Nitrogen Turnover by Sequential Immobilization and Mineralization during Residue Decomposition in Soils

Additions of ¹⁵N-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 ¹⁵NO⁻₃ 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 ¹⁵N-labeled NO⁻₃ and were incubated for 90 d. Evolution of CO₂ and concentrations and isotopic composition of NO⁻₃ were monitored. Increases in rates of NO⁻₃ 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 NO⁻₃ by nonlabeled NO⁻₃. 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 ¹⁵N-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. Journal Paper no. J-15788 of the Iowa Agric. and Home Economics Exp. Stn., Ames. Project no. 2995. This work was supported in part by Cooperative Agreement no. CX820311 between the U.S. Environmental Protection Agency and the Iowa Department of Natural Resources and in part by the Integrated Farm Management Demonstration Program of the Agricultural Energy Management Fund, state of Iowa, through the Iowa Dep. of Agriculture and Land Stewardship.