Research shows soil temperature modulated millet agriculture in Neolithic East Asia

Millet has been an important crop in East Asia for much of the Holocene, a period beginning about 11,700 years ago. To better understand how environmental conditions may have shaped the development of millet agriculture, researchers from the Institute of Earth Environment of the Chinese Academy of Sciences, and their collaborators in and aboard China investigated loess deposits from the Chinese Loess Plateau (CLP).

The study, which was published in PNAS on May 4, suggests that fluctuations in growing-season soil temperature played an important role in modulating the development and geographic spread of millet agriculture in East Asia. The researchers found that mid-Holocene soil cooling from about 7,500 to 6,000 years ago likely reduced thermally suitable zones for millet cultivation, contributing to a southward displacement of farming and a delayed large-scale agricultural expansion until subsequent soil temperature recovery after around 6,000 years ago.

To conduct the research, the scientists analyzed loess deposits from the Longgugou (LGG) section of the CLP. They combined 14 radiocarbon and 18 optically stimulated luminescence dates to develop a high‑resolution chronology spanning approximately 12,300 to 2,800 years ago. Based on this chronology, they performed biomarker analyses on 114 samples to reconstruct growing-season soil temperature and vegetation conditions. By integrating these proxy reconstructions with archaeological datasets and transient climate simulations, the researchers explored how Holocene soil temperature variations may have influenced the spatiotemporal evolution of millet agriculture in Neolithic East Asia.

The results indicate that between approximately 12,300 and 7,500 years ago, soil temperatures were relatively high, accompanied by comparatively low moisture and sparse vegetation. From 7,500 to 6,000 years ago, soil temperatures progressively declined under wetter conditions with denser vegetation. After 6,000 years ago, soil temperatures rapidly recovered and then remained relatively stable for millennia, while moisture and vegetation cover decreased.

By comparing these climate and vegetation reconstructions with archeological evidence for millet agriculture, the researchers suggest that, between 8,000 and 7,500 years ago, relatively warmer soils may have supported early millet cultivation Yanshan-Liaoning region, where communities may have faced greater subsistence pressure. In the CLP and lower-latitude regions, millet use appears to have remained limited and likely co-existed with hunting, gathering, and other subsistence practices.

In contrast, from 7,500 to 6,000 years ago, cooler soils, wetter conditions, and denser vegetation may have narrowed reliable cultivation space, particularly near the lower thermal thresholds for frost-sensitive millets. These conditions likely contributed to a southward shift of millet farming toward the CLP and surrounding regions. From 6,000 to 4,000 years ago, recovering and more stable soil temperatures, along with reduced vegetation cover, and advances in cultivation practices, might have facilitated millet expansion and broader late Neolithic development.

This study provides a high-resolution Holocene growing-season soil temperature record from a core region of early millet agriculture in East Asia and highlights soil temperature as an underappreciated climatic constraint on millet agricultural development. The findings suggest that large-amplitude soil temperature fluctuations helped modulate the geographic distribution and developmental trajectory of early millet farming, offering refined insights into climate-society interactions during the Neolithic period in East Asia.