Long-term experiments (LTEs) have provided data to modellers and agronomists to investigate changes and dynamics of soil organic carbon (SOC) under different cropping systems. As treatment changes have occurred due to agricultural advancements, so too have analytical soil methods. This may lead to method bias over time, which could affect the robust interpretation of data and conclusions drawn. This study aims to quantify differences in SOC due to changes in dry combustion methods over time, using soil samples of a LTE established in 1963 that focuses on mineral and organic fertilizer management in the temperate zone of Northeast Germany. For this purpose, 1059 soil samples, collected between 1976 and 2008, have been analysed twice, once with their historical laboratory method right after sampling, and a second time in 2016 when all samples were analysed using the same elementary analyser. In 9 of 11 soil sampling campaigns, a paired t-test provided evidence for significant differences in the historical SOC values when compared with the re-analysed concentrations of the same LTE sample. In the sampling years 1988 and 2004, the historical analysis obtained about 0.9 g kg⁻¹ lower SOC compared with the re-analysed one. For 1990 and 1998, this difference was about 0.4 g kg⁻¹. Correction factors, an approach often used to correct for different analytical techniques, could only be applied for 5 of 11 sampling campaigns to account for constant and proportional systematic method error. For this particular LTE, the interpretation of SOC changes due to agronomic management (here fertilization) deviates depending on the analytical method used, which may weaken the explanatory power of the historical data. We demonstrate that analytical method changes over time present one of many challenges in the interpretation of time series data of SOC dynamics. Therefore, LTE site managers need to ensure providing all necessary protocols and data in order to retrace method changes and if necessary recalculate SOC.

中文翻译：

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长期试验土壤有机碳分析方法变化的困境

长期实验 (LTE) 已为建模人员和农学家提供数据，以研究不同种植制度下土壤有机碳 (SOC) 的变化和动态。随着农业的进步，处理方式发生了变化，土壤分析方法也发生了变化。随着时间的推移，这可能会导致方法偏差，从而影响对数据和结论的可靠解释。本研究旨在使用 1963 年建立的专注于德国东北部温带矿物和有机肥料管理的 LTE 的土壤样本，量化由于干式燃烧方法随时间的变化而导致的 SOC 差异。为此，对 1976 年至 2008 年间采集的 1059 份土壤样本进行了两次分析，一次是在采样后立即使用历史实验室方法，第二次是在 2016 年，当时所有样品都使用相同的元素分析仪进行了分析。在 11 次土壤采样活动中的 9 次中，一对与同一 LTE 样本的重新分析浓度相比，t检验提供了历史 SOC 值显着差异的证据。在 1988 年和 2004 年的采样年中，历史分析获得的SOC 比重新分析的低约 0.9 g kg ^{-1 。}对于 1990 年和 1998 年，这个差异约为 0.4 g kg ⁻¹. 校正因子是一种经常用于校正不同分析技术的方法，只能应用于 11 次抽样活动中的 5 次，以说明恒定和成比例的系统方法误差。对于这个特定的 LTE，由于农艺管理（此处施肥）导致的 SOC 变化的解释因所使用的分析方法而异，这可能会削弱历史数据的解释力。我们证明，分析方法随时间的变化是解释 SOC 动力学时间序列数据的众多挑战之一。因此，LTE 站点管理员需要确保提供所有必要的协议和数据，以便追溯方法更改，并在必要时重新计算 SOC。