The potential of stable carbon and nitrogen isotope analysis of foxtail and broomcorn millets for investigating ancient farming systems

Abstract

Foxtail and broomcorn millets are the most important crops in northern China since the early Neolithic. However, little evidence is available on how people managed these two crops in the past, especially in prehistory. Previous research on major C₃ crops in western Eurasia demonstrated the potential of stable carbon and nitrogen isotope analysis of charred archaeobotanical remains to reveal the management of water and manure, respectively. Here, we evaluate the feasibility of a similar approach to C₄ millets. Foxtail and broomcorn millet plants grown in pots in a greenhouse under different manuring and watering regimes were analysed to test the effects of management on stable carbon and nitrogen isotope values of grains. Stable nitrogen isotope values of both millets increased as manuring level increased, ranging from 1.7 ‰ to 5.8 ‰ in different conditions; hence, it appears a feasible tool to identify manuring practices, in agreement with results from recent field studies. However, the two millets exhibit opposing trends in stable carbon isotope values as watering level increased. The shift in stable carbon isotope values of millets is also smaller than that observed in wheat grown in the same experimental environment, making it difficult to identify millet water status archaeologically. In addition, we charred millet grains at different temperatures and for varying durations to replicate macro-botanical remains recovered archaeologically, and to evaluate the offsets in carbon and nitrogen isotope values induced by charring. We found that the stable nitrogen isotope values of foxtail millet and broomcorn millet can shift up to 1-2 ‰ when charred, while the stable carbon isotope values change less than 0.3 ‰. Overall, we demonstrate that stable nitrogen isotope values of charred foxtail and broomcorn millet seeds could provide insight into past field management practices, and both carbon and nitrogen isotope values can together inform palaeodietary reconstruction.

Keywords: archaeology; broomcorn millet; carbon; charring; farming; foxtail millet; nitrogen; stable isotopes.

Figure 1

The carbon (A) and nitrogen isotope values (B) of foxtail millet (Setaria italica), broomcorn millet (Panicum miliaceum) and wheat (Triticum aestivum) grains grown under different manuring and watering conditions (for growing conditions, 1% and 5% are referring to the manuring levels, and low, medium, and high are referring to watering levels, for example, ‘1% low’ represents ‘1% manuring + low watering level’).

Figure 2

Charred foxtail (upper, Setaria italica) and broomcorn millet (lower, Panicum miliaceum) seeds under different charring conditions. The red box approximately delineates seeds comparable to well preserved archaeological remains.

Figure 3

The change of carbon and nitrogen isotope values of foxtail (Setaria italica) and broomcorn millet (Panicum miliaceum) grains when heated at different temperatures and durations.