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Brief Report

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

by
Bingbing Liu
1,
Yongxiu Lu
2,3,*,
Yishi Yang
1,
Wenyu Wei
2,3 and
Guoke Chen
1,*
1
Gansu Provincial Institute of Cultural Relics and Archaeology, Lanzhou 730000, China
2
Key Laboratory of Western China’s Environmental System (Ministry of Education), Lanzhou University, Lanzhou 730000, China
3
College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(6), 3581; https://doi.org/10.3390/su14063581
Submission received: 29 January 2022 / Revised: 15 March 2022 / Accepted: 16 March 2022 / Published: 18 March 2022
(This article belongs to the Special Issue Socio-Economic and Demographic Impacts of Climate Change from Pre-Historic to Modern Times)
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Abstract

:
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.

1. Introduction

An archaeobotanical analysis is an effective approach that can be implemented to explore some of the most commonly examined archaeological concerns from the prehistoric and historical periods, such as the history of crop domestication or crop diffusion around the world [1,2,3,4] and human–environment interaction [5,6,7]. A large number of archaeobotanical data have been reported in recent decades [8,9,10,11], with most studies focusing on prehistoric sites. While most archaeobotanical analysis has centered on identifying species information from plant remains, the measurement of the grain sizes of different species is increasingly becoming a concern, as this may provide important evidence through which we can understand the history of crop domestication [12,13], crop processing [14,15], social organization, and dietary practices [16,17] in different areas of Eurasia. Moreover, stable carbon and nitrogen analysis has been used to study crop growth environments. The δ13C and δ15N values of charred grains are the primary means by which we can infer the water and fertilizer status of crop cultivation [18,19], both of which would provide us with further information on local environmental changes and human activities [20,21,22]. However, the comprehensive application of these methods to examine variations in crop grain size and their relationship to the water and fertilizer conditions that result in crop growth has rarely been reported [23].
Previous studies have revealed that broomcorn millet (Panicum miliaceum) and foxtail millet (Setaria italica) were the main plant subsistence in northern China during the Neolithic Age, both of which were domesticated around 10,000 BP (before present, defined as 1950 AD) [2,24]. The cultivation of these two crops probably acted as a source of auxiliary livelihoods in most areas of northern China between 10,000 and 7000 BP [25,26,27], became the primary subsistence strategy between 7000 and 6000 BP [28,29], and provided an important economic basis for the cultural evolution that occurred during the late Neolithic period in the Yellow River valley [30,31,32]. However, a proportion of the crops that were introduced into plant subsistence, such as barley and wheat, exceeded the proportion of the indigenous foxtail and broomcorn millet in vast areas of northwestern China during the Bronze Age [33,34,35,36]. Until the Tang dynasty (618–907 AD), foxtail and broomcorn millet were the most important staple foods in central northern China [37,38]. With the rise of the Han Empire (202 BC–220 AD) and the Tang Empire, Gansu Province came to be controlled by the Chinese central government, which, in turn, affected the livelihoods in the area, including in the Hexi Corridor [39]. According to historical documents, foxtail and broomcorn millet acted as the primary plant subsistence in the Hexi Corridor during the sixth and seventh centuries [40]. However, a systematic archaeobotanical analysis of these key regions during this period has not yet been carried out in detail.
Recently, massive uncharred plant remains, mostly chaffs of foxtail and broomcorn millet, were unearthed from the Chashancun cemetery. Surprisingly, the sizes of each type of millet chaff differ visibly from one another, but the influencing factors are unclear. Thus, this paper aims to explore the potential influence of the water and fertilizer conditions on millet chaff size using size measurements and stable isotopic analysis. Our findings may be valuable for understanding living strategies in northwest China during the early Tang dynasty.

2. Materials and Methods

The Chashancun cemetery (Figure 1) is located in Chashan village, Tianzhu Tibetan Autonomous County, Gansu Province. The cemetery includes one brick chamber and has yielded a large amount of pottery, various metalware, wood lacquerware, stone tools, silk fabrics, leather objects, sacrificial animals, and food crops [41]. According to an unearthed epitaph, the cemetery can be dated back to the second year of the Tianshou era (691 AD), and the cemetery’s occupant has been identified as Murong Zhi, a member of the Tuyuhun royal family. During the excavation of the Chashancun cemetery, which was conducted between September and December 2019, nine silk bags (Wugunang, 五谷囊) containing plant remains were unearthed from a coffin chamber in the northwest part of the cemetery (Figure 1D). Interestingly, they were strung on a wood stick, something that is not commonly found in excavated Tang dynasty graves. The composition of the four broken silk bags shows that all of the silk bags contained the same mixed plant remains. Some of the mixed plant remains from these four broken silk bags were scattered on the ground during the cleaning process, which made it difficult to assign these plants back to their original silk bags. Therefore, these mixtures of mixed crops from the four broken silk bags were transferred into new numbered sample bags by the archaeological team. (Figure 1E,F).
The three sample bags were collected from the Gansu Provincial Institute of Cultural Relics and Archaeology and transferred to Lanzhou University for further research. Considering that each sample bag has similar plant contents and proportions (confirmed by a discussion with the head of the excavation team and the identification of the plant remains, unpublished), the millet chaffs collected from them are discussed together. A total of 1819 foxtail millet chaffs and 1814 broomcorn millet chaffs were chosen to explore the relationship between millet chaff size and their carbon and nitrogen isotope values. Each type of millet was divided into two groups (large vs. small) according to the morphological variations that could be determined with the naked eye before starting the experiment. The length, width, and thickness of the large and small millet chaffs were measured using a vernier caliper at the MOE Key Laboratory of Western China’s Environmental Systems, Lanzhou University, Gansu Province, China. All of the millet chaffs were observed and photographed under a stereo binocular low-power microscope.
Isotopic analysis was conducted on 60 samples that were randomly chosen from each millet group according to the measured width data. These isotopic samples were washed several times using distilled water in an ultrasonic cleaner for 30 min and then dried in an oven at 70 °C for 12 h. Subsequently, the dried samples were crushed to powder using an agate mortar/pestle and were weighed into tin containers (0.2–0.4 mg) for isotopic measurements. The samples were processed in an automated carbon and nitrogen analyzer linked to a Thermo Finnigan Flash DELTA-plus XL mass spectrometer (Finnigan, Germany) at the MOE Key Laboratory of Western China’s Environmental Systems, Lanzhou University. After 10 samples had been presented, the standards (IAEA-600, δ15N: 1‰; Protein, δ15N: 5.94‰; Puge, δ15N: 5.6‰) were entered into the nitrogen sample list, and graphite (δ13C: −16‰) was entered into the carbon sample list for isotopic calibration and to monitor the stability, respectively. The C and N isotopes were measured relative to the international standards—namely, Vienna Pee Dee Belemnite (V-PDB) and Ambient Inhalable Reservoir (AIR), respectively. The isotopic analytical precision was ±0.2‰.

3. Results

3.1. Chaff Size of Foxtail and Broomcorn Millet Remains from the Chashancun Cemetery

The detailed morphological characteristics of each type of millet chaffs without caryopses are shown in Figure 2. The mean values, ranges, and standard deviations for the length, width, and thickness of the foxtail and broomcorn millet chaffs were calculated (Table 1; Figure 3). A total of 1819 broomcorn millet chaffs (910 large broomcorn millet chaffs and 909 small ones) and 1814 foxtail millet chaffs (904 large foxtail millet chaffs and 910 small ones) were measured in this study. The width and thickness of the broomcorn millet chaffs ranged from 1.11 to 2.38 mm (mean = 1.85 ± 0.26 mm) and from 0.95 to 2.24 mm (mean = 1.50 ± 0.25 mm), while the foxtail millet chaffs ranged from 0.95 to 1.94 mm (mean = 1.50 ± 0.20 mm) and from 0.69 to 1.90 mm (mean = 1.19 ± 0.21 mm), respectively. As shown in Figure 3, we found that more than 80% of the large foxtail millet chaffs had widths that ranged from 1.55 to 1.79 mm, while the widths of the small foxtail millet ranged from 1.18 to 1.49 mm. Additionally, widths of more than 80% of the large broomcorn millet chaffs ranged from 1.93 to 2.19 mm, while widths of the small broomcorn millet chaffs ranged from 1.43 to 1.82 mm. Therefore, the isotopic samples from each group were reselected according to the measured width data from 10% of the large millet chaffs and 90% of the small millet chaffs. The data that can distinguish between the large and small millet only apply to the millet chaffs from the Chashancun cemetery. In addition, three pieces of broomcorn millet chaff data were excluded from further analysis due to them being inconsistent with most of the other data, a factor that probably was due to a wrong measurement being recorded during the experiment (see the red rectangle in Figure 3C,D).

3.2. Carbon and Nitrogen Isotopes of Foxtail and Broomcorn Millet Chaffs from the Chashancun Cemetery

Stable isotopic analyses of the 30 foxtail millet samples and 30 broomcorn millet samples are presented in Table 2 and Figure 4. The δ13C and δ15N values of the broomcorn millet samples (n = 30, including 15 large broomcorn millet samples and 15 small ones) ranged from −13.0‰ to −12.0‰ (mean = −12.5‰ ± 0.3‰) and from 15.7‰ to 17.7‰ (mean = 16.6‰ ± 0.7‰), respectively. The δ13C and δ15N values of the foxtail millet samples (n = 30, including 15 large foxtail millet samples and 15 small ones) ranged from −14.0‰ to −12.9‰ (mean = −13.4‰ ± 0.3‰) and from 15.7‰ to 18.8‰ (mean = 17.6‰ ± 1.0‰), respectively.

4. Discussion

Based on measurements of the foxtail and broomcorn millet chaffs that were unearthed from the Chashancun cemetery, there are notable differences in the chaff size (Figure 2 and Figure 3). While the variations in length among the chaffs of these two millet crops are insignificant, there are evident differences in their widths and thicknesses (Figure 3). This distribution characteristic of the size of the large and small millet chaffs is consistent with that of the grain size in modern millet samples, which are divided into two mature groups [42]. However, the factors that influence morphological variation in millet chaffs that are relatively large or small have rarely been reported.
The grain size of crops may be influenced by multiple factors, such as domestication and hereditary properties [12,43], crop processing [14,44], maturity [45], and the environmental conditions of crop growth [46,47]. The crop grain size varies between wild and domesticated crops and spans over a long-term scale (more than 1000 years; the grain processing technology was highly developed during the historical period in the Hexi Corridor, especially during the Tang Empire. Therefore, crop domestication and crop processing can be excluded as factors influencing the chaff size differentiation in the foxtail and broomcorn millet from the Chashancun cemetery.
Immaturity has been proposed as another key influencing factor resulting in the small grain size of the millet preserved in archaeological sites [45]. The morphological characteristics of millet seeds, the embryonic shape, and the length of the millet grains should be used to identify the maturity of both foxtail and broomcorn millet [42]. The foxtail and broomcorn millet grains that were collected from the Chashancun cemetery are uncharred chaffs (Figure 2), in contrast to the charred grains that are unearthed from most archaeological sites, wherein the features of the embryo are well preserved, and whose maturity cannot easily be determined. Funerary objects are often featured as important signs that demonstrate the status of the cemetery occupant [48,49]. As the foxtail and broomcorn millet grains in the Chashancun cemetery were buried as funerary objects for Mu Rong Zhi, a royal descendant of the ancient Tuyuhun Kingdom, it is reasonable to infer that these buried millet crops are likely mature. Moreover, agricultural techniques, including sowing techniques, management techniques, and reaping and processing techniques, underwent significant development during the Tang dynasty [50], resulting in a large proportion of mature grains. However, the proportion of millet chaffs of varying sizes that were unearthed from the Chashancun cemetery was almost accordant, indicating that the grains were very likely mature seeds. Therefore, the major factors influencing the chaff sizes of these buried millet crops are most likely related to their growing environment, such as water stress and soil fertility.
The δ13C values of the plants were affected by their various environmental conditions, including the concentrations of CO2 in the atmosphere, as well as the temperature, sunlight, and water status [51,52,53,54,55]. During the seventh century, the ancient Tubo Kingdom (633–842 BC) became powerful and conquered Tuyuhun, eventually taking over the entirety of Tuyuhun [56]. Thus, it is unlikely that millet grains from the Chashancun cemetery came from the Tibetan Plateau, which was occupied by the enemies of Tuyuhun. The Hexi Corridor and its surrounding areas, with the exception of the Tibetan Plateau, are located in arid and semiarid regions in a middle temperate zone and have accordant climate conditions [57], and these areas were likely the cultivation areas of the foxtail and broomcorn millet samples from the Chashancun cemetery. Therefore, the carbon isotopes of these buried millet chaffs would have mainly been affected by water status. While the relationship between water status and C4 plants is more complicated than that of C3 plants, the δ13C values are largely negatively correlated [58,59,60]. Though some case studies have suggested that the δ13C values of C4 plants are also negatively correlated with water conditions, most studies have proposed that the δ13C values of C4 plants generally increase with improved water status [61,62,63]. Previous studies have also suggested that the δ13C values of millet crops show a positive correlation with high precipitation [64,65]. The δ15N isotope values of crops are mainly affected by soil fertility, which may be related to natural environmental conditions, such as temperature, precipitation, and soil parent materials [66,67,68]. Moreover, the δ15N values of the remains of crops that are unearthed from archaeological sites might have been significantly influenced by manuring activities [69,70,71]. Based on the analysis of nitrogen isotopes of millet remains from late Neolithic sites in Shaanxi Province, Wang et al. [72] proposed that millet manuring facilitated an agricultural expansion in northern China during the late Neolithic period.
When comparing the results obtained from the millet chaffs collected from the Chashancun cemetery with those obtained from other archaeological sites, the ranges of the δ13C values were consistent with those of other excavated millets, while the ranges of the δ15N values were much higher (15.7~18.8‰). For example, the δ15N isotope values of the millet seeds collected in Shaanxi Province from the Late Neolithic period ranged from 3.3‰ to 6.9‰ [72], those from the west Loess Plateau during the Tang dynasty ranged from 3.8‰ to 7.8‰ [23], and those from the Shanxi Province during the Northern Wei dynasty ranged from 4.3‰ to 5.9‰ [73]. All in all, the δ15N values of charred millet seeds from different spatiotemporal areas were less than 8‰. The dry climate may have caused the increase in the δ15N isotope values in arid areas [74,75]. The Hexi Corridor has an arid climate, with the annual precipitation being 80–180 mm and the annual evapotranspiration, 180–230 mm [76,77]. Therefore, one of the reasons for the high δ15N in millet chaffs is likely due to the Hexi Corridor being in a more arid region, compared with Shaanxi Province, Shanxi Province, and the Loess Plateau. Based on previous studies, the anthropogenic manuring of crops can lead to higher δ15N values [72,78], and mature fertilization technology was used extensively during the Tang dynasty [79]. Moreover, domestic animals (cattle, sheep, and horses) grazing on cultivated fields after the crops have been harvested can produce intensive dung that can enrich soil fertility [80,81]. Animal husbandry was well developed during the Sui and Tang periods in northwestern China [82]. Therefore, another possible explanation for the high δ15N values is that good soil fertility can unintentionally increase the δ15N values of plants. However, whether these abnormally high values are related to uncharred millet chaffs requires further studies of the same material in the future. In this paper, we discussed the relative nitrogen isotope discrepancies between the large and small millet chaffs of each species.
The δ13C and δ15N values of the foxtail and broomcorn millet chaffs from the Chashancun cemetery suggest that the chaff size of those buried crop remains is correlated with water stress and soil fertility. As shown in Figure 4, the δ13C values of the large foxtail millet chaffs range between −13.5‰ and −12.9‰, and these values are higher than those of the small foxtail millet chaffs, which range between −14.0‰ and −13.4‰, suggesting that the water status required for the growth of large foxtail millet chaffs was superior to that of small foxtail millet chaffs. The δ15N values for the large foxtail millet chaffs range between 16.6‰ and 18.5‰; this is higher than the values determined for some of the small foxtail millet chaffs, which range between 15.7‰ and 16.6‰ but are lower than the values determined for other small foxtail millet chaffs, which range between 18.3‰ and 18.8‰ (Figure 4). It is possible that millet crops that were cultivated in different areas were mixed together and buried in the Chashancun cemetery. These data imply either that water stress was more important than soil fertility for the growth of foxtail millets in the Chashancun cemetery, or that water fertilizer coupling was a determinant for the yield of foxtail millet. Regarding buried broomcorn millet chaffs in the Chashancun cemetery, the δ13C values of the large and small broomcorn samples mainly range from −13.0‰ to −12.0‰, suggesting that water stress was probably not the major factor determining the chaff size of the broomcorn millet buried in the Chashancun cemetery. However, the δ15N values of the large broomcorn millet chaffs (17.0–17.7‰) are much higher than those of the small broomcorn millet chaffs (15.7–16.2‰) (Figure 4), indicating that soil fertility was the primary factor impacting the yield of broomcorn millet. Nevertheless, we are unable to determine whether the different water and fertilizer conditions affecting the cultivation of the millet chaffs buried in the Chashancun cemetery were induced by human management practices or by natural environmental conditions, for example, different geomorphic locations or regions, as the irrigating systems and fertilization technologies were well–developed during the early Tang dynasty period in northwest China according to historical records [79,83,84,85].

5. Conclusions

Our study provides a new archaeobotanical analysis of the Chashancun cemetery in Wuwei, a key region of the east Hexi Corridor. The application of size measurements and isotopic analysis to study foxtail and broomcorn millet chaffs buried in the Chashancun cemetery suggest that water stress and soil fertility were likely the primary environmental factors impacting the yield of foxtail and broomcorn millet in this area during the late seventh century. This case study demonstrates the importance of appropriate water fertilizer coupling to ensure the quality of cultivated millet crops during the early Tang dynasty. However, the forces shaping the water and fertilizer conditions affecting millet chaffs are not clear. Therefore, the anthropogenic and natural agents underlying these factors need to be examined in greater detail in future research.

Author Contributions

Conceptualization, G.C.; methodology, B.L., Y.Y. and W.W.; formal analysis, B.L. and Y.L.; investigation, B.L., Y.Y. and G.C.; resources, W.W.; supervision, G.C.; visualization, Y.L.; writing—original draft preparation, B.L., Y.Y., G.C., and Y.L.; writing—review and editing, all authors; project administration, G.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by a study on the cemetery of Murong Zhi, a member of the Tuyuhun royal family, during the Wu Zhou Period (Grant No. 20AKG007); Archaeology China-The archaeological research of Tuyuhun royal cemeteries during the Tang dynasty; Priority Research Program of the Chinese Academy of Sciences, Pan-Third Pole Environment Study for a Green Silk Road (Pan-TPE) (Grant No. XDA2004010101); Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (Grant No. 2019QZKK0601).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used in the current study are available from the corresponding authors.

Acknowledgments

We acknowledge the editor and reviewers of this paper, for their constructive comments and suggestions; we also thank the workers involved in the excavation of the Chashancun cemetery for the samples used in this study.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of the Chashancun cemetery (A–C) and the plant remains in the cemetery (D–F).
Figure 1. Location of the Chashancun cemetery (A–C) and the plant remains in the cemetery (D–F).
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Figure 2. Millet chaff remains found at the Chashancun cemetery.
Figure 2. Millet chaff remains found at the Chashancun cemetery.
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Figure 3. Scatter diagram and box plot detailing the length, width, and thickness of foxtail and broomcorn millet chaffs. (A,B) represent the result of foxtail millet chaffs; (C,D) represent the results of broomcorn millet chaffs.
Figure 3. Scatter diagram and box plot detailing the length, width, and thickness of foxtail and broomcorn millet chaffs. (A,B) represent the result of foxtail millet chaffs; (C,D) represent the results of broomcorn millet chaffs.
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Figure 4. Scatter diagram of carbon and nitrogen isotope values for millet chaffs.
Figure 4. Scatter diagram of carbon and nitrogen isotope values for millet chaffs.
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Table
Table 1. Statistical results regarding length, width, and thickness of single millet chaff.
Table 1. Statistical results regarding length, width, and thickness of single millet chaff.
SpeciesTypeNumberMean (mm)SDRange (mm)
Large broomcorn millet chaffLength9103.110.142.60–3.51
Width9102.060.121.66–2.38
Thickness9101.710.131.32–2.24
Small broomcorn millet chaffLength9093.180.182.65–3.62
Width9091.630.151.11–2.01
Thickness9091.300.150.95–1.76
Large foxtail millet chaffLength9042.330.151.71–2.78
Width9041.670.101.35–1.94
Thickness9041.340.151.01–1.90
Small foxtail millet chaffLength9102.360.191.76–2.90
Width9101.340.120.95–1.66
Thickness9101.040.140.69–1.47
Table
Table 2. Summary results of δ13C and δ15N.
Table 2. Summary results of δ13C and δ15N.
SpeciesNumberδ15N (‰)δ13C (‰)
MeanSDRangeMeanSDRange
Large broomcorn millet chaff1517.30.217.0–17.7−12.50.3−13.0–−12.0
Small broomcorn millet chaff1515.90.115.7–16.2−12.40.2−12.8–−12.0
Large foxtail millet chaff1517.70.716.6–18.5−13.20.2−13.5–−12.9
Small foxtail millet chaff1517.41.215.7–18.8−13.70.2−14.0–−13.4
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Liu, B.; Lu, Y.; Yang, Y.; Wei, W.; Chen, G. 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. Sustainability 2022, 14, 3581. https://doi.org/10.3390/su14063581

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Liu B, Lu Y, Yang Y, Wei W, Chen G. 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. Sustainability. 2022; 14(6):3581. https://doi.org/10.3390/su14063581

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Liu, Bingbing, Yongxiu Lu, Yishi Yang, Wenyu Wei, and Guoke Chen. 2022. "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" Sustainability 14, no. 6: 3581. https://doi.org/10.3390/su14063581

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