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

Millet agriculture, which originated in northern China, alongside rice agriculture, have nurtured the Chinese civilization. Prehistoric manuring practices likely promoted and maintained sustainable millet agricultural production in the loess area of northern China. However, ongoing controversy exists regarding the indicators of prehistoric manuring intensity of foxtail millet (Setaria italica) and common millet (Panicum miliaceum). Here, we present the results of pot and field fertilization experiments on two millet types. Our findings suggest that manuring can significantly increase the δ¹⁵N values of foxtail millet, and its δ¹⁵N values increase with increasing manuring levels. The δ¹⁵N values of foxtail millet leaves are systematically greater than those of grains by approximately 1.6‰. Manuring could have a long-term residual impact on increasing the δ¹⁵N values of soil and millet crops. Combined with previous crop fertilization experiment results, we propose that the impact of manuring on the δ¹⁵N values of non-nitrogen-fixing crops is roughly consistent. The δ¹⁵N value and amount of manure are key factors determining the extent of change in plant δ¹⁵N values. The millet grain δ¹⁵N values can serve as reliable indicators of manuring practices. Finally, we provide an interpretive framework for assessing the correlation between manuring levels and the δ¹⁵N values of archaeological millet remains. The δ¹⁵N values of ancient millet grains suggest widespread and intensive manuring practices in prehistoric millet agriculture in northern China, spanning from the early Yangshao period to the Longshan period.