The cost-efficiency and reliability of two methods for soil organic C accounting

Sequestering organic carbon (C) in soil can help to combat land degradation, improve food security, and mitigate greenhouse gas emissions and climate change. But we need reliable, cost-efficient methods to assess, monitor, and verify the change. Here, we compared two methods for the direct measurement of soil organic C stocks and for monitoring the change. Our aims were to quantify the soil organic C stock in two carbon estimation areas, under cropping and grazing, using composite sampling with two designs and proximal sensing. We compared the two schemes in terms of the (a) accuracy of the estimated C stocks, the total cost, and the cost-efficiency, calculated as the ratio of the accuracy of the estimate and the total cost, and (b) uncertainty of the estimated standard error of the estimated C stocks. We found that compositing was cheaper but more inaccurate than sensing. Sensing was 1.2 to 2.1 times more cost-efficient than compositing. We also found that the uncertainty of the estimated standard errors from compositing was large and unreliable, which can hinder the quantification of a minimum detectable difference in organic C stocks. We show that the sensor-derived spatially explicit data can also be used to map the C stocks, which can help to optimise the sampling design in subsequent monitoring rounds. Our findings have important implications for the development of C measurement and monitoring methodologies. Visible-near infrared and gamma attenuation sensing can accurately, cost-efficiently, and reliably monitor and verify changes in soil C stocks.