Soil organic carbon variability in croplands: Implications for sampling design

The variation of soil organic carbon (SOC) concentration is important not only for indicating the uncertainty of SOC stock at spatial scales, but also for calculating the minimum sample size to detect SOC concentration change temporally. The coefficient of variation (CV), an index often used to express the variation of SOC concentration, is affected by multiple sampling factors. Yet, minimization of the SOC concentration CV remains elusive. A total of 117 observations of the CV for SOC concentration in croplands were collected from 41 published studies. Pearson correlation analysis was used to determine the effect of three sampling factors, that is, sample size, sampling area, and sampling interval, on SOC concentration variation. Results showed that the SOC concentration of croplands was mostly low (CV, <15%) to moderately (CV, 15%-35%) variable. Significant linear relationships existed between the CV and sample size, and between the CV and log (sampling area), suggesting that stratified sampling may reduce SOC variation. A significant linear relationship also existed between the CV and log (sampling interval), indicating that soil spatial variability contributed to the high CV of SOC concentration. A maximum of 2 km of sampling interval was recommended for SOC stock investigations in croplands. These findings provide some guidance for optimum sampling design on SOC dynamics research.