Natural 15N Abundance in Soil, Leaves, and Grain as Influenced by Long Term Additions of Fertilizer N at Several Rates

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Abstract
Soil, leaf, and grain natural 15N abundance was measured in corn (Zea mays L.) and soybean (Glycine max L.) plots which had received various rates of fertilizer N for 20 years. Soil 15N abundance did not significantly reflect the amount of fertilizer N applied and should not be used to estimate fertilizer N additions. Corn leaf and grain 15N abundance reflected additions of fertilizer N only at low rates of fertilizer N applied. Soybean leaf 15N abundance reflected a decrease in symbiotic N fixation with additional increments of fertilizer N applied.