Carbon and nitrogen isotopic variability in foxtail millet (Setaria italica) with watering regime

Abstract

Rationale Carbonised plant remains are analysed for reconstruction of past climates and agricultural regimes. Several recent studies have used C4 plants to address related questions, and correlations between modern C4 plant δ13C values and rainfall have been found. The millets were important food crops in prehistoric Eurasia, yet little is known about causes of isotopic variation within millet species. Previous research has shown there to be significant isotopic variation between millet accessions. Here we compare isotope ratios from plants grown under different watering regimes. This allows for a consideration of whether or not Setaria italica is a good proxy for environmental reconstruction. Methods We compare stable isotope ratios of Setaria italica plants grown in a controlled environment chamber with different watering regimes. We compare the carbon isotope ratios of leaves and grains, and the nitrogen isotope ratios of grains, from 12 accessions of Setaria italica. Results We find significant isotopic variability between watering regimes. Carbon isotope ratios are positively correlated with water availability, and on average vary by 1.9‰ and 1.7‰ for leaves and grains, respectively. Grain nitrogen isotope ratios also vary with watering regime; however, the highest isotope ratios are found with the 130‐mL watering regime. Conclusions The carbon isotope ratios of Setaria italica are strongly correlated with water availability. However, the correlation is the opposite to that seen in studies of C3 plants. The difference in isotopic ratio due to watering regime is comparable with that seen between different accessions; thus distinguishing between changing varieties of Setaria italica and changing climate is problematic. In terms of grain nitrogen isotope ratios, the highest δ15N values were not associated with the lowest watering regime. Again, δ15N variation is comparable with that which would be expected from an aridity effect or a manuring effect, and thus distinguishing between these factors is probably problematic.

The effect of water availability on the carbon and nitrogen isotope composition of a C4 plant (pearl millet, Pennisetum glaucum)

“…The weak signals observed here within accessions are likely to be obscured by environmental and genetic individualities among plants, which differ between environmentally-controlled greenhouses and "real-life" agricultural set-ups. Our results are consistent with the recent findings of Lightfoot et al (2020) for foxtail millet (Setaria italica). Differences in δ 13 C values produced by different watering treatments are not large enough in magnitude to be distinguishable from the isotopic variability between millet accessions.…”

“…This relationship has also been observed in other C 4 species, including foxtail millets (Setaria italica) (An et al, 2015;Lightfoot et al, 2020) and grasses of the Aristida genus (Schulze et al, 1996;Murphy & Bowman, 2009), and was previously reported for P. glaucum in an observational study (Reid et al, 2018). Previous studies have attributed this relationship to C 4 plant physiology, particularly to the equilibrium isotope effect between dissolved CO 2 and bicarbonate in mesophyll cells (e.g., Schulze et al, 1996).…”

“…In a controlled experimental set-up, δ 13 C values in maize (Zea mays L.) have been shown to be positively correlated with water availability (Dercon et al, 2006). Additionally, a positive relationship between carbon isotope ratios and water availability was found recently in foxtail millet (Setaria italica) varieties exposed to different watering regimes in a growth chamber experiment (Lightfoot et al, 2020).…”

Pastures and fodder for feeding equids 3000 years ago: The Can Roqueta site (Barcelona, Spain) as a model of equine herd management

“…However, in millet, the correlation between isotopic signals and water availability is not apparent. Millet's C4‐type physiology allows its cultivation in more arid environments, and it is therefore less likely that they were cultivated in areas of water availability (Lightfoot et al, 2020). The difference between the millet‐fed equids in Group 2b and the cows with high δ 13 C and δ 15 N isotopic values could again be due to increased fertilization of crops grown for cattle feed (Figure 5).…”

“…Wallace et al (2013) have calculated a δ 13 C offset of +1.9‰ for wheat and +1.7‰ for barley. In millet S. italica , Lightfoot et al (2020) found similar differences of up to +2‰. Regarding nitrogen signature, Fraser et al (2011) have calculated a mean value for the grain‐rachis difference of +2.4‰ in cereal crops in a range of zones and under varying conditions of irrigation and fertilization intensity.…”