Organic, conventional and hydroponic vegetables: Can 15N natural abundance of farm N inputs differentiate mode of production?

Abstract Organic and synthetic N-fertilizers normally show quite different δ15N values, which can be imprinted in agricultural soils and growing plants. Therefore, δ15N values have been considered as a promising marker to differentiate organic plant products from other farming systems that do not exclude synthetic N-fertilizer use. The aim of this study was to evaluate the δ15N values of fertilizer N sources from different farming systems and the δ15N values of the plant products produced in situ. Samples of soil, substrates, N sources, and plant products (lettuce, tomato, and strawberry) were sampled from well documented organic (n = 2), conventional (n = 4) and hydroponic (n = 2) farms in the southern Brazil. Plant parts were also compared. δ15N values of N-inputs (substrates, organic and synthetic fertilizers) overlapped, within the range of −2 to +8 ‰. Soil samples from conventional farms (+5.3 to +9.8 ‰) showed higher δ15N values than from organic systems (+4.9 to +6.5 ‰). As a consequence, δ 15N values had limited value as a tool to differentiate plant products from different farming systems. However, it may be a potentially more useful technique for strawberries but not for tomatoes and lettuce. The inconsistency found in the differentiation between farming systems can be attributed to the overlapping of δ15N values of the fertilizer N sources used on those farms.