Nitrogen transformations in a flooded soil in the presence and absence of rice plants: 1. Nitrification

Nitrification rates (n) in the floodwater of an alkaline clay were measured in the absence or presence of rice plants by inhibition of ammonium oxidation and ¹⁵N-dilution techniques. Floodwater nitrate concentrations in control treatments showed a marked diurnal variation, and were higher than in the inhibitor treatments after the first day. Ammonium concentrations in floodwater declined exponentially in all treatments, being markedly affected by diffusion and NH₃ volatilization but little affected by nitrification and plant uptake. Nitrification rates in floodwater estimated by ¹⁵N-dilution were generally higher than the rates estimated by the inhibitor method. Estimates of n were generally higher during daylight hours than at night, and did not differ significantly between planted and unplanted pots. Microbial immobilisation of labelled ammonium and gross N immobilisation were not affected by addition of the nitrification inhibitor 2-ethynylpyridine.