Microbial immobilization of ammonium and nitrate in cultivated soils

Abstract

The microbial immobilization of ammonium and nitrate was measured by 13N organic measurements after the application of labelled urea, (NH 4) 2SO 4, KNO 3 (KN) or NH 4NO 3 with or without glucose in four different soils. In the soils incubated without glucose, the microbial immobilization of the added ammonium varied between 1.5 and 4 mg N kg −1 soil. No immobilization occurred at the expense of NO 3 when KN was applied. When glucose was added at the rate 500 mg C kg −1 soil, the immobilization was very active between the first and the third day, at 10°C. The maximal amounts of 13N immobilized were much higher for the [ 15N]urea, 15(NH 4) 2SO 4, 15NH 4NO 3 and 15NO 3K. treatments than for the NH 4 15NO 3 application. This preferential immobilization of NH 4 was also observed in pure cultures of bacteria isolated from one of the soils and attributed to the inhibition of nitrate uptake by ammonium. The immobilization ratio, immobilized N: decomposed C, was calculated for glucose, accounting for pool substitution effects and immobilization due to native C. It was independent of the form of N applied and similar between soils, c 45–48 mg N g −1 C.