Seasonal variations in abundance and activity of nitrifiers in four arable cropping systems.

Ammonium and nitrite oxidizers were counted with the most probable number (MPN) method and potential ammonium- and nitrite-oxidation rates were determined with a chlorate inhibition technique in an arable soil over a 3-year period. Samples were taken from the topsoil once a month for 2 years and a few times during a third year in four cropping systems: unfertilized lucerne ley and barley, and nitrate fertilized grass ley and barley. The distribution of nitrifiers was determined and their activities measured at various soil depths and between and within plant rows of fertilized barley.The numbers and activities of ammonium oxidizers were highest in the spring and autumn samples. Numbers of ammonium oxidizers ranged from 0.2 to 19×10(4) and nitrite oxidizers from 3 to 870×10(4) cells g(-1) dry soil. Potential ammonium-oxidizer activities ranged from 120 to 1,060 and nitrite-oxidizer activities ranged from 280 to 680 ng N g(-1) dry soil hour(-1). Lucerne and grass leys generally showed the highest, whereas unfertilized barley had the lowest, abundances and activities.Abundance estimates and activities were 10-20 times higher in the plow layer than in underlying sand and clay layers. A strong correlation was found between organic matter content vs numbers and activities of both ammonium and nitrite oxidizers. Only nitrite oxidizer counts were significantly higher within plant rows compared to between plant rows.