Effects of marine biofertilisation on Celtic bean carbon, nitrogen and sulphur isotopes: implications for reconstructing past diet and farming practices

Abstract

Rationale: The application of fertilisers to crops can be monitored and assessed using stable isotope ratios. However, the application of marine biofertilisers (e.g. fish, macroalgae/seaweed) on crop stable isotope ratios has been rarely studied, despite widespread archaeological and historical evidence for the use of marine resources as a soil amendment. Methods: A heritage variety of Celtic bean, similar in size and shape to archaeobotanical macrofossils of Vicia faba L., was grown in three 1 x 0.5 m outdoor plots under three soil conditions: natural soil (control); natural soil mixed with macroalgae (seaweed); and 15 cm of natural soil placed on a layer of fish carcasses (Atlantic cod). These experiments were performed over two growing seasons in the same plots. At the end of each growing season, the plants were sampled, measured and analysed for carbon, nitrogen and sulphur stable isotope ratios (δ13C, δ15N, δ34S). Results: The bean plants freely uptake the newly bioavailable nutrients (nitrogen and sulphur) and incorporate a marine isotopic ratio into all tissues. The bean δ15N values ranged between 0.8 ‰ and 1.0 ‰ in the control experiment compared with 2 ‰ to 3 ‰ in the macroalgae crop and 8 ‰ to 17 ‰ in the cod fish experiment. Their δ34S values ranged between 5 ‰ and 7 ‰ in the control compared with 15 ‰ to 16 ‰ in the macroalgae crop and 9 ‰ to 12 ‰ in the cod fish crop. The beans became more 13C‐depleted (δ13C values: 1–1.5 ‰ lower) due to crop management practices. Conclusions: Humans and animals consuming plants grown with marine biofertilisers will incorporate a marine signature. Isotopic enrichment in nitrogen and sulphur using marine resources has significant implications when reconstructing diets and farming practices in archaeological populations.