Evaluation of the Potential for Soil Organic Carbon Content Monitoring With Farmers

Increasing soil organic carbon (SOC) content is crucial for soil quality and climate changemitigation. SOC monitoring is indispensable to the corresponding policies and shouldprovide results at farm scale to allow for incentives. In Switzerland, farmers performmandatory analyses of the SOC content of the 0–20 cm topsoil of every field, based ona composite sample, at least every 10 years. The corresponding results are stored in adatabase in canton of Geneva. These data may be relevant for topsoil SOC monitoring,in particular for carbon sequestration policies, provided that they show appropriatequality, which is analyzed in this study. The minimum detectable change (MDC) of pastresults calculated based on the observed SOC changes was 0.013% g g−1at cantonscale (2,700 fields). Based on extended sampling of three representative fields, differentsampling strategies were simulated to determine the best future sampling guidelines forfarmers. Collecting 20 aliquots with a gouge on the field diagonals was considered thebest sampling compromise with field MDC of∼0.1% g g−1and a sampling durationof 20 min. Compared to this procedure, former farmers’ sampling was not biased inaverage but showed a variance of 0.22% g g−1due to smaller number of aliquotsand varying sampling depths. Based on the best sampling results and assumptions onfarm-scale SOC variance or SOC differences, the MDCs at farm scale ranged from0.21 to 0.12% g g−1(5 fields) and 0.09 to 0.05% g g−1(30 fields), respectively.These MDCs are small compared to published monitoring networks MDCs and allowdetermining SOC change rates at farm scale, thus offering perspectives for inexpensiveand efficient monitoring in the frame of soil quality or climate mitigation incentives. Forthe latter, however, additional information with equivalent soil mass and deeper-layercarbon content would be necessary.