Measuring and monitoring soil carbon.

Abstract

Soils are the largest terrestrial reservoir of organic carbon, yet great uncertainty remains in estimates of soil organic carbon (SOC) at global, continental, regional and local scales. Compared with biomass carbon, changes in SOC associated with changes in land use and management, or climate change, must be monitored over longer periods. The changes are small relative to the very large stocks present in the soil, as is their inherent variability. This requires sensitive measurement techniques and due consideration for the minimum detectable difference (MDD). Relationships between environmental and management factors and SOC dynamics can be established using experimental field trials, chronosequence studies and monitoring networks. Soil monitoring networks (SMNs), for example, can provide information on direct changes of SOC stocks through repeated measurements at a given site, as well as data to parameterize and test biophysical models at plot scale. Further, they can provide a set of point observations that represent the (mapped) variation in climate/soil/land use and management at national scale, allowing for upscaling. SMNs must be designed to detect changes in soil properties over relevant spatial and temporal scales, with adequate precision and statistical power. Most SMNs, however, are in the planning or early stages of implementation; few networks are located in developing countries, where most deforestation and land-use change is occurring. Within these monitoring networks, sites may be organized according to different sampling schemes, for example regular grid, stratified approach or randomized; different statistical methods should be associated with each of these sampling designs. Overall, there is a need for globally consistent protocols and tools to measure, monitor and model SOC and greenhouse gas emission changes to allow funding agencies and other organizations to assess uniformly the possible effects of the impacts of land-use interventions, and the associated uncertainties, across the range of world climate, soils and land uses.