Soil processes and func tions in critical zone observatories: Hypotheses and experimental design

European Union policy on soil threats and soil protection has prioritized new research to address global soil threats. This research draws on the methodology of Critical Zone Observatories (CZOs) to focus a critical mass of international, multidisciplinary expertise at specific field sites. These CZOs were selected as part of an experimental design to study soil processes and ecosystem function along a hypothesized soil life cycle-from incipient soil formation where new parent material is being deposited, to highly degraded soils that have experienced millennia of intensive land use. Further CZOs have been selected to broaden the range of soil environments and data sets to test soil process models that represent the stages of the soil life cycle. The scientific methodology for this research focuses on the central role of soil structure and soil aggregate formation and stability in soil processes. Research methods include detailed analysis and mathematical modeling of soil properties related to aggregate formation and their relation to key processes of reactive transport, nutrient transformation, and C and food web dynamics in soil ecosystems. Within this program of research, quantification of soil processes across an international network of CZOs is focused on understanding soil ecosystem services including their quantitative monetary valuation within the soil life cycle. Further experimental design at the global scale is enabled by this type of international CZO network. One example is a proposed experiment to study soil ecosystem services along planetary-scale environmental gradients. This would allow scientists to gain insight into the responses of soil processes to increasing human pressures on Earth's critical zone that arise through rapidly changing land use and climate.