Complexities of nitrogen isotope biogeochemistry in plant-soil systems: implications for the study of ancient agricultural and animal management practices

Abstract

Nitrogen isotopic studies have the potential to shed light on the structure of ancient ecosystems, agropastoral regimes, and human-environment interactions. Until relatively recently, however, little attention was paid to the complexities of nitrogen transformations in ancient plant-soil systems and their potential impact on plant and animal tissue nitrogen isotopic compositions. This paper discusses the importance of understanding nitrogen dynamics in ancient contexts, and highlights several key areas of archaeology where a more detailed understanding of these processes may enable us to answer some fundamental questions. This paper explores two larger themes that are prominent in archaeological studies using stable nitrogen isotope analysis: (1) agricultural practices (use of animal fertilizers, burning of vegetation or shifting cultivation, and tillage) and (2) animal domestication and husbandry (grazing intensity/stocking rate and the foddering of domestic animals with cultigens). The paucity of plant material in ancient deposits necessitates that these issues are addressed primarily through the isotopic analysis of skeletal material rather than the plants themselves, but the interpretation of these data hinges on a thorough understanding of the underlying biogeochemical processes in plant-soil systems. Building on studies conducted in modern ecosystems and under controlled conditions, these processes are reviewed, and their relevance discussed for ancient contexts.

Effects of Depth and Land Cover on Soil Properties as Indicated by Carbon and Nitrogen-Stable Isotope Analysis

“…As illustrated in Table 2, the subsoil (i.e., 60-90 cm depth) had signifi cantly higher δ N at the topsoil in this study indicated that N was a product of m ultiple variables such as N fi xation, precipitation, rainstorm, and the use of chemical fertilizers. Several investigators had also demonstrated similar results, e.g., [66][67][68]. The authors mentioned above attributed this to atmospheric N 2 , which is usually the raw material for fertilizer production, and δ…”

“…In contrast, when the soil depth profi le was increased, the value of δ 15 N became signifi cantly higher. According to [56,66,[74][75] It is noteworthy that this increase in δ…”

Stable isotope ratio analysis of bone collagen as indicator of different dietary habits and environmental conditions in northeastern Iberia during the 4th and 3rd millennium cal B.C.

“…Otherwise,δ 15 N values are able to provide information about the trophic level that an specific organism holds in the food chain Minagawa and Wada 1986). They are considered to increase generally between 3 and 5‰ with each trophic level up inside the food web Schwarcz and Schoeniger 1991), although some researchers suggest this range could be wider (Hedges and Reynard 2007;Szpak 2014).…”

“…This δ 13 C variability is not only recorded in plants, as it is introduced also in higher trophic levels through their consumption, and plays an important role in the study of shepherding and livestock management (Tornero et al 2016a). Plants also take organic nitrogen from the soil, resulting in their δ 15 N values showing variations depending on natural or anthropic factors which affect the soil-plant system (Szpak 2014). Among the natural factors, the presence of organic material in the soil, temperature and water availability could be the most significant ones (Ambrose 1991;Handley et al 1999).…”

Stable isotopes and selective forces: examples in biocultural and environmental anthropology

“…Although first seen as a source of error in paleodietary research, fertilization systematics are now being used to reconstruct the rise of intensive agriculture and its link to animal domestication, husbandry practices (e.g., complete captivity, controlled herds, and pasturing locales), and land use patterns (Balasse 2014, Hamilton & Thomas 2012, Makarewicz & Tuross 2012, Szpak 2014b). …”