Learning Center

Dr. Richard Mulvaney
Department of Natural Resources and Environmental Sciences
University of Illinois - Champaign/Urbana

Tracer techniques using isotopic N have been employed since the 1940s, to investigate such topics as N transformations and cycling in agricultural and other soils, N sources for crop uptake, the fate and behavior of fertilizer N applied to soils, the nature and extent of soil and fertilizer N losses, and fertilizer N uptake efficiency. The value of these techniques is apparent from their effect on analytical sensitivity, which can be a crucial issue in defining the topic under investigation. For example, tracer techniques provide the only means to detect microbial or chemical immobilization, or to distinguish between plant uptake of soil versus fertilizer N. Numerous studies have demonstrated that a substantial proportion of fertilizer N, typically between 10 and 30%, remains in the soil after the growing season, and that, even when highly fertilized, the soil itself often supplies the majority of N removed at harvest. There is growing evidence to implicate biological mineralization as the key process affecting soil N availability, and thus fertilizer N requirements. Moreover, recent work suggests that a soil’s capacity for mineralization can be estimated from alkali-labile N determined by simple diffusion methodology, which provides new hope that fertilizer N recommendations can become more accurate through the use of soil testing. This methodology may be employed for N-isotope analyses by direct combustion or the classical Rittenberg process, creating numerous opportunities for tracer research to advance our understanding of soil N cycling and improve fertilizer N management.