Variation of 15N in Corn and Soil Following Application of Fertilizer Nitrogen

Samples of corn (Zea mays L.) plant tissue (grain and leaf) were obtained from the University of Illinois South Farm plots and from the University of Illinois Morrow plots. Soil samples were also obtained from each of the South Farm plots. The ¹⁵N:¹⁴N ratio of the nitrogen in these corn plants is shown to be sensitive to the source of nitrogen taken up by the plants. A systematic decline in δ¹⁵N (per mill excess ¹⁵N) was observed within a narrow range of variation (about 4 δ¹⁵N units) with increasing applications of commercial nitrogen fertilizer. This result is consistent with increasing contributions of fertilizer nitrogen to the plant, as the fertilizer had a δ¹⁵N value close to zero. Similarly, the influence of a small amount of manure nitrogen (40 kg nitrogen/ha) applied over many years was detected in samples grown on the University of Illinois' historic Morrow plots. The results indicate that the nitrogen of grain and the leaf tissue of young plants is derived at least in part from different pools reflecting different proportions of fertilizer and soil nitrogen or fertilizer nitrogen of different δ¹⁵N, due to the time course of its fractionation. Unexplained differences between the influence of soybeans and clover on δ¹⁵N were also observed. Neither the quantity nor the δ¹⁵N of the total nitrogen was influenced by short term (3 years) addition of large quantities of nitrogen (448 kg/ha) to the soil on the South Farm plots. A relatively high value of δ¹⁵N was obtained for NO₃⁻ in the soil profile in the fall following spring application of 448 kg nitrogen/ha. This suggests that the observed nitrate nitrogen was not derived from nitrogen molecules applied as fertilizer in the spring, but originated rather from the exchange of fertilizer nitrogen for soil nitrogen and/or the “priming effect” (release of extra soil nitrogen as a result of the application of fertilizer nitrogen).