Identifying N fertilizer regime and vegetable production system in tropical Brazil using 15 N natural abundance

BACKGROUND This study was conducted in areas of vegetable production in tropical Brazil, with the objectives of (i) measuring the variation in δ 15  N in soils, organic N fertilizer sources and lettuce ( Lactuca sativa L.) from different farming systems, (ii) measuring whether plant δ 15  N can differentiate organic versus conventional lettuce and (iii) identifying the factors affecting lettuce δ 15  N . RESULTS Samples of soil, lettuce and organic inputs were taken from two organic, one conventional and one hydroponic farm. The two organic farms had different N‐sources with δ 15  N values ranging from 0.0 to +14.9‰ (e.g. leguminous green manure and animal manure compost, respectively), and differed significantly ( P < 0.05) in lettuce δ 15  N (+9.2 ± 1.1‰ and +14.3 ± 1.0‰). Conventional lettuce δ 15  N (+8.5 ± 2.7‰) differed from hydroponic lettuce δ 15  N (+4.5 ± 0.2‰) due to manure inputs. The N from leguminous green manure made a small contribution to the N nutrition of lettuce in the multi‐N‐source organic farm. CONCLUSION To differentiate organic versus conventional farms using δ 15  N the several subsets of mode of fertilization should be considered. Comparisons of δ 15  N of soil, organic inputs and lettuce allowed a qualitative analysis of the relative importance of different N inputs. © 2015 Society of Chemical Industry