To meet the goals of the Climate Agreement, policy makers consider incentivizing soil carbon sequestration by carbon credits to offset greenhouse gas emissions from agriculture, industry and other sectors. Therefore, the interest in monitoring soil organic matter dynamics is growing rapidly, yet factors affecting the precision of the monitoring results are rarely quantified. We used the rhetorical scheme “The seven circumstances” to structure the methodological aspects of setting up a soil organic matter (SOM) monitoring program. The rhetorical scheme was applied during four years on two grass-based dairy farms in the Netherlands to assess in detail: conversion of SOM weight concentrations to SOM stocks (HOW), the effect of soil depth and the integration of scales field vs. farm (WHERE), and the effect of sampling date and sampling frequency on SOM estimates (WHEN). We found that all three circumstances affect conclusions on SOM stocks or SOM dynamics strongly. Considerable variation was found in the relationship between soil bulk density and SOM weight concentration (i.e. pedotransfer curves) among fields, depth and literature reference equations. Therefore, preferably a site specific pedotransfer curve should be used when comparing SOM stocks based on SOM weight concentrations across sites. Large differences in trends of SOM stock changes over time were found between fields and sampling depths. We conclude that a sampling depth in grassland soils up to 60 cm may be relevant to capture the dynamics in deeper layers. Furthermore, for quantitative underpinning of carbon payment schemes, the whole farm should be monitored rather than a few fields as trends between fields are highly variable.

为实现气候协议的目标，政策制定者考虑通过碳信用激励土壤固碳，以抵消农业、工业和其他部门的温室气体排放。因此，监测土壤有机质动态的兴趣正在迅速增长，但影响监测结果精度的因素很少被量化。我们使用修辞方案“七种情况”来构建建立土壤有机质 (SOM) 监测计划的方法学方面。该修辞方案在荷兰的两个草地奶牛场应用了四年，以详细评估：将 SOM 重量浓度转化为 SOM 存量（HOW）、土壤深度的影响以及田间与农场规模的整合（在哪里），以及采样日期和采样频率对 SOM 估计值 (WHEN) 的影响。我们发现所有三种情况都会强烈影响对 SOM 股票或 SOM 动态的结论。在田间、深度和文献参考方程之间，土壤容重和 SOM 重量浓度（即土壤转移曲线）之间的关系存在相当大的变化。因此，在基于站点间 SOM 重量浓度比较 SOM 存量时，最好使用站点特定的 pedotransfer 曲线。在田间和采样深度之间发现 SOM 库随时间变化的趋势存在很大差异。我们得出结论，草原土壤中高达 60 厘米的采样深度可能与捕获更深层的动态有关。此外，对于碳支付计划的定量支持，