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Variations in the Natural Abundance of ¹⁵N of Wheat Plants in Relation to Fertilizer Nitrogen Applications¹

ABSTRACT

Measurements were made by two laboratories of the ¹⁵N (per mill ¹⁵N excess) of winter wheat plants (Triticum aestivum L.) grown at five locations in Pennsylvania on experimental plots. The plots were fertilized with N at various rates. The results from both laboratories showed a consistent decline in ¹⁵N with increasing rates of N application. Such a decrease in ¹⁵N is consistent with increasing contributions of fertilizer N to the plants as the rate of application increased, given that fertilizer N has a lower ¹⁵N content than the soil N. The coefficients of regression of ¹⁵N of wheat on N application rate were always negative and usually significantly different from zero. The regression coefficients computed from the results of the two laboratories were not significantly different from each other in 12 of 16 experiments. There was, however, a systematic, unexplained difference in the results from the two laboratories. A regression of one set of data on the other resulted in a regression coefficient significantly different from one, the theoretically expected value.

¹ The work was supported in part by the National Science Foundation under the Project Number GI-39009A 2.

² Research Associate, Center for the Biology of Natural Systems, Washington University, St. Louis, MO., 63130; Soil Scientist, Plant Nutrition Lab., USDA, Agricultural Research Service, NE Region, Agricultural Research Center, Beltsville, MD., 20705.

Received for publication February 5, 1975. Accepted for publication June 9, 1975.