IMMOBILIZATION OF AMMONIUM AND NITRATE AND THEIR INTERACTION WITH NATIVE N IN 3 ILLINOIS MOLLISOLS

Three Illinois Mollisols were incubated for 2 weeks at 25-degrees-C after treatment with different amounts of glucose and/or N-15-labelled (NH4)2SO4 or N-15-labelled KNO3. The objectives were: (1) to compare the immobilization and interaction of NH4+-N and NO3--N with the native soil N, and (2) to study the relationship between immobilization of applied N and the added N interaction. As determined, immobilized N refers to forms not extractable with 2 M KCl (immobilized N-15 + clay-fixed (NH4+)-N-15). In all cases, both NH4+-N and NO3--N were actively immobilized and transformed into organic forms in the presence of glucose. In the absence of glucose, a higher proportion of NH4+ than NO3- was recovered in organic forms. Although the three soils differed considerably in the amounts of applied N immobilized, similar trends in N immobilization were observed. A positive added N interaction occurred with all soils, the magnitude increasing with the rate of N addition. In the absence of glucose, higher added N interactions were obtained for NH4+ than NO3-, whereas there was very little difference between NH4+ and NO3- in the presence of glucose. The results indicate that under conditions of rapid immobilization (eg., in the presence of glucose), NH4+ and NO3- will show comparable interaction with the native soil N, whereas in unamended soil, the extent of this interaction will be greater with NH4+ than with NO3-. Significant correlations were observed between applied N immobilized and the added N interaction only in one soil having a high initial mineral N content.