not differ and nitrate accumulates. Extractable ammonium concentrations initially increase but usually drop below their initial level. The stable isotope ratio of 15-N in nitrate also changes rapidly after disturbance. In soils with high rates of net nitrification, the 15-N nitrate pool is initially depleted but moves towards the enriched state of the total soil N pool. Sampling and mixing stimulates nitrification by i) interrupting other in situ processes of ammonium consumption and ii) by bringing the ammonium and the autotrophic oxidizers into closer contact.