Crop stable isotope ratios: new approaches to palaeodietary and agricultural reconstruction

考古学家通常使用人类骨胶原中稳定氮同位素的比例（15 N：14 N）来评估过去的饮食主要是动物还是植物。这一推论源于广泛观察到的较重同位素（15 N）从食物链的一个层次到下一个层次的富集。因此，食草动物，包括素食的人类，其比率将高于植物，而肉食的人类（主要涉及食草动物的消费）将具有更高的比率，因为它们比植物高两个营养级。然而，这种模式过于简单化，需要一种更细致的方法来可靠地重建过去的饮食。特别是，该模型假设所有植物都具有相似的氮同位素比率，但已知这些比率差异很大。特别值得关注的是，从农业的引入（新石器时代）开始，人类管理引起的作物氮同位素比率的变化。一项试点研究表明，施肥的做法（使用动物粪便作为肥料，以提高作物产量）导致作物15 N的大量富集，以至于以施肥谷物为基础的饮食通常被解释为以动物为基础。此外，其他研究表明，早期耕作往往涉及劳力密集型投入，如施肥。事实上，最早的农民施肥可以解释欧洲新石器时代遗址中人类骨骼的高15 N：14 N比率，那里有广泛的农作物种植证据。关于作物氮同位素比率的信息不仅对于将人类饮食重建置于更坚实的基础上至关重要，而且对于确定早期农业实践的强度也至关重要。传统观点认为，农业是从低投入的“粗放型”系统（如轮垦，在新开垦的地块上种植几年，然后转向新的地块）逐步演变为劳动密集型的耕作形式（如连作）。然而，越来越多的考古学证据表明，早期的农业并不像轮垦，而是以“固定”的田地为特色，从一代到下一代密集种植。通常用于推断作物管理方法的证据是间接的，其依据是观察到可耕地杂草的不同组合与不同的做法有关。一种直接从作物本身的遗骸推断耕作强度的方法将大大扩大可以评估耕作强度的考古遗址的范围，因为只有少数考古植物组合含有足够数量的可耕地杂草种子来解决耕作实践。因此，该项目的第一个目标是确定施肥如何影响当今作物的氮同位素比率。通过了解新石器时代出现的谷类和豆类作物的这种影响，我们可以提高考古学中饮食评估的可靠性，并开发一种新的工具，通过施肥的实践直接识别集约化的作物管理。该项目的第二个目标涉及考古学中关于新石器时代人类饮食变化和青铜时代农业强度变化的主要辩论。我们将通过整合从西南亚到北方欧洲的新石器时代考古遗址的农作物遗骸、动物骨骼和人类骨骼的氮同位素比率，来解决新石器时代农业饮食主要是以农作物还是以牲畜为基础的问题。对于随后的青铜时代时期，我们将类似地分析从叙利亚到北方欧洲考古遗址的作物，动物和人类遗骸，以确定与早期复杂社会的兴起有关的农业变化的程度和方向。

Archaeologists routinely use ratios of stable nitrogen isotopes (15N:14N) in human bone collagen to assess whether past diets were primarily animal- or plant-based. This inference stems from widely observed enrichment of the heavier isotope (15N) from one level in the food chain to the next. Thus, herbivorous animals, including humans with a vegetarian diet, would have ratios higher than plants, whereas humans having a meat diet (which would largely involve consumption of herbivores) would have even higher ratios, since they are two trophic levels above plants. This model is an over-simplification, however, and a more nuanced approach is needed to arrive at reliable reconstructions of past diet. In particular, the model assumes that all plants have similar nitrogen isotope ratios, but these are known to vary widely. Of specific concern are alterations to crop nitrogen isotope ratios caused by human management from the introduction of farming (Neolithic period) onwards. A pilot study has shown that the practice of manuring (use of animal dung as fertiliser to enhance crop yields) causes substantial enrichment of crop 15N, to the extent that a diet based on manured cereals would conventionally be interpreted as animal-based. Moreover, other research has indicated that early farming tended to involve labour-intensive inputs such as manuring. In fact, manuring by the earliest farmers may explain high 15N:14N ratios for human bones from a number of European Neolithic sites, where there is widespread evidence for crop cultivation. Information on crop nitrogen isotope ratios is crucial not only for placing human dietary reconstruction on a firmer foundation, but also for identifying the intensity of early farming practice. The conventional view is that agriculture evolved from low-input 'extensive' systems (e.g. shifting cultivation, with cropping of newly cleared plots for a few years, followed by a shift to new plots) through progressively more labour-intensive forms of farming (e.g. continuous cropping). There is growing archaeological evidence, however, to suggest that early farming did not resemble shifting cultivation but rather featured 'fixed' fields that were intensively cropped from one generation to the next. The evidence generally used to infer the methods of crop management is indirect, being based on the observation that different combinations of arable weeds are associated with different practices. A method for inferring cultivation intensity directly from the remains of the crops themselves would greatly expand the range of archaeological sites where farming intensity can be assessed, since only a minority of archaeobotanical assemblages contain sufficient quantities of arable weed seeds to address cultivation practices. The first aim of this project, therefore, is to establish how manuring affects nitrogen isotope ratios in present-day crops. By understanding this effect in the range of cereal and pulse crops that emerged in the Neolithic period, we can improve the reliability of dietary assessment in archaeology and develop a new tool for directly identifying intensive crop management through the practice of manuring. The second aim of the project relates to major debates in archaeology concerning human dietary change in the Neolithic period and change in the intensity of farming during the Bronze Age. We will address the question of whether Neolithic farming diets were primarily crop- or livestock-based by integrating nitrogen isotope ratios for crop remains, animal bones and human bones from archaeological sites of the Neolithic period extending from south-west Asia to northern Europe. For the subsequent Bronze Age period we will similarly analyse crop, animal and human remains from archaeological sites from Syria to northern Europe to determine the extent and direction of changes in farming that were associated with the rise of early complex societies.