What can crop stable isotopes ever do for us? An experimental perspective on using cereal carbon stable isotope values for reconstructing water availability in semi-arid and arid environments

Vegetation History and Archaeobotany https://doi.org/10.1007/s00334-018-0708-5 ORIGINAL ARTICLE What can crop stable isotopes ever do for us? An experimental perspective on using cereal carbon stable isotope values for reconstructing water availability in semi-arid and arid environments Pascal Flohr1,2 · Emma Jenkins3 · Helen R. S. Williams1 · Khalil Jamjoum4 · Sameeh Nuimat5,6 · Gundula Müldner1 Received: 25 August 2018 / Accepted: 4 December 2018 © The Author(s) 2019 Abstract This study re-assesses and refines the use of crop carbon stable isotope values (Δ13C) to reconstruct past water availability. Triticum turgidum ssp. durum (durum wheat), Hordeum vulgare (six-row barley) and Sorghum bicolor (sorghum) were experimentally grown at three crop research stations in Jordan for up to three years under five different irrigation regimes: 0% (rainfall only), 40%, 80%, 100% and 120% of the crops’ optimum water requirements. The results show a large variation in carbon stable isotope values of crops that received similar amounts of water, either as absolute water input or as percentage of crop requirements. We conclude that C 3 crop carbon stable isotope composition should be assessed using a climate zone specific framework. In addition, we argue that interpretation should be done in terms of extremely high values showing an abundance of water versus low values indicating water stress, with values in between these extremes best interpreted in conjunction with other proxy evidence. Carbon stable isotope values of the C4 crop Sorghum were not found to be useful for the reconstruction of water availability. Keywords Plant carbon stable isotope values · Experimental crop growing · Water availability · Water management Introduction Communicated by F. Antolín. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00334-018-0708-5) contains supplementary material, which is available to authorized users. * Pascal Flohr 1 Department of Archaeology, University of Reading, Whiteknights, Reading RG6 6AB, UK 2 Present Address: School of Archaeology, University of Oxford, 1‑2 South Parks Road, Oxford OX1 3TG, UK 3 School of Applied Sciences, Bournemouth University, Fern Barrow, Poole BH12 5BB, UK 4 National Centre for Agricultural Research and Extension, Al‑Baqah, Amman 19381, Jordan 5 Ministry of Agriculture, Queen Rania Al Abdullah Street 39, Amman 11181, Jordan 6 Present Address: International Union for Conservation of Nature (IUCN) in Jordan, Hasan Bakar Al Azazi Building No 20, Sweifiyeh, PO Box 942230, Amman 11194, Jordan The reconstruction of water availability is essential for understanding past societies, especially in semi-arid and arid environments where fluctuations in aridity can have considerable effects on food production and, by implication, social and economic security. Profound droughts have, for example, been linked to the abandonment of sites as well as social, economic or political ‘collapse’ (Kaniewski et al. 2013; Weiss 2015). Water management strategies, such as floodwater farming and irrigation, have been employed since prehistory to ensure stable harvests and to generate agricultural surplus (Finlayson et al. 2011), both of which arguably underpin the development of complex societies. The reconstruction of past water availability is therefore central to many important questions in archaeology. The application of stable carbon isotope discrimination (Δ13C) of archaeobotanical remains to infer past water availability was pioneered in the 1990s (Araus and Buxó 1993; Araus et al. 1997a, b) and has since been regularly applied (Ferrio et al. 2005; Araus et al. 2007; Fiorentino et al. 2008; 13 Vol.:(0123456789) Vegetation History and Archaeobotany Riehl et al. 2008; Roberts et al. 2011; Masi et al. 2014; Caracuta et al. 2015; Mora-González et al. 2018). The advantages of the method are that crop remains are frequently present in the archaeological record, can be directly dated, and are easily linked to other archaeological remains. The carbon stable isotope composition of plant tissues primarily reflects water availability, so water management can be inferred by integrating the isotope data with palaeoclimate indicators for the time period in question. If the crop isotope signal suggests that water availability was greater than expected according to the climate proxies, it is likely that the crops received water in a form other than rainfall, such as through irrigation, artificial watering or by cultivation on alluvial fans (Ferrio et al. 2005). The method has a solid theoretical basis (Farquhar et al. 1982, 1989). During uptake and assimilation of CO2, plants discriminate against the heavier isotope 13C. The magnitude of discrimination is largely dependent on the plants’ photosynthetic pathway ( C3, C4, CAM), but is also affected by environmental factors, most notably water availability, as confirmed by studies on modern plants such as wheat and barley (Craufurd et al. 1991; Araus et al. 1997a, b, 1999; Merah et al. 2001; Ferrio et al. 2005, 2007; Monneveux et al. 2005; Wallace et al. 2013). Following this, archaeobotanical crop Δ13C values have been linked with specific amounts of water input through a regression equation (Araus et al. 1997b, 2014). However, slopes and intercepts of the regression lines vary between studies (ESM 1), indicating that crop Δ13C values are not solely determined by water input. Indeed, research has shown that large variability can exist in Δ13C of crops grown under similar amounts of rainfall or irrigation (Flohr et al. 2011; Wallace et al. 2013), especially in semi-arid and arid environments (Riehl et al. 2014). This variability could be due to related factors including evapotranspiration, which is in turn affected by temperature and wind speed, and soil characteristics like soil type, depth and ability to retain water, as is further investigated in this paper. In addition, plant Δ13C has been shown to be affected by several other variables not directly related to water availability, most notably salinity (Isla et al. 1998; Shaheen and Hood-Nowotny 2005; Yousfi et al. 2010), temperature, also when unrelated to water availability (O’Leary 1995), light intensity (Mulkey 1986; O’Leary 1995; Yakir and Israeli 1995) and nutrient supply (Toft et al. 1989; Choi et al. 2005; Cabrera-Bosquet et al. 2007, 2009; Serret et al. 2008) (but see Condon et al. 1992 and references therein). A correlation of Δ13C with altitude has also been demonstrated (Körner et al. 1988, 1991; Sparks and Ehleringer 1997, but see Friend et al. 1989; van de Water et al. 2002; Wang et al. 2010), although this is probably the result of a combination of different environmental factors which vary with altitude, most notably precipitation and temperature (Friend et al. 1989). In environments with closed canopies, 13 such as dense forests, as much as 5–8‰ higher plant Δ13C values have been observed, possibly (...truncated)