Nitrogen studies on black soils form the Darling Downs, Queensland. II. The nitrifying activity of subsurface horizons

Abstract

Rates of nitrification of a black clay soil were studied with the percolation technique, using samples from various depths down to 3 ft. There was a marked decrease in nitrification rate with increasing depth, the successive curves showing longer lag periods. Deep samples, which contain more chloride and have a higher pH value than those from the surface, accumulated large quantities of nitrite. Chloride, added to the percolators in concentrations equal to those found in the profile, had no marked effect on nitrite accumulation but slightly reduced the rate of nitrate production. The percolation of surface samples with ammonium sulphate solutions with the pH increased by alkali additions resulted in marked nitrite accumulation comparable with that found in deeper layers in presence of ammonium sulphate only. Continued repercolation at high pH values gave evidence of adaptation of the soil organisms to alkaline conditions; the percolate pH at which maximal nitrite accumulation was recorded rose with successive treatments. It was concluded that the decrease in nitrifying capacity with depth is related to reduction in numbers of active organisms and to the partially selective effect of high pH on the oxidation of nitrite.