Increased uncertainty in soil carbon stock measurement with spatial scale and sampling profile depth in world grasslands: A systematic analysis

We observed that spatial scale and soil profile depth combined to explain 44% of the variability of SOC stock uncertainty, as measured by the coefficient of variation (CV). More specifically, the CV increased with the spatial scale and soil profile depth measured. However, the sampling depth effect is less certain due to lack of data at deeper depths. This uncertainty associated with SOC stock measurements has ramifications for SOC stock change detection. For example, at a fixed 0-30 cm soil profile depth, the minimum detectable change (MDC) of SOC stocks between two sampling dates (50 samples) was 14, 20 and 29% at 0.1, 10 and 10 000 km2, respectively. At a fixed spatial scale of 0.1 km2, the MDC was 12, 14 and 18% for soil profiles of 0-10, 0-30 and 0-100 cm, respectively. Finally, this study provides global estimates of the uncertainty that will be useful for planning sampling strategies for SOC stock measurement in various projects across the world and to evaluate the feasibility of SOC stock measurement for different investment levels and timescales.