Biophysical Framework for Modeling Soil Aggregation.

Soil aggregation is a concept coined to define the spatial arrangement of soil constituents and the degree with which they are bound together. It also refers to the formation of a complex network of pores, which span wide range of sizes and connectivity. Soil aggregation is heuristically linked with a myriad of ecological and agricultural functions of soils—including resistance to erosion, transport of fluids and heat, microbial and root habitats, biogeochemical cycling, as well as feedbacks among these functions. It is one of the most sensitive soil qualities that readily responds to disturbances such as cultivation, fire, drought, flooding, and changes in vegetation. These changes are commonly quantified and incorporated in soil models indirectly as alterations in carbon content and type, bulk density, aeration, permeability, as well as water retention characteristics. However, soil aggregation as the key unifying mechanism remains poorly quantified and is rarely included in predictive soil models. Here we provide a biophysical framework for quantitative and predictive modeling of soil aggregation and its attendant soil characteristics. The framework treats aggregates as hotspots of biological, chemical and physical processes centered around roots and root residue. We keep track of the life cycle of an individual aggregate from it genesis in the rhizosphere, fueled by rhizodeposition and mediated by vigorous microbial activity, until its disappearance when the root-derived resources are depleted. The framework synthesizes current understanding of microbial life in porous media; water holding and soil binding capacity of biopolymers; and environmental controls on soil organic matter dynamics. The framework paves a way for integration of processes that are presently modeled as disparate or poorly coupled processes, including storage and protection of carbon, microbial activity, greenhouse gas fluxes, movement and storage of water, resistance of soils against erosion.