Manured or not manured? New baselines for re-interpreting cereal C and N stable isotope data from Neolithic sites in the NW Mediterranean

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• C and N stable isotope analyses of grain and chaff from experimental fields done. • Crop rotation without manuring and poorly manured fields available. • Crop rotation fields yielded values around 0‰ and 1.2‰ • Long-term poorly manured fields yielded δ 15 N values between −3 and −4‰ • Neolithic δ 15 N values ca. 3–8 ‰ interpreted as moderate-manuring intensive farming. The NW Mediterranean region saw the arrival of agriculture around 5800 cal. BCE. The paucity of weeds in the Neolithic archaeobotanical record in the area makes it difficult to characterise agricultural practices. Stable carbon and nitrogen isotope analyses of archaeobotanical remains provide an opportunity to discuss the availability of water and nutrients during cereal growth. This type of approach has been attempted for this region in recent publications. However, there is a lack of sufficient local modern baselines to confirm the archaeological interpretations. In order to improve this situation, experimentally grown crops from the Archaeological Museum of l’Esquerda (Roda de Ter, Barcelona) have been analysed. The museum has a long-term experimental project in which different crops have been grown under different conditions. Winter and spring sowing in a 3-year rotation without fertiliser, and occasionally manured fields have been tested with different crops such as einkorn, emmer, spelt, faba beans, barley and rye. Modern samples of spring and autumn sown rye and emmer, and autumn sown einkorn and spelt, both from chaff and grain, were analysed for stable carbon and nitrogen isotopes. Based on the results, we agree with previous work that modern crops from unmanured fields give δ 15 N values below 2 ‰, while fields under crop rotation without manure show negative values up to −4‰. For the NW Mediterranean region, δ 15 N values around 3–8 ‰ combined with Δ 13 C values around 16–18 ‰ in the archaeobotanical record should be interpreted as indicative of moderate manuring.