Isotopic and Nonisotopic Estimations of Fertilizer Nitrogen Uptake by Sudangrass in Field Experiments

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Abstract
Recoveries of fertilizer N in above‐ground portions of crops grown in field experiments were estimated by: (i) difference method; (ii) linear regression of total N in crops on rates of applied N; (iii) 15N isotopic tracer method; and (iv) linear regression of labeled fertilizer N uptake on rates of labeled fertilizer N. In order to evaluate the use of the isotope, these four methods were applied to data from previously reported experiments (Westerman, Kurtz, and Hauck, 1972) which involved two 15N‐labeled fertilizers (urea and oxamide) at rates of 0, 56, 112, and 168 kg N/ha. The test crop was Sudax (SX11) sorghum‐sudangrass hybrid (Sorghum sudanense) and harvests were made at approximately 30‐day intervals during the cropping seasons.The difference method overestimated recovery of urea and oxamide nitrogen in sudangrass 35 and 31%, respectively, in 1966 and 23 and 35% in 1967 when compared to the isotopic tracer method. Similarly, linear regression of total N in crops on rates of N overestimated recoveries of urea and oxamide nitrogen 32 and 17%, respectively, in 1966 and 17 and 21% in 1967 when compared to linear regressions of fertilizer N in crops on rates of 15N labeled nitrogen. Isotopically‐labeled fertilizers are almost a necessity for estimating N recovery in many agronomic experiments.

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