Soil quality in Life Cycle Assessment: Towards development of an indicator

Soils are an essential resource in both managed and natural systems, and maintaining soil quality is critical to the development of sustainable development of human activities, in particular agriculture. The assessment of impacts of human activities on soil quality needs a universal practical approach that can be incorporated in global environmental assessments. But so far, no synthetic indicators have been proposed that would combine the main soil characteristics in a simple formula valid for all types of soils and climates. The existence of numerous interrelated soil characteristics and their complex interactions with agricultural practices has postponed development of a universal soil-quality impact indicator in environmental assessment, especially in Life Cycle Assessment (LCA). LCA is an internationally normalized method widely used to evaluate potential impacts of products. The inability to represent impacts on soil quality remains one of the unresolved problems in LCA because of soilʹs spatial and temporal variability and the complex interactions between soil properties. It is crucial to consider soil quality in the environmental assessment of products, especially those with a majority of their life cycle in biological processes (such as agriculture and forestry). After presenting a general overview of the soil quality concept, we review the existing LCA approaches that consider soil quality in their inventory and impact-assessment phases, and outline the challenges to refining soil quality impacts in LCA. These challenges include estimation of complex soil quality impacts that depend on fluctuating soil characteristics, consideration of both on-site and off-site agricultural soils, and aggregation of impacts both spatially and temporally. The inventory items describing soil quality can be soil properties, functions or processes (e.g., erosion, compaction). Their quantification implies that they be (1) relevant for calculating impacts on soil quality, (2) related to one or more functional units, (3) as accurate as possible and (4) calculated with available data at a global scale. Inventory items based on processes fulfill these four needs and can be considered as midpoint indicators in LCA. Furthermore, indicators based on processes allow an incremental improvement of the method through the inclusion of new impacts. Efforts first must focus on developing robust impact indicators for individual soil processes before attempting to aggregate them into a single indicator. Nonetheless, several aggregation approaches are presented. The appropriate level of complexity and comprehensiveness for assessing impacts on soil quality should result from a compromise between oversimplified and overcomplicated descriptions of the multiple functions and properties of soils.