2019

Watered or not? That is the question

In recent decades, the study of prehistoric food globalization has provided us with a vivid picture of the translocation of crops domesticated at both ends of Eurasia from pre-5000 BC to around 1500 BC. Agriculture, at the intersection of nature and culture, manifests a joint force of anthropogenic and environmental influences. Thus, the diffusion of crops is never solely the movement of domesticated species, but the underlying techniques and ideologies surrounding the use of these crops may also be transmitted as essential accessories. Nevertheless, the cultural traditions and ecological situations of the “new territory” may reshape these “accessories” as re-adaption strategies in a novel context.

After Wallace, M., et al. (2013). “Stable carbon isotope analysis as a direct means of inferring crop water status and water management practices.” World Archaeology 45(3): 388-409 Yufeng Sun

Compared to the indigenous dry-land millet agriculture of North China, the Southwest Asian domesticated crops, wheat and barley, have higher water demands, especially for wheat. Thus, water supply constitutes one of the most restrictive factors on their eastward journey into China. Climatically, this “Crescent-shaped Cultural Transmission Belt”, in the archaeological sense, also largely overlaps with the demarcation line of the monsoon and non-monsoon regions. Outside the belt, low precipitation and high evaporation is the core feature of the non-monsoon region; whereas inside the belt, the monsoonal climate brings an uneven distribution of precipitation seasonally, with a concentrated summer precipitation in contrast to the concentrated winter precipitation of Southwest Asia. So, what was the watering condition of the earliest wheat and barley that came into China? How did people ameliorate the environmental challenge of moving into a new region?

Carbon isotopic analysis was employed on these ancient plants to address these questions. Principally, when plants photosynthesize in good watering conditions, the fractionation of 13C toward CO2 in the air is encouraged ( resulting in lower δ13C values) because the stomata on the leaves have a higher level of conductancy, and vice versa. Combined with δ13C values in the air during different historical periods, Δ13C values can be calculated to evaluate the water status when crops growing. In this study, Wallace et al. (2013)’s criterion against the water status of wheat and barley was referred to for further analysis.

Samples for this study come from over 20 sites, ranging from the westernmost province of Xinjiang to the coastal province of Shandong in North China and spanning from c. 2000 BC to around the beginning of the Christian Era. After testing with Mass Spectrometer, experimental data indicate that most of the early wheat and barley in China were well-watered, regardless of their location in non-monsoonal or monsoonal regions. Previous studies have revealed that wheat and barley were treated significantly differently in prehistoric sites of the Eastern Mediterranean and South Asia. Δ13C values indicate that barley tended to be poorly-watered because of its drought-tolerant physiological properties. However, in this study, only slight differences in the watering status between wheat and barley were observed, especially for those from the non-monsoonal region. Interestingly, barley may be cultivated as a dry-land crop in monsoonal regions.

In the non-monsoonal region, Δ13C values suggest most wheat and barley samples were well-watered. With the consideration of limited precipitation and high evaporation, it is highly possible that intentional irrigation toward wheat and barley had been adopted at low intensity. At the very least, it seems that people cultivated wheat and barley in landscapes rich with water resources, such as field plots with high underground water tables or near flowing water or a body of water on the surface. The negative correlation between Δ13C values and annual and seasonal precipitation of the sites where samples came from in the non-monsoonal region further supports our conclusions. In the monsoonal region, wheat samples have higher Δ13C than those of the non-monsoonal region. Considering the enriched summer precipitation, the use of irrigation cannot be directly inferred.

In order to explain these phenomena, we must contextualize the samples and data with their archaeological and environmental backgrounds. We take the Hexi Corridor as an example for the non-monsoonal region. This region–where an arid environment and vulnerable ecology restricted peoples’ living space–was the frontier for communication and conflict, serving as the bridge between Central China and the Continental Interior. The patch-like oases along the piedmont plain were the only homelands for ancient people and facilitated an intensive agricultural strategy – to gain more yield per unit — on the limited arable land. Additionally, the frequent migration of groups and available land tenure in the 2nd and 1st millennium BC may have exacerbated tensions over arable land resources. As a result of this situation, finer water management strategies, especially for wheat and barley which have high water demands, can be expected.

In contrast,  in Central China, arable land was not a scarce resource, due to depopulation during the Bronze Age and early Iron Age. In most cases, an extensive strategy—to increase the yield by increasing the area of cultivated land – may have been the choice. With the emergence of early cities and the growth of populations, a spatially uneven investment in crops in terms of water and nutrient management was likely, especially in  mega-sites such as Yanshishengcheng, Zhouyuan and Yinxu as the capital city of Shang or Zhou Dynasty in the Bronze Age. Currently, more data and more evidence are expected to elucidate this hypothesis.   

 

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