Agronomic conditions and crop evolution in ancient Near East agriculture

ARTICLE Received 24 Dec 2013 | Accepted 24 Apr 2014 | Published 23 May 2014 DOI: 10.1038/ncomms4953 Agronomic conditions and crop evolution in ancient Near East agriculture José L. Araus1,*, Juan P. Ferrio2,*, Jordi Voltas2,*, Mònica Aguilera2 & Ramón Buxó3 The appearance of agriculture in the Fertile Crescent propelled the development of Western civilization. Here we investigate the evolution of agronomic conditions in this region by reconstructing cereal kernel weight and using stable carbon and nitrogen isotope signatures of kernels and charcoal from a set of 11 Upper Mesopotamia archaeological sites, with chronologies spanning from the onset of agriculture to the turn of the era. We show that water availability for crops, inferred from carbon isotope discrimination (D13C), was two- to fourfold higher in the past than at present, with a maximum between 10,000 and 8,000 cal BP. Nitrogen isotope composition (d15N) decreased over time, which suggests cultivation occurring under gradually less-fertile soil conditions. Domesticated cereals showed a progressive increase in kernel weight over several millennia following domestication. Our results provide a first comprehensive view of agricultural evolution in the Near East inferred directly from archaeobotanical remains. 1 Department of Plant Biology, University of Barcelona, E-08028 Barcelona, Spain. 2 Department of Crop and Forest Sciences—AGROTECNIO Center, Universitat de Lleida, Avda Rovira Roure 191, E-25198 Lleida, Spain. 3 Museu d’Arqueologia de Catalunya, Pedret 95, E-17007 Girona, Spain. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to J.L.A. (email: ) or to J.P.F. (email: ) or to J.V. (email: ). NATURE COMMUNICATIONS | 5:3953 | DOI: 10.1038/ncomms4953 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. 1 ARTICLE T NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4953 he study of how growing conditions, farming practices and crop domestication evolved from the beginning of agriculture is crucial for revealing the complexity and dynamics of ancient societies and to understand the current Mediterranean landscapes. The initial steps towards plant domestication in the Fertile Crescent can be pushed back to the 12th millennium cal BP, with different cereals being the main staples. Plant domestication in this region was a slow process with crop cultivation appearing at least 1,000 years before the earliest morphological changes supporting domestication (for example, changes in seed shedding, increase in kernel size)1. However, the evolution of agricultural systems after domestication, including water availability, soil fertility and potential yields achieved, as well as the pace of ‘post-domestication’ progress in kernel size, remains mostly unknown2. The carbon isotope discrimination (D13C) of charred kernels has been used to reconstruct water conditions3, including cereal water inputs of Neolithic4,5 and Bronze Age sites6,7 in the Near East. The comparison of D13C values from cultivated plant remains (charred seeds) and forest trees (charcoal) has aided the discrimination between climate and anthropogenic effects on crop performance3,8. Since water availability is the main factor limiting cereal productivity in Mediterranean conditions and, in turn, the D13C of plant biomass integrates water conditions during growth9, information on potential crop productivity can be gained by combining 13C signatures10 and kernel weight11. Shortages of soil nutrients probably constituted important constraints on growth in prehistoric agriculture12. Measures to enhance the availability of nutrients for crops included burning of native vegetation and application of nutrient-enriched sediments, domestic waste, and green and animal manure13. However, there is very little direct evidence relating to the history of ancient manuring. Intensive manuring may be taken as an indicator of marked social changes in land use and tenure during the Neolithic14. Manuring crops involves a long-term investment in arable land associated with adoption of a sedentary way of life. In general, any agronomic practice aiming to improve soil fertility, such as the use of animal and green manure or fallow, would increase the nitrogen isotope composition (d15N) of both soil and plants. Different studies suggest that the long-term application of manure to permanently cultivated sites leaves a recognizable N-isotopic imprint on kernels and straw15. However, analysis of d15N in kernels as a proxy to infer soil management practices and fertility in ancient agriculture is only a recent development4,11,14,15. Kernel weight is a key factor determining nutritional quality in cereals and, as such, has profound economic implications1. Archaebotanists interested in the evolution of kernel size at the origin of agriculture usually report on the breadth and thickness of charred kernels1. However, kernel dimensions may suffer alterations because of carbonization. Alternatively, the original weight of kernels can be inferred from archaeological (charred) kernel dimensions, after considering the impact of carbonization on three-dimensional deviations5,11. This allows data of archaeological kernels to be compared with the huge amount of information available on kernel weight for present-day agriculture. Three main objectives were pursued in this study. The first was to characterize the evolution of water availability and nutritional status of cereal crops in the Near East from the beginnings of agriculture to the turn of the era. Second, the increase in kernel weight (ultimately linked to directional selection) was tracked to determine when values comparable to those of present-time cereals were achieved by domesticated barley and naked wheat. Finally, the third objective was to estimate cereal yields in the past 2 as compared with current values for rainfed agriculture in the region. The study shows crop water availability, inferred from D13C, being two- to fourfold higher in the past than at present, with a maximum between 10,000 and 8,000 cal BP, whereas grain yield values are comparable to those achieved now-a-days in the region. The decrease of d15N over time suggests cultivation occurring under gradually less-fertile soil conditions. A progressive increase in kernel weight over several millennia following domestication is also accounted. Results Studied kernels. Size, d15N and D13C of charred kernels from five cereal species (including wild and domesticated) (Supplementary Fig. 1), together with charcoal D13C records of two forest trees (Quercus sp. and Pistacia sp.), were studied. These remains belong to a set of 11 Near East archaeological sites (from Upper Mesopotamia or Al-Jazira) spanning 11,000 years from the onset of agriculture in the region (13th millennium cal BP) (Fig. 1). Crop water status. The D13C of ancient cereals and trees was significantly (...truncated)