The life of soils: Integrating the who and how of multifunctionality

Capturing the complexity of soil life for soil quality assessments is one of the most challenging paradoxes of contemporary soil science. Soil biota perform a plethora of processes that are fundamental to soil quality. As the concept of soil quality developed, so have the attempts to integrate soil biological measurements into monitoring schemes from field to regional scale. To date, however, soil science has not yet succeeded to provide flexible yet objective biological indicator methods to assess soil multifunctionality, customised to the user's context. We present an integrative framework and elucidate the who and how of soil multifunctionality. The framework encompasses the current scientific understanding of the role of soil biota in supporting the many soil processes that underly soil quality. We specified these relationships for four soil functions (Carbon and Climate Regulation, Water Regulation and Purification, Nutrient Cycling, and Disease and Pest Regulation). We identify challenges often encountered in soil quality assessment and monitoring schemes and discuss how the framework can be applied to provide a flexible selection tool. Soil quality assessments are conducted in different contexts. As assessment objectives range from mechanistic understanding, to functional land management and large spatial scale monitoring so will the practical and logistical constraints for method selection vary. Biological assessments need to move beyond the quest for a one-size-fits-all minimum dataset, and adopt a more nuanced selection approach founded in soil biology. We stress that biological attributes should not be considered in isolation but alongside soil chemical and physical attributes, as well as management and environmental contextualisation. The presented framework offers a structure to further quantify, understand and communicate the who and how of soil biology in defining multifunctionality.