Nitrogen isotope values of Pennisetum glaucum (pearl millet) grains: towards a reconstruction of past cultivation conditions in the Sahel, West Africa

Abstract

The nitrogen isotopic composition of charred wheat and barley grains reflect manuring intensity and have been used to reconstruct manuring practices at archaeological sites across Europe and western Asia. To assess whether this analytical method can be applied to a staple crop in the West African Sahel, pearl millet (Pennisetum glaucum), the nitrogen isotope value of pearl millet grains in this region were determined and the effect of charring ascertained. Pearl millet ears were collected from fields in northeast Senegal, where the fertilisation histories of the plots (manure and/or household waste) were known. The nitrogen isotope values of these millet grains provide an insight into the values to expect for pearl millet grains grown with low-moderate addition of manure/household waste in a similar climate. Charring of grains by heating at 215-260°C for 4-24 hours increases the nitrogen isotope values of pearl millet grains by a maximum of 0.34‰. In light of these modern data, the nitrogen isotope values of millet grains recovered from the archaeological settlement mound of Tongo Maaré Diabal, Mali can be interpreted as evidence for modest levels of manure/household waste input throughout the occupation of the site from cal A.D. 500-1150. This study demonstrates the potential for nitrogen isotope values of pearl millet grains to shed light on past farming practices in West Africa.

Direct evidence for agricultural intensification during the first two millennia AD in northeast Burkina Faso

“…The %N and %C content and δ 15 N and δ 13 C values of each pearl millet grain sample included in this study are given in Supplementary material (Table S2). Prior to graphing and analysis, these δ 15 N and δ 13 C values were corrected for charring by subtracting 0.34 ‰ and 0.11 ‰, respectively, from the determined δ 15 N and δ 13 C values (after Styring et al, 2019). Only the first analysed of the duplicate samples (i.e.…”

“…(shaded region in Fig 2; values are taken from Styring et al, 2019). One of the samples has a higher %N content and others have slightly lower %N contents than the modern charred millet grains.…”

“…One of the samples has a higher %N content and others have slightly lower %N contents than the modern charred millet grains. However, the range of %N contents of modern samples is derived from only one study by Styring et al (2019), which considered modern commercial cultivars, and it is likely that ancient pearl millet had a more diverse range of %N contents. Furthermore, the δ 15 N values of these samples do not appear anomalous and therefore we have included these in further analysis and interpretation.…”

“…The increasing frequency of cultivation would have removed more nutrients from the soil, reducing its fertility and therefore potentially reducing crop yields. It has been speculated that soil nutrients were replaced by addition of cattle manure, likely through grazing of the fields in the dry season, following the harvest (Höhn and Neumann, 2012), as occurs today and as occurred in the recent past (Krings 1991;Le Houérou, 1989;Prudencio 1993;Styring et al, 2019). Cultivated fields close to homesteads (within c. 500 m) today also receive inputs of household waste (including animal manure and ash), which is generally spread by hand (Adderley et al, 2004;Pelissier 1966;Prudencio 1993).…”

“…A linear model was therefore developed to allow the manuring level of an archaeological cereal grain sample to be predicted based on its δ 15 N value and the estimated annual rainfall of its growing location (Styring, Charles, et al, 2017). While the linear model was primarily based on the δ 15 N values of wheat and barley grains sampled from fields in Eurasia, a recent study found that the δ 15 N values of modern pearl millet grains growing in plots receiving modest levels of manure/household waste in three regions in Senegal (mean annual rainfall between 494 and 565 mm) fit well into the low and medium manuring levels defined by the model (Styring et al, 2019). Since then, Reid et al (2018) found that the δ 15 N values of pearl millet grown in four different years at the Germplasm Introduction and Research Unit, Kingshill, United States Virgin Islands, with annual rainfall ranging from 807-1353 mm, correlated positively with annual rainfall (an opposite relationship to that observed globally among all plant types).…”