To quantify the spatial distribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) and to determine nitrification activity in soil aggregates along a landscape, soil samples were collected from three landscape positions (shoulder, backslope, and toeslope) at two pasture sites with contrasting climatic conditions. The abundance of AOB and AOA was estimated by quantifying their respective bacterial and archaeal amoA gene copies using real-time polymerase chain reaction. Soil organic C (SOC), total N (TN), and the potential nitrification rate (PNR) were measured in aggregate size ranges (4–1, 1–0.25, and 0.25–0.05 mm). At site 1, a decreasing trend in PNR was observed as the size of aggregates decreased. Both bacterial and archaeal amoA genes were higher in the macroaggregates (4–1 and 1–0.25 mm) than in the microaggregates (0.25–0.05 mm) along the landscape. At site 2, PNR was higher in the smallest size of aggregates. In the 0.25–0.05-mm fraction, the abundance of bacterial and archaeal amoA genes was equal to, or greater than, those found in larger aggregate sizes. The relative abundance of archaeal amoA gene and the PNR correlated with relative SOC and TN contents along the landscapes. The positive relationship between relative archaeal amoA gene abundance and PNR suggests that nitrification in the studied pastures is probably driven by ammonia-oxidizing Thaumarchaeota.
