Crop fertility conditions in North-Eastern Gaul during the La Tène and Roman periods: a combined stable isotope analysis of archaeobotanical and archaeozoological remains.

Considerable archaeobotanical datasets describe cereal cultivation in north-eastern France, from the Iron Age to the Roman period. This study aims to complement these by using stable isotope analysis on charred cereal grains. Soil fertility was investigated through d15N and d13C analyses of 1480 charred cereal grains, dated from the Late La Tène to the Late Antiquity periods. In the Île-de-France, charred grain D13C values suggested good hydric conditions, with drier episodes in the 1st and 3rd century AD; while in Champagne, the lower D13C values for spelt reflect the lower water holding capacity of the chalky soils. A wide range of cereal d15N values (0.8 - 8.7 ¿) implies a wide range of soil fertility conditions. Jouars-Pontchartrain and Palaiseau (Île-de-France) yielded the highest cereal d15N values, whereas Acy-Romance (Champagne) delivered among the lowest. From these three sites, the d15N values of red deer bone collagen were used to estimate the reference d15N values for unmanured plants. Unlike in Acy-Romance, there were significant differences in Palaiseau and Jouars-Pontchartrain, indicating that the cultivated cereals inherited their high d15N values from manured soil. At Jouars-Pontchartrain, the d15N value (almost 9¿) suggested a high trophic level manuring source, possibly from pig and/or human faeces