Controls on nitrification in a water-limited ecosystem: experimental inhibition of ammonia-oxidising bacteria in the Patagonian steppe

We studied controls on nitrification in an undisturbed water-limited ecosystem by inhibiting autotrophic nitrifying bacteria in soils with varying levels of vegetative cover. The activity of nitrifying bacteria was disrupted using nitrapyrin, 2-chloro-6-(trichloromethyl)-pyridine, under field conditions in three microenvironments (underneath shrubs, next to grasses and in bare soil). Ammonia-oxidising bacteria were detected by PCR analysis of DNA in soils. The inhibition of nitrification changed the concentrations of NO3 2 and NH4 þ in the soil, while the microenvironment was most important in determining the response of bacteria to the inhibitor. Nitrapyrin application resulted in a significant ðp , 0:05Þ reduction in soil NO3 concentration (39%) and a significant increase ðp , 0:001Þ in soil NH4 concentration (41%). Untreated bare-soil microenvironments had the lowest concentrations of NH4 þ (1.57 mg/g of dry soil) and NO3 2 (0.49 mg/g of dry soil) when compared to the other microenvironments, and showed the highest impacts of nitrification inhibition. For example, NH4 þ concentrations increased 288% and NO3 2 concentrations decreased 60% in inhibited bare-soil microenvironments. In contrast, untreated microenvironments underneath shrubs had the highest levels of NH4 þ (10.01 mg/g of dry soil) and NO3 2 (0.69 mg/g of dry soil), but showed no significant effects of inhibition of nitrification on soil nitrogen concentrations. q 2003 Elsevier Ltd. All rights reserved.