Short-term population dynamics of ammonia oxidizing bacteria in an agricultural soil

Ammonia oxidizing bacteria (AOB) control the rate limiting step of nitrification, the conversion of ammonia (NH4+) to nitrite (NO2−). The AOB therefore have an important role to play in regulating soil nitrogen cycling. Tillage aerates the soil, stimulating rapid changes in soil N cycling and microbial communities. Here we report results of a study of the short term responses of AOB and net nitrification to simulated tillage and NH4+ addition to soil. The intensively farmed vegetable soils of the Salinas Valley, California, provide the context for this study. These soils are cultivated frequently, receive large N fertilizer inputs and there are regional concerns about groundwater N concentrations. An understanding of N dynamics in these systems is therefore important. AOB population sizes were quantified using a real-time PCR approach. In a 15 day experiment AOB populations, increased rapidly following tillage and NH4+ addition and persisted after the depletion of soil NH4+. AOB population sizes increased to a similar degree, over a 1.5-day period, irrespective of the amount of NH4+ supplied. These data suggest selection of an AOB community in this intensively farmed and C-limited soil, that rapidly uses NH4+ that becomes available. These data also suggest that mineralization may play an especially important role in regulating AOB populations where NH4+ pool sizes are very low. Methodological considerations in the study of soil AOB communities are also discussed.