Ammonia Oxidizers in a Grazing Land with a History of Poultry Litter Application.

Poultry litter (PL) is widely applied to grazing lands in GA, which is the top poultry producer in the US. Apart from being a source of important nutrients, PL contains metals that come from feed additives to control chicken diseases. It is not clear how repeated application of PL affect important nutrient transformations and soil microorganisms that mediate these processes. In this study, we examined changes in the community structure and activity of ammonia oxidizers (AO) in a grazing land with a history of PL application. Soil samples were collected in 2009 (after 15 years of PL application) and 2013 (after 2 years of stopping PL application) at the depths of 0-5 and 0-15 cm. The abundance and community composition of AO were determined with molecular methods that included quantitative polymerase chain reaction, denaturing gradient gel electrophoresis (DGGE) and amoAC intergenic spacer analysis (AISA). Nitrification potential measurement was used for measuring activity of AO. In 2009, the abundance of AO was significantly higher in PL treated plots than the controls, with bacteria being numerically dominant over archaea. This was in agreement with the nitrification potential rates, which were higher in PL treated plots than controls. In 2013, the numerical dominance of bacteria over archaea decreased significantly in response to the discontinuation of PL application. AISA profiling of ammonia oxidizing bacteria (AOB) indicated that the majority were from Nitrosospira spp in both types of plots. Shannon index of diversity (H) calculated from AISA showed significant differences between 2009 and 2013 samples at 0-15 cm for both types of plots. However, the H-values calculated for ammonia oxidizing archaea (AOA) using DGGE did not show any significant differences between the two depths in 2013 for both types of plots. In general, the impact of PL was mainly felt on AO abundance, not composition.