IMMOBILIZATION AND UPTAKE OF NITROGEN APPLIED TO CORN AS STARTER FERTILIZER

One goal of best management practices for N fertilization is to obtain high utilization of applied fertilizer. This experiment was conducted to determine plant uptake and the degree of microbial immobilization of NH4-N and NO3-N in fluid starter fertilizer applied to corn (Zea mays L.). In addition, two modifications of the fumigation-incubation method for biomass C and N determinations were evaluated for their ability to quantify immobilization of fertilizer N by soil microbial populations. Starter fertilizers tagged with (NH4NO)-N-14 or (NH4NO3)-N-15 were used to measure effects related to the fertilizer NH4 and NO3 components. Plant and soil samples were collected at the V3 and V8 growth stages. In both years, NO3 in the starter fertilizer was leached out of the emerging plant's root zone before it could be fully utilized. Ammonium underwent rapid nitrification and subsequently also was leached out of the topsoil, limiting immobilization and plant uptake. In the second year of the study, dicyandiamide (DCD) was included in the treatments, which maintained more N in the NH4 form, resulting in greater crop uptake and more microbial immobilization of fertilizer N. Under environments that tend to be conducive to leaching, it may be necessary to add a nitrification inhibitor to starter fertilizers to ensure that fertilizer N remains positionally available to young corn plants. The Shen, Pruden, and Jenkinson method for estimating biomass N gave immobilization values that were consistent with values anticipated from isotopic analyses of soil N. The Voroney and Paul technique appeared to overestimate biomass N by 1.5- to seven fold for our soils and climatic conditions.