Effect of dung, ash and runoff water on wheat and barley grain sizes and stable isotope ratios: Experimental studies in ancient desert agriculture (Negev, Israel)

Archaeological excavations in the central Negev desert in terraced wadi fields at Horvat Haluqim revealed remains of two ancient fertilizers: charred plant ash and animal dung. Average annual rainfall in the area is 94 mm. Runoff rainwater from natural hillside catchments, captured by terrace walls, augmented soil moisture in valleys to enable agriculture. Some terraced fields are farmed by Bedouin, who grow wheat and barley. Using these cereal varieties, we conducted novel investigations in this arid desert environment about the effect of plant ash, sheep dung and runoff water on grain sizes, δ¹³C, Δ¹³C, and δ¹⁵N. Our study included both controlled pot experiments and traditional runoff farming by Bedouin. The pots were filled with local desert loess soil to investigate the effect of four different fertilizer treatments – (1) “None” for baseline data, (2) “Ash”, (3) “Dung”, (4) “Ash & Dung combined” – on the above cereal varieties. The largest cereal grains were produced by treatment 4 (ash & dung), which is a remarkable result, because it independently corroborates the archaeological findings. The pots received equal amounts of tap water, totalling 240 mm for barley and 325 mm for wheat. The Δ¹³C values of cereal grains in the pot experiments ranged from 15.62 to 17.47‰. Concerning δ¹⁵N, sheep dung produced a small increase, as compared to the baseline data, but plant ash fertilizer caused a decrease. Ash and dung together (treatment 4) yielded variable δ¹⁵N results. Stable isotopes of the same cereal varieties were also studied in the context of traditional runoff farming by Bedouin in terraced wadi fields in the area. Runoff water reception by terraced fields is by nature highly variable. A negative correlation was found between δ¹³C of cereal grains and runoff soil moisture. The Δ¹³C values ranged from 12.59 to 17.44‰. Concerning δ¹⁵N, cereal grains from the drier fields had comparatively high values, while the wetter fields yielded the two lowest δ¹⁵N values. Nevertheless, other δ¹⁵N values from wetter fields were quite high, indicating the effect of additional factors besides runoff water. Though the Bedouin do not add fertilizers to the terraced fields, their sheep and goats graze the cereal stubble after the harvest. This leads to a spatially random and spotty distribution of manure, which may explain the diverging δ¹⁵N values.