Further investigation into the impact of manuring on stable carbon (δ13C) and nitrogen isotope (δ15N) values in pulses: a four-year experiment examining Celtic bean (Vicia faba)

Plant stable carbon (delta C-13) and nitrogen (delta N-15) isotope values can be used to directly investigate crop husbandry practices such as manuring; a key variable in understanding the scale and intensity of past farming practices. We present new results from a four-year experiment examining the impact of manuring on the delta C-13 and delta N-15 values of a heritage cultivar of the broad or faba bean (Vicia faba), the 'Celtic black broad bean'. This paper builds upon our previous pilot study (Treasure ER, Church MJ, Gr & ouml;cke DR (2016) The influence of manuring on stable isotopes (delta C-13 and delta N-15) in Celtic bean (Vicia faba L.): archaeobotanical and palaeodietary implications. Archaeol Anthropol 8:555-562. https://doi.org/10.1007/s12520-015-0243-6). Our results indicate that manuring increased delta N-15 values in beans, with a mean value of 0.5 +/- 0.4 parts per thousand in control samples compared to a mean value of 2.1 +/- 1.3 parts per thousand in the most intensively manured plots. It was not always possible to distinguish between an unmanured and manured crop on the basis of delta N-15 values, although manured crops consistently exhibited larger variation in delta N-15. Bean delta C-13 values show no clear relationship with manuring, although large variability (> 3 parts per thousand) was found within crops cultivated under the same conditions. Manuring also increased plant biomass production, seed-size and the bean per plant ratio. Overall, we suggest that high delta N-15 values (> 1.5 parts per thousand) in archaeobotanical remains of pulses can be attributed to small-scale, intensive cultivation that is typical of gardens where manuring rates are very intensive.