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

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. 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. 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.