RESIDUAL FORMS OF FERTILIZER NITROGEN IN A GRASSLAND SOIL

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Abstract
We characterized residual 15N-enriched fertilizer in a Temvik silt loam (fine-silty, mixed Typic Haploboroll) containing an established stand of crested wheatgrass (Agropyron desertorum, var. Mandan). The purpose of the study was to investigate the sequential fate and behavior of residual N associated with perennial grassland production in the northern Great Plains. Fertilizer enriched with 15N was applied at an 84-kg N/ha rate to a different set of field microplots each spring during the 5-yr experiment. The remaining microplots received 84 kg nonenriched N/ha annually. After the 5th-yr harvest, the soil was removed from the microplots, divided into three sections.sbd.0 to 10, 10 to 30, and 30 to 60 cm deep.sbd.and subjected to analysis for various N forms. Our results indicate that residual N, which constituted about 30 to 50% of the N15-enriched fertilizer applied, was incorporated primarily into surface soil organic N forms during the first season of application. Essentially no potential existed for leaching of residual N after harvest, because relatively little remained in the original ammonium nitrate form. That the residual N was so susceptible to mineralization (3 to 10.times.), compared with the soil N, is interpreted to mean that much is probably incorporated into newly formed, more accessible, and labile organomineral complexes similar in composition to those already present in the soil. Even after five growing seasons, the relative distribution of residual N in some organic forms was not the same as that of the soil N.