The fertilization of fields in the northern half of France (600 BCE−500 CE), approached by N isotope analyses of cereal remains

Abstract

The economic development of societies in Gaul during the Second Iron Age and the Roman period relied on agricultural system transformations. Hundreds of carpological studies carried out in the northern half of France over the last four decades have thoroughly documented the evolution of crop production and farming practices during this period. The increase in agricultural production required to sustain urbanization and trade expansion led to a diversification of cropping systems, resulting in the emergence of extensive systems. Weed ecological studies have shown how fertilization methods for cultivated soils changed between the earlier Gallic period, when highly fertile soils were common, and the Roman period, with a diversification of situations. This insight is only relevant on a global scale and required a more in-depth analysis. The present study investigates organic fertilization techniques at the crop scale, through nitrogen isotope analyses (δ15N), which are used as an indicator of soil fertility and the use of fertilizing organic matter (such as manure). The study was conducted on a sample of 124 crop remains, consisting of 6,490 archaeological cereal grains (emmer wheat, spelt wheat, free-threshing wheats and hulled barley) recovered from 68 archaeological sites located across the northern half of France, dating between the 6th century BC and the 5th century AD. To evaluate δ15N values of archaeological cereals in terms of fertilization, we compared published experimental data of modern cereals (grown under different fertilization regimes) for temperate Europe with published and new isotopic analyses of archaeological deer bone collagen from northern France as a proxy for wild unfertilised flora. Discriminant analysis and ordinal logistic regression were used for the classification of archaeological grain δ15N values by degree of fertilization. The results show that field fertilization was a common practice in Gaul, generally on a moderate scale. However, variations are observed in fertilization use within farms, with some intensively fertilized fields and others with less or no fertilization at all. In addition, regional differences emerged, particularly in the chalky Champagne region, where farmers used significantly less fertilizer than in other regions. Chronological trends over the studied period remain unclear. Disparities in the fertilization levels of different cereal species are noted. The δ15N values show considerable variations in soil fertility for barley and spelt, while fertilization regimes for free-threshing wheats and emmer only diversified during the 2nd and 1st centuries BC. These observations concur with the extensification of cultivation as inferred from carpological and weed ecology studies. Interspecific variations can also be seen in treatments applied to free-threshing wheats, which were fertilized much more commonly than the other three cereals from the 1st c. BC/1st c. AD, when these crops were in full expansion.