The single-layer equivalent soil mass method for the evaluation of soil organic carbon stocks: Sources of errors, simplification, and associated detectable change

It has been well established that for consistent soil organic carbon stock (SOCS) monitoring over time, the SOCS must be determined in an equivalent soil mass (ESM) layer rather than a depth layer. This work focused on the single-layer ESM method, its cost, accuracy and simplification. With the objective of simplifying ESM method and reducing costs, we sampled 393 fields in western Switzerland and experimentally questioned the sampling methods, the sources of variance and the minimum detectable change (MDC) of SOCS, and possible simplification. The layer mass was accurately determined using gouge-augers, thus overcoming a major drawback in performing ESM SOCS evaluation. The relationships between number of aliquots and estimated SOCS variance was determined. Sampling at fixed depth and layer bulk density resulted in unacceptable errors and MDCs exceeding several decades when converted to years before a change was detectable. We introduced a new procedure to fully consider the coarse fraction volume in the layer, which remains a major source of error on the SOCS when the coarse fraction volume exceeds 10 % of the layer volume, while not impacting the detectability of the change. The single-layer ESM method provided an MDC corresponding to less than 10 years before SOCS change detectability under the average regional conditions, and mass correction results in negligible increase in MDC compared to that of the 0–30 cm layer. Simplifying this method by using the average soil organic carbon content of the layer mass correction only slightly increased the MDC, thus providing an opportunity to decrease the cost of SOCS monitoring significantly.