Abstract. To build confidence in the efficacy of soil carbon (C) crediting programs, precise quantification of soil organic carbon C (SOC) is critical. Detecting a true change in SOC after a management shift has occurred, specifically in agricultural lands, is difficult as it requires robust soil sampling and soil processing procedures. Informative and meaningful comparisons across spatial and temporal time scales can only be made with reliable soil C measurements and estimates, which begin on the ground and in soil testing facilities. To gauge soil C measurement inter-variability, we conducted a blind external service laboratory comparison across eight laboratories selected based on status and involvement in SOC quantification for C markets. To better understand how soil processing procedures and quantification methods commonly used in soil testing laboratories affect soil C concentration measurements, we designed an internal experiment assessing the individual effect of several alternative procedures (i.e., sieving, fine grinding, and drying) and quantification methods on total (TC), inorganic (SIC), and organic (SOC) soil C concentration estimates. We analyzed 12 different agricultural soils using 11 procedures that varied either in the sieving, fine grinding, drying, or quantification step. We found that a mechanical grinder, the most commonly used method for sieving in service laboratories, did not effectively remove coarse materials (i.e., roots and rocks), thus resulted in higher variability and significantly different C concentration measurements from the other sieving procedures (i.e., 8 + 2 mm, 4 mm, and 2 mm with rolling pin). A finer grind generally resulted in a lower coefficient of variance where the finest grind to < 125 µm had the lowest coefficient of variance, followed by the < 250 µm grind, and lastly the < 2000 µm grind. Not drying soils in an oven (at 105 °C) prior to elemental analysis on average resulted in a relative difference of 3.5 % lower TC, and 5 % lower SOC due to inadequate removal of moisture. Compared to the reference method used in our study where % TC was quantified by dry combustion on an elemental analyzer, % SIC was measured using a pressure transducer, and % SOC was calculated by the difference of % TC and % SIC, predictions of all three soil properties (% TC, % SIC, % SOC) using Fourier Transformed Infrared Spectroscopy were in high agreement (R² = 0.97, 0.99, 0.90, respectively). For % SOC, quantification by loss on ignition had a low coefficient of variance (5.42 ± 3.06 %) but the least agreement (R² = 0.83) with the reference method.

中文翻译：

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由于样品处理，常见土壤碳测量存在较大误差


摘要。为了建立对土壤碳 (C) 计入计划有效性的信心，土壤有机碳 (SOC) 的精确量化至关重要。在管理转变发生后，检测 SOC 的真正变化非常困难，特别是在农业用地中，因为它需要强大的土壤采样和土壤处理程序。只有通过可靠的土壤碳测量和估算，才能在空间和时间时间尺度上进行信息丰富且有意义的比较，这些测量和估算从地面和土壤测试设施开始。为了衡量土壤 C 测量的相互变异性，我们根据 C 市场 SOC 量化的状况和参与情况选择了八个实验室，对这些实验室进行了盲法外部服务实验室比较。为了更好地了解土壤测试实验室常用的土壤处理程序和量化方法如何影响土壤碳浓度测量，我们设计了一项内部实验，评估几种替代程序（即筛分、细磨和干燥）和量化方法对土壤碳浓度测量的单独影响。总 (TC)、无机 (SIC) 和有机 (SOC) 土壤碳浓度估算。我们使用 11 种不同的程序（筛分、细磨、干燥或定量步骤）分析了 12 种不同的农业土壤。我们发现，机械研磨机（服务实验室中最常用的筛分方法）无法有效去除粗物质（即根和岩石），因此导致变异性较高，并且 C 浓度测量值与其他筛分程序（即碳浓度）显着不同。 、8 + 2 毫米、4 毫米和 2 毫米（带擀面杖）。 更精细的研磨通常会导致更低的变异系数，其中最细研磨至 < 125 µm 的变异系数最低，其次是 < 250 µm 研磨，最后是 < 2000 µm 研磨。由于水分去除不充分，在元素分析之前未在烘箱（105 °C）中干燥土壤平均会导致 TC 降低 3.5%，SOC 降低 5%。与我们研究中使用的参考方法相比，% TC 通过元素分析仪上的干燃烧进行量化，% SIC 使用压力传感器测量，% SOC 通过 % TC 和 % SIC 的差异计算，这三个方法的预测使用傅里叶变换红外光谱法测得的土壤特性（% TC、% SIC、% SOC）高度一致（R ² 分别 = 0.97、0.99、0.90）。对于 % SOC，通过烧失量进行量化的方差系数较低 (5.42 ± 3.06 %)，但与参考方法的一致性最低 (R ² = 0.83)。