Immobilization of ammonium and nitrate and their interaction with native N in three Illinois Mollisols

Summary

Three Illinois Mollisols were incubated for 2 weeks at 25°C after treatment with different amounts of glucose and/or ¹⁵N-labelled (NH₄)₂SO₄ or ¹⁵N-labelled KNO₃. The objectives were: (1) to compare the immobilization and interaction of NH ^sup+_inf4 −N and NO ^sup-_inf3 −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 MKCl (immobilized ¹⁵N+clay-fixed ¹⁵NH ^sup+_inf4 ). In all cases, both NH ^sup+_inf4 −N and NO ^sup-_inf3 −N were actively immobilized and transformed into organic forms in the presence of glucose. In the absence of glucose, a higher proportion of NH ^sup+_inf4 than NO ^sup-_inf3 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 NH ^sup+_inf4 than NO ^sup-_inf3 , whereas there was very little difference between NH ^sup+_inf4 and NO ^sup-_inf3 in the presence of glucose. The results indicate that under conditions of rapid immobilization (e.g., in the presence of glucose), NH ^sup+_inf4 and NO ^sup-_inf3 will show comparable interaction with the native soil N, whereas in unamended soil, the extent of this interaction will be greater with NH ^sup+_inf4 than with NO ^sup-_inf3 . Significant correlations were observed between applied N immobilized and the added N interaction only in one soil having a high initial mineral N content.