The δ15N values of foxtail millet (Setaria italica) and common millet (Panicum miliaceum) are reliable indicators of manuring practices

Foxtail millet (Setaria italica) and common millet (Panicum miliaceum) were domesticated in northern China around 10,000 years ago. Between 10,000 and 4,000 years ago, during the Neolithic period, China gradually established an agricultural pattern of "rice in the south and millet in the north." In the Yangtze River Basin, rice cultivation prevailed, while the Yellow River Basin was primarily focused on millet farming. These two distinct agricultural systems jointly contributed to the rise of Chinese civilization. In the millet-based agricultural system, which was centered on millet cultivation alongside domestic pig (Sus scrofa domesticus) husbandry, the use of pig manure as fertilizer likely played a key role in establishing a positive feedback loop. This, in turn, enabled the sustainable intensification of agriculture in the Loess Plateau of northern China, where soil fertility is naturally limited.

The stable nitrogen isotope composition (δ¹⁵N) of charred foxtail and common millet grains from archaeological sites is often used to indicate ancient manuring practices. However, various controversies have arisen due to the lack of modern fertilization experiments. Science China: Earth Sciences published the results of fertilization experiments on foxtail and common millets by Professor Xiaoyan Yang's team at Lanzhou University. The team conducted field and pot experiments to establish the relationship between manuring intensity and millet δ¹⁵N values. Pot experiments revealed that δ¹⁵N values of foxtail millet increased with the amount of manure applied, with leaves consistently having δ¹⁵N values about 1.6‰ higher than grains. Field experiments showed no significant difference in δ¹⁵N values between the control group, where manure application was halted for 2–3 years, and the manured experimental group. This indicates that residual manure continues to influence the δ¹⁵N values of foxtail and common millets even after manuring practice ceases, confirming the long-term residual effects of manure.

Based on published data from modern fertilization experiments, the δ¹⁵N values of manure and its application amount are key factors in determining changes in plant δ¹⁵N. Manuring can significantly increase the δ¹⁵N values of millet, with long-term, heavy applications increasing them by approximately 7‰. Archaeological millet grains with δ¹⁵N values lower than +1‰ likely indicate low or no manure use, values between +1‰ and +4‰ suggest medium manuring, and values above +4‰ indicate heavy manuring. Prehistoric millet grains from northern China generally exceed the +4‰ threshold, suggesting widespread and intensive manuring practices from the early Yangshao to the late Longshan periods.

See the article:

Yang, J., Yang, X., You, T. and Chen, F., 2024. The δ¹⁵N values of foxtail millet (Setaria italica) and common millet (Panicum miliaceum) are reliable indicators of manuring practices. Science China Earth Science, 67(9): 2910–2923. https://doi.org/10.1360/SSTe-2023-0312