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Understanding the crop management practices of millet agriculture is pivotal for comprehending the development of Chinese civilization. Recent studies have indicated that organic manuring plays a crucial role in ensuring sustainable millet cultivation on the Loess Plateau. However, uncertainties still exist regarding how manuring practices impact the δ15N values of C4 millets compared to C3 cereals. Furthermore, inadequate information on crop δ15N at archaeological sites has led to controversial interpretations of animal and human diets. In this study, we present new findings from an experimental archaeological research conducted in actual loess farmland in China to explore the potential range of variability in grain δ15N values of millets. Our results demonstrate that animal manure significantly increases Setaria and Panicum grain δ15N values, ranging from 2.7 to 9.3‰. Considering trophic enrichment effects on nitrogen isotopes, humans consuming manured millets may yield values ranging from 5.7 to 12.3‰, suggesting alternative explanations for high δ15N values other than animal protein consumption. Opposite to the general hypothesis, the grain δ15N values are systematically lower than those of leaves. The difference between the values of Setaria and Panicum and the process of manure influencing the grain δ15N values are also discussed. Our study provides novel insights into the nitrogen stable isotopic indicator of millet manuring and will serve to set reconstructions of past manuring and diet practices in northern China on a firmer foundation. [ABSTRACT FROM AUTHOR]