The role of plant species and soil condition in the structural development of the rhizosphere.

Jon R Helliwell,Sacha J Mooney,W Richard Whalley,Anthony J Miller,Craig J Sturrock

doi:10.1111/pce.13529

Abstract

Roots naturally exert axial and radial pressures during growth, which alter the structural arrangement of soil at the root–soil interface. However, empirical models suggest soil densification, which can have negative impacts on water and nutrient uptake, occurs at the immediate root surface with decreasing distance from the root. Here, we spatially map structural gradients in the soil surrounding roots using non‐invasive imaging, to ascertain the role of root growth in early stage formation of soil structure. X‐ray computed tomography provided a means not only to visualize a root system in situ and in 3‐D but also to assess the precise root‐induced alterations to soil structure close to, and at selected distances away from the root–soil interface. We spatially quantified the changes in soil structure generated by three common but contrasting plant species (pea, tomato, and wheat) under different soil texture and compaction treatments. Across the three plant types, significant increases in porosity at the immediate root surface were found in both clay loam and loamy sand soils and not soil densification, the currently assumed norm. Densification of the soil was recorded, at some distance away from the root, dependent on soil texture and plant type. There was a significant soil texture × bulk density × plant species interaction for the root convex hull, a measure of the extent to which root systems explore the soil, which suggested pea and wheat grew better in the clay soil when at a high bulk density, compared with tomato, which preferred lower bulk density soils. These results, only revealed by high resolution non‐destructive imagery, show that although the root penetration mechanisms can lead to soil densification (which could have a negative impact on growth), the immediate root–soil interface is actually a zone of high porosity, which is very important for several key rhizosphere processes occurring at this scale including water and nutrient uptake and gaseous diffusion.

Highlights

IntroductionThe dynamic nature of the rhizosphere (the zone of soil surrounding a growing root which is influenced by it) provides a niche environment which exhibits biophysical and chemical gradients very different to those found away from the soil immediately influenced by the root, referred to as the bulk soil
The dynamic nature of the rhizosphere provides a niche environment which exhibits biophysical and chemical gradients very different to those found away from the soil immediately influenced by the root, referred to as the bulk soil
At the same bulk density in the clay loam, there was no significant change in porosity from the bulk soil value for the tomato treatment (Figure 1d), but significant reductions in porosity of 7.5 and 9.5 % compared to the bulk soil value to 23.6 and 23.1 % in the wheat and pea species respectively (Figure 1b, f; P

Summary

Introduction

The dynamic nature of the rhizosphere (the zone of soil surrounding a growing root which is influenced by it) provides a niche environment which exhibits biophysical and chemical gradients very different to those found away from the soil immediately influenced by the root, referred to as the bulk soil. These gradients control root activity through a combination of root-derived exudations and physical structural alterations, influencing water and nutrient uptake, gaseous exchange, particle rearrangement and wettability at the immediate root surface. Around an actively growing root, have been largely limited to theoretical models

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