Evaluating Water Fertilizer Coupling on the Variations in Millet Chaff Size during the Late Seventh Century in Northwest China: Morphological and Carbon and Nitrogen Isotopic Evidence from the Chashancun Cemetery

Stable isotopic analyses of the remains of plants that have been unearthed from archaeological sites are often featured as key indicators of crop cultivation and the living environment. However, systematic archaeobotanical studies have not been applied widely in Chinese historical sites, especially in those from the Tang dynasty. This paper aims to use carbon and nitrogen isotopic analyses to reveal the potential influence of water and fertilizer conditions on the size of millet chaffs that were excavated from the Chashancun cemetery. To achieve this, >3600 uncharred broomcorn and foxtail millet chaff remains were measured. Furthermore, 30 broomcorn millet samples and 30 foxtail millet samples were selected to analyze the carbon and nitrogen isotopes, respectively. The widths and thicknesses of the broomcorn millet chaffs ranged from 1.11 to 2.38 mm and from 0.95 to 2.24 mm, respectively, while those of the foxtail millet chaffs ranged from 0.95 to 1.94 mm and from 0.69 to 1.90 mm, respectively. The δ¹³C and δ¹⁵N values of the broomcorn millet chaffs ranged from −13.0‰ to −12.0‰ and from 15.7‰ to 17.8‰, respectively, while those of the foxtail millet chaffs ranged from −14.0‰ to −12.9‰ and from 15.7‰ to 18.8‰, respectively. The results show correlations between the millet chaff size and the carbon/nitrogen isotopic values, suggesting that water and fertilizer conditions might have significantly affected millet grain yield during the late seventh century in northwestern China.