Plant Roots for Sustainable Soil Structure Management in Cropping Systems

Soil structure is dependent on a dynamic interplay between plants, microbes, and primary soil constituents (e.g. particle size distribution, organic matter), shaping the physical environment for crop growth. Degradation of soil structure is also one of the key reasons for the exacerbating threats from erosion and loss of fertility in arable soils. The structure comprises the solid phase of soil aggregates as well as the water/gas phase of the secondary pore system between aggregates. In most soils, both structural components follow a hierarchical architecture covering 103 (aggregates) to 104 (pores) orders of magnitude. Distinct organic- and inorganic-binding agents drive the formation and stabilization of soil structure. In this, plant roots are the main factor relevant to all size classes, involving gluing of mineral particles by rhizodeposition (microaggregation), aggregation around decomposing particulate root debris (macroaggregation), enmeshment by living roots (macroaggregate stabilization) as well as indirect (microaggregate) effects via rhizosphere microbiota. Roots also modify the pore-size distribution and pore geometry, thereby changing soil hydraulic properties, particularly in the near-saturated macropore range. The type and extent of root structural impacts differ with the rooting type (taproot vs. fibrous), time (living vs. decomposed), environmental conditions (e.g. texture, moisture, temperature), and plant family (e.g. rhizosheath formation, microbial associations). Due to its dynamic and biological nature, mechanical soil loosening by different tillage intensity is insufficient to manage soil structure. Successful root-mediated structure-improving measures include biopore formation by tap-rooted species, which provides better access to subsoil resources, and aggregate enmeshment by fibrous root systems for mitigating the risk of soil erosion. High crop diversity in the rotation combines species-specific root effects to influence the individual mechanisms underlying a stable, crumby aggregate structure, and a multi-functional pore system. Rotations with a minimum duration of bare fallow periods and integration of cover crop mixtures contribute to optimizing structural soil properties. Advances in root ecology, with novel methods of biochemical characterization of root/rhizosphere carbon inputs, root-zone imaging, and pore-scale modelling, will contribute the mechanistic knowledge on pathways of root-induced soil structure dynamics to facilitate more targeted design and decision support of management measures for soil aggregation and porosity in sustainable cropping system intensification.