As interest in the voluntary soil carbon market surges, carbon registries have been developing new soil carbon measurement, reporting, and verification (MRV) protocols. These protocols are inconsistent in their approaches to measuring soil organic carbon (SOC). Two areas of concern include the type of SOC stock accounting method (fixed-depth (FD) vs. equivalent soil mass (ESM)) and sampling depth requirement. Despite evidence that fixed-depth measurements can result in error because of changes in soil bulk density and that sampling to 30 cm neglects a significant portion of the soil profile’s SOC stock, most MRV protocols do not specify which sampling method to use and only require sampling to 30 cm. Using data from UC Davis’s Century Experiment (“Century”) and UW Madison’s Wisconsin Integrated Cropping Systems Trial (WICST), we quantify differences in SOC stock changes estimated by FD and ESM over 20 years, investigate how sampling at-depth (> 30 cm) affects SOC stock change estimates, and estimate how crediting outcomes taking an empirical sampling-only crediting approach differ when stocks are calculated using ESM or FD at different depths. We find that FD and ESM estimates of stock change can differ by over 100 percent and that, as expected, much of this difference is associated with changes in bulk density in surface soils (e.g., r = 0.90 for Century maize treatments). This led to substantial differences in crediting outcomes between ESM and FD-based stocks, although many treatments did not receive credits due to declines in SOC stocks over time. While increased variability of soils at depth makes it challenging to accurately quantify stocks across the profile, sampling to 60 cm can capture changes in bulk density, potential SOC redistribution, and a larger proportion of the overall SOC stock. ESM accounting and sampling to 60 cm (using multiple depth increments) should be considered best practice when quantifying change in SOC stocks in annual, row crop agroecosystems. For carbon markets, the cost of achieving an accurate estimate of SOC stocks that reflect management impacts on soils at-depth should be reflected in the price of carbon credits.

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核算方法和采样深度对于估算土壤碳储量变化的重要性

随着人们对自愿土壤碳市场的兴趣激增，碳登记机构一直在开发新的土壤碳测量、报告和验证（MRV）协议。这些协议测量土壤有机碳（SOC）的方法不一致。值得关注的两个领域包括 SOC 库核算方法的类型（固定深度 (FD) 与等效土体质量 (ESM)）和采样深度要求。尽管有证据表明，由于土壤容重的变化，固定深度测量可能会导致误差，并且采样到 30 厘米忽略了土壤剖面 SOC 储量的很大一部分，但大多数 MRV 协议没有指定使用哪种采样方法，只要求采样至 30 厘米。使用加州大学戴维斯分校的世纪实验（“Century”）和威斯康星大学麦迪逊分校的威斯康星州综合种植系统试验（WICST）的数据，我们量化了 20 年来 FD 和 ESM 估计的 SOC 库变化的差异，研究了如何在深度（> 30 cm）采样) 影响 SOC 库变化估计，并估计当使用不同深度的 ESM 或 FD 计算库时，采用仅经验抽样计入方法的计入结果有何不同。我们发现，FD 和 ESM 对库变化的估计可能相差超过 100%，而且正如预期的那样，这种差异大部分与表层土壤容重的变化有关（例如，世纪玉米处理的 r = 0.90）。这导致 ESM 和基于 FD 的股票之间的信用结果存在显着差异，尽管许多治疗方法由于 SOC 库存随着时间的推移而下降而没有获得信用。虽然土壤深度变异性的增加使得准确量化整个剖面的储量变得具有挑战性，但采样至 60 厘米可以捕获容重、潜在 SOC 重新分布以及总体 SOC 储量的较大比例的变化。在量化一年生中耕作物农业生态系统中 SOC 储量的变化时，应将 ESM 核算和采样至 60 厘米（使用多个深度增量）视为最佳实践。对于碳市场而言，准确估算 SOC 储量（反映管理对土壤的影响）的成本应反映在碳信用额的价格中。