Rhizosheath: Distinct features and environmental functions

Interactions between plant roots and the surrounding soils are crucial for plant growth and soil health. The rhizosheath, which is the sheath-like soil that is tightly attached to the plant root surface, is increasingly acknowledged to exhibit distinct features and critical environmental functions for soil, plants, and microbes. However, the differences between the terms "rhizosheath" and "rhizosphere soil" remain ambiguous across the literature, and not all features and functions of rhizosheaths have reached a consensus. Here, we summarize the terminology, methodology, formation mechanisms, and findings of the features and functions of rhizosheaths via an integrated and quantitative approach. First, we propose a comprehensive framework for the terminology of different rhizosphere compartments, which allows for consistent definitions and improved comparability across studies. We also summarize the current knowledge of the mechanisms underlying rhizosheath formation. Moreover, by collecting evidence from the literature, we highlight the distinct physical, chemical, and microbial features of rhizosheath soils, such as 66% higher water content, 34% higher soil organic carbon, 56% higher water-extractable organic carbon, and 71% higher microbial biomass nitrogen in rhizosheath soils than those in other rhizosphere counterparts. We further discuss how these features lead to the environmental and biogeochemical functions of rhizosheaths, including sand fixation, soil aggregation, water and nutrient uptake, carbon dynamics, and microbial colonization. We highlight the significance of these functions from micro- to ecosystem-scale. Finally, we propose future research perspectives to better understand and harness the environmental benefits of rhizosheaths.