Tillage and Fertilization Influences on Autotrophic Nitrifiers in Agricultural Soil.

Tillage and Fertilization Influences on Autotrophic Nitrifiers in Agricultural Soil

Abstract

Nitrification is a biological oxidation of NH₃ to NO₂^- (catalyzed by ammonia-oxidizing nitrifiers) and then to NO₃^- (catalyzed by nitrite-oxidizing nitrifiers). Reducing nitrification could increase N fertilizer use efficiency by decreasing NO₃^- leaching and reducing NO and N₂O emissions, which also benefits the environment. Generally, nitrogen fertilization stimulates nitrification, but there is no agreement about the influence of tillage. Our objectives were to determine if fertilization and tillage changed the relative proportions of nitrifier groups, their composition, and the nitrification rate within a long-term continuous maize monoculture soil. The study site is a long-term (>50 years) field of continuous maize (Zea mays L.) with three nitrogen fertilization rates (0, 168, and 336 kg ha^-1) and either no-tillage (NT) or plow tillage (PT). We used Most Probable Number (MPN) and Denaturing Gradient Gel Electrophoresis (DGGE) analysis to evaluate the number and composition of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) and performed ammonia-oxidation assays to determine potential nitrification rates. Our data suggested that the number of AOB and NOB increased (approximately   20% and 50%, respectively) with increasing fertilization rate in NT and PT treatments, while the ratio of AOB to NOB decreased from 1.1 to 0.9 in PT and from 1.2 to 1 in NT. The nitrification rate in NT was 44% higher than that in PT without fertilizer and became similar with high N fertilization. Nitrification rates and specific enzyme activity increased with increasing fertilization. This indicated that not all nitrifiers equally contribute to nitrification. DGGE analysis showed that the nitrifier community structure shifted among the different soil treatments.  Building a link between soil tillage, fertilization, and nitrifier community structure will help improve N fertilizer efficiency and reduce environmental contamination through more refined soil management.