NITROGEN TURNOVER BY SEQUENTIAL IMMOBILIZATION AND MINERALIZATION DURING RESIDUE DECOMPOSITION IN SOILS

Additions of N-15-labeled fertilizers to soils often induce mineralization of nonlabeled N, and such mineralization poses problems when interpreting the results of isotopic analyses. We studied the effects of added (NO3-)-N-15 on turnover of N during the decomposition of corn (Zea mays L.) residues in soils. Samples of a Galva silty clay loam (fine-silty, mixed, mesic Typic Hapludoll) were treated with various levels of corn stover and various levels of N-15-labeled NO3- and were incubated for 90 d. Evolution of CO2 and concentrations and isotopic composition of NO3- w ere monitored. Increases in rates of NO3- addition increased initial rates of immobilization of labeled N and subsequent rates of mineralization of nonlabeled N. The sequential immobilization of labeled N and mineralization of nonlabeled N resulted in substantial replacement of labeled NO3- by nonlabeled NO3-. Early added-N-induced immobilization of labeled N often was offset by subsequent added-N-induced mineralization of nonlabeled N before the end of the study. Sequential immobilization and mineralization should be recognized as a potential source of error in N-15-tracer studies because sequential processes violate the commonly held assumptions that mineralization and immobilization occur simultaneously. This error could be avoided by recognizing that sequential immobilization and mineralization can occur and that the sequential processes result in a different distribution of isotopes than does simultaneous mineralization-immobilization turnover.