Integrating Soil Structure and Soil Function: Tales of Aggregates, Pores, Biota, and Organic Matter.

Soil structure (the arrangement of solids and pores in soil) exerts significant controls on the soil physical and chemical environment.  Further, it provides the habitat for plant roots and soil biota, thereby affecting their growth, function, turnover, and diversity.  At the same time, soil biota (including roots) engineer and modify their own environment, largely through their interactions with soil minerals and organic matter in a self-organized feedback system.  Their activities can alter the architecture of soil particles and pores and the stability of secondary aggregated structures, which further affects aeration, water and resource availability, and the cycling of soil organic matter.  The basic interactions and dependencies of this feedback system and its importance to life on earth have been intuitively recognized for a long time.  However, progress on identifying the controlling mechanisms and quantifying their impacts on integrated system processes has been limited by the complexity of the interactions and our ability to observe and measure them across multiple scales of organization.  I will discuss how the conceptualization of the porosity exclusion principle and the mechanisms promoting aggregate hierarchy some 25 years ago stimulated a new generation of research that has taken us on a path leading from a focus on understanding aggregate formation and stabilization mechanisms, to studies of the role of aggregates in the protection of organic matter from decomposition, to investigations of soil structural controls on the diversity and function of soil organisms.  With each successive step in understanding and with the advent of new analytical tools, significant progress has occurred, yet many questions remain.  Holistic understanding of this feedback system and predictions of its impacts on soil function will require the integrated efforts of multiple disciplines and their measurement capabilities.