Evaluation of the 15Nitrogen Isotope Dilution Technique for Estimating Crop Nitrogen Uptake from Organic Residues.

A field experiment was conducted to test the new approach for estimating crop nitrogen (N) uptake from organic inputs. The soil was prelabeled with 15N by applying 15N fertilizer to sunflower crop (Helianthus annuus L. var. Viki). The 14N plots, which received unlabelled fertilizer, were also set up. At harvest, 15N labeled residues were added to the unlabeled soils at a rate of 73 kg N ha-1 (direct technique) and unlabeled residues were added to the 15N-labeled soils at the same rate (indirect technique). Control plots without residues were also established. All plots were sown with the wheat (Triticum aestivum L. var merchouch)-fababean (Vicia faba L.)-wheat (Triticum aestivum L. var merchouch) cropping sequence. In the cropping sequence, the first, second and third crop derived respectively 12.01, 2.4, and 1.93 kg N ha-1 from crop residues estimated by the direct method and 14.77, 3.3, and 1.85 kg N ha-1 estimated by the indirect method. The results showed no significant difference between the two techniques, which suggests that the new soil prelabeling technique compares well with the direct technique. [ABSTRACT FROM AUTHOR]