Designing a national soil carbon monitoring network to support climate change policy: a case example for US agricultural lands

Soils contain the largest terrestrial pool of carbon, and have large annual transfers of carbon with biomass pools and the atmosphere. Agricultural land use and management, and changes in climate have significant impacts on soil carbon, and if managed with conservation practices agricultural soils could be enhanced while sequestering carbon and mitigating greenhouse gas emissions. To better inform national climate change policy decisions for agricultural lands, robust and accurate estimates of soil organic carbon (SOC) stock changes are needed at regional to national scales. The design of a national soil monitoring network for carbon on agricultural lands is discussed including determination of sample size, allocation, and site-scale plot design. A quantitative case study is presented using modeled estimates of SOC stock change variability and a set of soil sample measurements to evaluate a potential network design for U.S. agricultural lands. Stratification by climate, soil, and land use with sites allocated based on modeled SOC stock change variability could effectively reduce the national standard error of SOC stock change. Sampling additional sites rather than multiple cores per site is recommended. Soil carbon monitoring can support climate change policy and reduce uncertainties in SOC stock changes for national greenhouse gas inventories.