Limitations and usefulness of nitrogen-15 techniques in measuring mineralization/immobilization and plant uptake of soil and fertilizer nitroge

Interpretation of fertilizer efficiency experiments utilizing $sp{15}$N is complicated by biologically cycling, resulting in exchange of soil microbial $sp{14}$N with added $sp{15}$N. The impact of several environmental factors on microbial nitrogen cycling processes and consequent plant $sp{15}$N recoveries were studied to determine the limitations and usefulness of $sp{15}$N in plant N recovery studies. A field experiment with spring wheat tested the effects of chemical fertilizer source and placement. Experiments with paddy rice, performed in India, assessed green manure plant part and decomposition time treatments in flooded soils. And a greenhouse experiment compared green manure and chemical fertilizer as nitrogen sources for paddy rice.;In the wheat experiment, data from fertilizer source treatments were inconclusive. However, fertilizer placement treatments significantly affected $sp{15}$N levels without altering inorganic soil N. Broadcast placement enhanced microbial cycling of $sp{15}$N relative to the band placement, by increasing the potential for interactions between soil microorganisms and fertilizer N.;Experiments using Sesbania green manure indicated that ready carbon substrates stimulated microbial growth and nitrogen cycling, resulting in significant isotopic exchange. The extent of exchange varied with carbon levels in the green manure materials, as in the case of Sesbania roots, tops and roots + tops; and in the length of time for microbial cycling, as in the decomposition time treatments. Less isotopic exchange in urea treatments relative to green manure was attributed to rapid plant N uptake and localized placement of the fertilizer rather than differences in carbon substrate levels.;In all cases except one, plant $sp{15}$N recovery results were not consistent with yield, or with plant and soil N data. Evaluating only plant $sp{15}$N recoveries, without considering nitrogen cycling processes and isotopic exchange, results in erroneous conclusions about nitrogen recovery. $sp{15}$N should not be used to evaluate fertilizer efficiency.;A better use of $sp{15}$N is as an indicator of nitrogen flows and fate within the soil-plant system. Observed declines in plant $sp{15}$N contents in several of these experiments indicate the need for further research to understand plant nitrogen utilization and how to keep nitrogen in the plant, once it has been taken up.