NITRIFICATION OF AMMONIUM IN DIFFERENT COMPONENTS OF A FLOODED RICE SOIL SYSTEM

Nitrification associated with the various components [subsurface soil from unplanted and planted (rhizosphere) fields, standing water and su rface soil from planted and unplanted fields and leaf sheath suspensio ns] of submerged rice paddies was examined in incubation experiments w ith solutions inoculated with soil or water samples. Substantial nitri fication occurred in all samples, standing water and surface soil samp les in particular, during their 40-day incubation with NH4+-N. Almost all the NH4+-N, disappeared during incubation with standing water, was recovered as NO3--N. This, compared to 70-80% from all soil samples a nd only 29% from leaf sheath suspensions. Significant loss of nitrogen , especially from leaf sheath suspensions, is probably due to nitrific ation-denitrification as evidenced by its complete recovery in the pre sence of N-Serve. Nitrification potential of the soil and water sample s varied with the crop growth stage and was more pronounced at tilleri ng and panicle inititation stages than at other stages. Nitrification potential of samples from green-manure-amended plots was distinctly le ss than that of samples from control and urea-amended plots. Most prob able number (MPN) estimates of ammonium-oxidizing bacteria were always higher in surface soil in both planted and unplanted plots at all sta ges of crop growth.