N15 tracer studies on the transformation of applied nitrogen in submerged rice soils

A study of the transformation of applied nitrogen in submerged rice soils, using N15 tagged compounds is reported. A part of the nitrogen applied as (N15H4)2SO4 and N15H4Cl was immobilized within ten days of submergence, due to microbial immobilization and the proportion of this remained fairly constant for a seventy-five day period of submergence. Fraction of the tagged nitrogen recovered in the ammonium fraction decreased with incubation. Application of extraneous inorganic nitrogen appeared to stimulate the mineralization of soil organic nitrogen. About 23–24% of the applied nitrogen was not accounted for at the end of seventy-five days of incubation. This loss is attributed to the oxidation of ammonium nitrogen to the nitrate form in the surface layer of the soil with subsequent leaching and denitrification in the reduced sub-surface zone.