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

Abstract

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.