PARENT SESSION
Poster Session #75: Nutrient Cycling II.
Friday, August 9. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Correlating gross nitrification rates with population dynamics of ammonia oxidizers in soil.

LeBauer, David^*,1, Okano, Yutaka¹, Scow, Kate¹, Jackson, Louise¹, ¹ University of California at Davis, Davis, Ca

ABSTRACT- Processes that control nitrification in agroecosystem soils are important factors determining the fates of ammonium fertilizer inputs. Responses of the autotrophic ammonia oxidizer (AAO) community in agricultural soil to simulated fertilization were measured in a microcosm experiment. Our objective was to quantify the relation between rates of gross nitrification and changes in soil extractable DNA encoding the active subunit of ammonia monooxygenase (amoA) of soil microbes. We collected soil from organically managed fields under tomato-corn rotation following tomato harvest at the Long Term Research on Agricultural Systems (LTRAS) facility. Soil was leached with three cation solution treatments to simulate two levels of ammonium fertilization and an unfertilized control. Microcosms were sampled at 0, 1, 3, 7, 14, and 28 days. Gross nitrification during a 24-hour incubation was measured using ¹⁵N-NO₃^- pool dilution. We developed a Taqman realtime quantitative PCR method to quantify the amoA gene as an indicator of AAO community size. Data suggest that in the control and low ammonium fertilization treatment, gross nitrification rates rapidly depleted available ammonium. An increase in gross nitrification rates and size of the AAO community in the high ammonium treatment after an initial delay may be due to the slow growth potential of nitrifiers. Measures of the response of the nitrifier community to ammonium inputs are important because nitrifiers compete for available ammonium with N-immobilizers including crop plants. A better understanding of the relationship between nitrification rates and nitrifier density will enable evaluation of approaches aimed at reducing nitrate losses.

KEY WORDS: ammonium fertilizer, nitrification, nitrate pool dilution, taqman realtime quantitative pcr