Structural differences between bulk and rhizosphere soil

Abstract

The physical characteristics of the soil at the root–soil interface are crucial because they determine both physical aspects of root function such as water and nutrient uptake and the microbial activity that is most relevant to root growth. Because of this we have studied how root activity modifies the structure and water retention characteristic of soil adjacent to the root for maize, wheat and barley. These plants were grown in pots for a 6-week growth period, then the soil adjacent to the root (rhizosphere soil) and bulk soil aggregates were harvested. These soil aggregates were then saturated and equilibrated at matric potentials between ~600 kPa and saturation, and the water retention characteristics were measured. From subsamples of these aggregates, thin sections were made and the porosity and pore-size distributions were studied with image analysis. Both image analysis and estimates of aggregated density showed that the rhizosphere soil and bulk soil had similar porosities. Growing different plants had a small but significant effect on the porosity of the soil aggregates. Image analysis showed that for all the plant species the structure of the rhizosphere soil was different to that of the bulk soil. The rhizosphere soil contained more larger pores. For maize and barley, water retention characteristics indicated that the rhizosphere soil tended to be drier at a given matric potential than bulk soil. This effect was particularly marked at greater matric potentials. The difference between the water retention characteristics of the bulk and rhizosphere soil for wheat was small. We compare the water retention characteristics with the data on pore-size distribution from image analysis. We suggest that differences in wetting angle and pore connectivity might partly explain the differences in water retention characteristic that we observed. The impact of differences between the water retention properties of the rhizosphere and bulk soil is discussed in terms of the likely impact on root growth

Do roots mind the gap?

“…We refer to several recent reviews on the specific physical, chemical, biological properties of the rhizosphere (Gregory 2006;Hinsinger et al 2009); to measurements of water content (Young 1995), structure (Whalley et al 2005), wettability (Read et al 2003), and mechanical stability (Czarnes et al 2000) of the rhizosphere compared to bulk soil; to in-situ infiltration in the rhizosphere (Hallet et al 2003); and to recent observations of unexpected water dynamics in the rhizosphere (Carminati et al 2010). These studies show several controversies, for example in whether the rhizosphere has increased or decreased water content and conductivity compared to the bulk soil.…”

Characterising the morphological properties and surface composition of radium contaminated particles: a means of interpreting origin and deposition

“…Several of the shape parameters have been previously used to determine aggregate characteristics in soils 22 . 125…”

High clay contents, dense soils, and spatial variability are the principal subsoil constraints to cropping the higher rainfall land in south-eastern Australia

“…While the volume occupied by roots in such soils may be small, the physical environment is quite different to that measured in the bulk soil. Whalley et al (2005) demonstrated that water retention and aeration properties of the rhizosphere soil are enhanced with respect to root growth compared to the properties of the bulk soil. Passioura (2002) describes the many interactions between root and soil that demonstrate the ability of plants to adapt to and survive in stressed situations.…”