Evaluation of organic sunflower fertilization using δ15N values

Abstract

This study was carried out to determine the effects of urea fertilization, legumes as a component of crop rotation, and soil type rich in soil organic matter on δ15N values of sunflower (Helianthus annuus L.) samples taken as traded or directly sampled from farms. The aim is to use δ15N values as a control of fraud related to inorganic N fertilization in organic farming systems in the near future. Median values for δ15N in sunflower samples separated into four origin/quality groups significantly increased in the order organically controlled (ΣOC) from China (1.0‰), ΣUrea fertilization (2.2‰), ΣOC lots (4.2‰), and ΣBio+ (4.6‰). The presence of alfalfa (Medicago sativa L.) in the crop rotation significantly decreased δ15N values in sunflower samples by 1.9‰. Regular FYM (and compost) application resulted in 0.3‰ higher values compared with the absence of organic fertilization. This increase was not significant, due to the small number of observations, contrasting numerous published studies. Growth on Chernozems led to an increase in δ15N values by 1.1‰ compared with Phaeozems. The measurement of δ15N values in sunflower samples is a useful but not a perfect tool to detect fraudulent inorganic N fertilization. At the OC level, 97.5% of the European samples were above 2.8 and even above 3.2 at the Bio+ level, so that 3.0 seems to be a reasonable threshold for taking further controlling action. This possibility will increase the willingness of producers to comply with the rules.