1. Compound-specific stable isotope analysis of individual amino acids (CSIA-AA) has emerged as a transformative approach to estimate consumer trophic positions (TP_CSIA) that are internally indexed to primary producer nitrogen isotope baselines. Central to accurate TP_CSIA estimation is an understanding of beta (β) values—the differences between trophic and source AA δ¹⁵N values in the primary producers at the base of a consumers’ food web. Growing evidence suggests higher taxonomic and tissue-specific β value variability than typically appreciated.
2. This meta-analysis fulfils a pressing need to comprehensively evaluate relevant sources of β value variability and its contribution to TP_CSIA uncertainty. We first synthesized all published primary producer AA δ¹⁵N data to investigate ecologically relevant sources of variability (e.g. taxonomy, tissue type, habitat type, mode of photosynthesis). We then reviewed the biogeochemical mechanisms underpinning AA δ¹⁵N and β value variability. Lastly, we evaluated the sensitivity of TP_CSIA estimates to uncertainty in mean β_Glx-Phe values and Glx-Phe trophic discrimination factors (TDF_Glx-Phe).
3. We show that variation in β_Glx-Phe values is two times greater than previously considered, with degree of vascularization, not habitat type (terrestrial vs. aquatic), providing the greatest source of variability (vascular autotroph = −6.6 ± 3.4‰; non-vascular autotroph = +3.3 ± 1.8‰). Within vascular plants, tissue type secondarily contributed to β_Glx-Phe value variability, but we found no clear distinction among C₃, C₄ and CAM plant β_Glx-Phe values. Notably, we found that vascular plant β_Glx-Lys values (+2.5 ± 1.6‰) are considerably less variable than β_Glx-Phe values, making Lys a useful AA tracer of primary production sources in terrestrial systems. Our multi-trophic level sensitivity analyses demonstrate that TP_CSIA estimates are highly sensitive to changes in both β_Glx-Phe and TDF_Glx-Phe values but that the relative influence of β values dissipates at higher trophic levels.
4. Our results highlight that primary producer β values are integral to accurate trophic position estimation. We outline four key recommendations for identifying, constraining and accounting for β value variability to improve TP_CSIA estimation accuracy and precision moving forward. We must ultimately expand libraries of primary producer AA δ¹⁵N values to better understand the mechanistic drivers of β value variation.

中文翻译：

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初级生产者氨基酸δ15N值的Meta分析及其对营养位置估计的影响

1. 单个氨基酸的化合物特异性稳定同位素分析 (CSIA-AA) 已成为一种估计消费者营养位置 (TP _CSIA )的变革性方法，这些位置在内部索引到初级生产者氮同位素基线。准确 TP _CSIA估计的核心是了解 beta ( β ) 值——消费者食物网基础的初级生产者中营养和来源 AA δ ¹⁵ N 值之间的差异。越来越多的证据表明，分类学和组织特异性β值的变异性比通常认为的更高。
2. 该荟萃分析满足了全面评估β值变异的相关来源及其对 TP _CSIA不确定性的贡献的迫切需要。我们首先合成了所有已发表的初级生产者 AA δ ¹⁵ N 数据，以调查与生态相关的变异来源（例如分类学、组织类型、栖息地类型、光合作用模式）。然后，我们回顾了支持 AA δ ¹⁵ N 和β值变异性的生物地球化学机制。最后，我们评估了 TP _CSIA估计对平均β _Glx-Phe值和 Glx-Phe 营养鉴别因子（TDF _Glx-Phe）不确定性的敏感性。
3. 我们表明β _Glx-Phe值的变化比以前考虑的要大两倍，血管化程度，而不是栖息地类型（陆地与水生），提供了最大的变异来源（血管自养生物 = -6.6 ± 3.4‰；非-血管自养=+3.3±1.8‰）。在维管植物中，组织类型对β _Glx-Phe值的变异性有次要作用，但我们发现 C ₃、C ₄和 CAM 植物β _Glx-Phe值之间没有明显区别。值得注意的是，我们发现维管植物β _Glx-Lys值 (+2.5 ± 1.6‰) 的变化远小于β _Glx-Phe值，使 Lys 成为陆地系统中初级生产源的有用 AA 示踪剂。我们的多营养级敏感性分析表明，TP _CSIA估计对β _Glx-Phe和 TDF _Glx-Phe值的变化高度敏感，但β值的相对影响在更高的营养级上消散。
4. 我们的结果强调初级生产者β值对于准确的营养位置估计是不可或缺的。我们概述了识别、约束和解释β值可变性以提高 TP _CSIA估计准确性和精度的四项关键建议。我们必须最终扩展初级生产者 AA δ ¹⁵ N 值的库，以更好地了解β值变化的机制驱动因素。