Manure for millet: Grain 815N values as indicators of prehistoric cropping intensity of Panicum miliaceum and Setaria italica

Broomcorn and foxtail millet are the only major domesticated plants indigenous to prehistoric Eurasia to rely on the C(4)photosynthetic pathway. Here we study the impact of animal manure (AM) on broomcorn millet (Panicum miliaceum) and foxtail millet (Setaria italica), grown in the Askov Long-Term Experiment, using unmanured soil and soil dressed with phosphorus plus potassium (PK) as reference treatments. Animal manure had a marked effect on yields and on the delta N-15 values of grains. For broomcorn millets grown on manured soil, the average delta N-15 value was 5.8%o. When grown on unmanured and PK-treated soils, the delta N-15 values were 0.4%o and 0.2%o, respectively. For foxtail millet the delta N-15 values also differed between grains from unmanured (-1.0%o), PK (0.7%o) and manured (6.3%o) treatments. Thus, when compared to unmanured soil, the offset due to manure was 7.3%o for Setaria and 5.3%o for the two Panicum varieties. In accordance with previous studies on C-3 crops, our study suggests that 815N values in charred millet grains recovered from archaeological sites could provide a robust indicator of prehistoric manuring intensity.