High expectations to harness agricultural soils as sinks for atmospheric carbon (C) necessitate methods for verifying changes in soil organic C (SOC) stocks and improving the accuracy of regional SOC stock estimates specific to various land‐use and management types. In this study we used a sample set included in the Finnish national soil monitoring network (n = 125) to determine the SOC stock and its vertical distribution in Finnish agricultural mineral soils using fixed depth (FD) and equivalent soil mass (ESM) methods. Further, we explored the within‐field variation in SOC stock and minimum sample size in the context of SOC sequestration. Boreal agricultural mineral soils were found to be rich in SOC. The average SOC stock ranged between 8.4 and 9.8 kg m⁻² in the top 30 cm depending on cropping system and soil type. Cultivation of perennial crops tended to enrich the C close to the soil surface in comparison to dominantly annual crops, which exhibited a less steep C gradient with depth. The commonly used van Bemmelen factor of 1.724 to convert results of loss on ignition to SOC was found to be appropriate for coarse‐textured Finnish agricultural soils. However, the results showed that in clay‐rich soils, it is critical to take into account the structural water bound in clay particles to avoid overestimation of the SOC content. The study also demonstrated that the minimum sample size required to detect the change of 0.1 kg C m⁻² at field level is typically several hundreds. Therefore empirical soil sampling is not at all cost‐effective enough for verifying the changes in SOC stocks with regard to compensation of C sequestration for farmers. Thus, alternative verification methods, such as SOC modelling in combination with field‐level data collected, for example, using remote sensing should be developed further.

中文翻译：

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估算北方农田土壤中的碳储量-方法论上的考虑

对利用农业土壤作为大气中碳（C）汇的寄予很高的期望，因此有必要采用方法来验证土壤有机碳（SOC）储量的变化，并提高针对各种土地利用和管理类型的区域SOC储量估计的准确性。在这项研究中，我们使用芬兰国家土壤监测网络（n = 125）中包含的样本集，使用固定深度（FD）和等效土壤质量（ESM）方法来确定芬兰农业矿物土壤中的SOC储量及其垂直分布。此外，我们探索了在SOC隔离的情况下SOC存量的内部变化和最小样本量。发现北方农业矿物土壤富含SOC。平均SOC存量在8.4至9.8 kg m ^-2之间在顶部30厘米处，具体取决于种植系统和土壤类型。与占主导地位的一年生作物相比，多年生作物的耕种趋向于使土壤表面附近的碳富集，而随着深度的变化，该作物的C梯度较小。常用的范贝梅伦系数1.724将着火损失的结果转换为SOC，适用于质地较粗的芬兰农业土壤。但是，结果表明，在粘土含量高的土壤中，考虑到结合在粘土颗粒中的结构水对于避免高估SOC含量至关重要。研究还表明，检测0.1 kg C m ^-2变化所需的最小样本量在现场级别通常为数百人。因此，经验性土壤采样根本不具有成本效益，不足以在补偿农民的固碳补偿方面验证SOC存量的变化。因此，应进一步开发替代验证方法，例如SOC建模与结合使用遥感收集的现场数据相结合的方法。