Isotopic and nonisotopic estimation of nitrogen uptake efficiency in container-grown woody ornamentals

Cornussericea L., Weigela florida (Bunge)A. DC., and Euonymus alatus (Thunb.) Sieb were grown outside in 3.8-L plastic containers for 345 days (1 Apr. 2001 to 11 Mar. 2002). Nitrogen (N) was applied at rates (NAR) of 25, 50, 100, 200, and 300 mg center dot L-1 and delivered as aqueous double-labeled N-15 depleted NH4NO3 (min 99.95% atom N-14). In all species, root, shoot, and total plant dry weight increased with increasing NARs while root to shoot ratios decreased. Similarly, root, shoot, and total plant N increased with NAR for each species, and at each NAR more N was stored in the roots than in the shoots. Estimation of fertilizer N uptake determined by the total N method was higher for all species and at each NAR than estimation of N uptake determined by the fertilizer N-15 tracer method. Fertilizer N uptake efficiency determined by the total N method was highest at 25 mg center dot L-1 and decreased as NARs increased. In contrast fertilizer N uptake efficiency determined by the fertilizer N-15 tracer method was lowest at 25 mg center dot L-1 and increased or remained relatively constant as NARs increased. Differences in N uptake and N uptake efficiency can be attributed to overestimation by the total N method due to the inclusion of nonfertilizer N and underestimation by the fertilizer N-15 tracer method due to pool substitution. Corrected N uptake efficiency values can be calculated by adjusting the original data (total Nor 15 N uptake) by the distance between the origin and the y intercept of the regression line representing the data.