Developmental morphology of cover crop species exhibit contrasting behaviour to changes in soil bulk density, revealed by X-ray computed tomography

Plant roots growing through soil typically encounter considerable structural heterogeneity, and local variations in soil dry bulk density. The way the in situ architecture of root systems of different species respond to such heterogeneity is poorly understood due to challenges in visualising roots growing in soil. The objective of this study was to visualise and quantify the impact of abrupt changes in soil bulk density on the roots of three cover crop species with contrasting inherent root morphologies, viz. tillage radish (Raphanus sativus), vetch (Vicia sativa) and black oat (Avena strigosa). The species were grown in soil columns containing a two-layer compaction treatment featuring a 1.2 g cm-3 (uncompacted) zone overlaying a 1.4 g cm-3 (compacted) zone. Three-dimensional visualisations of the root architecture were generated via X-ray computed tomography, and an automated root-segmentation imaging algorithm. Three classes of behaviour were manifest as a result of roots encountering the compacted interface, directly related to the species. For radish, there was switch from a single tap-root to multiple perpendicular roots which penetrated the compacted zone, whilst for vetch primary roots were diverted more horizontally with limited lateral growth at less acute angles. Black oat roots penetrated the compacted zone with no apparent deviation. Smaller root volume, surface area and lateral growth were consistently observed in the compacted zone in comparison to the uncompacted zone across all species. The rapid transition in soil bulk density had a large effect on root morphology that differed greatly between species, with major implications for how these cover crops will modify and interact with soil structure

Shaping 3D root system architecture

Plants are sessile organisms rooted in one place. The soil resources that plants require are often distributed in a highly heterogeneous pattern. To aid foraging, plants have evolved roots whose growth and development are highly responsive to soil signals. As a result, 3D root architecture is shaped by myriad environmental signals to ensure resource capture is optimised and unfavourable environments are avoided. The first signals sensed by newly germinating seeds — gravity and light — direct root growth into the soil to aid seedling establishment. Heterogeneous soil resources, such as water, nitrogen and phosphate, also act as signals that shape 3D root growth to optimise uptake. Root architecture is also modified through biotic interactions that include soil fungi and neighbouring plants. This developmental plasticity results in a ‘custom-made’ 3D root system that is best adapted to forage for resources in each soil environment that a plant colonises

Interactions between plant roots and the structure of compacted soils

Compaction is a form of soil physical degradation. It occurs through both human-induced and natural mechanisms such as the passage of heavy farm machinery in agricultural settings as well as the impact of animal trampling and glaciation in natural ecosystems. It results in the rearrangement of soil particles, resulting in an increase in bulk density and subsequent loss of porosity and pore connectivity. These changes in the soil matrix make it difficult for plant roots to penetrate deep into the soil profile as the connected macropore network which provides the path of least resistance has been destroyed. Currently, there are limited ways to remediate the impact of soil compaction, mechanical methods that exist only provide temporary relief and often exacerbate the problem at depth. The use of plant roots as a 'biotillage' tool could offer a natural means to help remediate soils subject to compaction. The objective of this study was to visualise and quantify the root architectures of selected cover crop and wild plant species grown in a variety of compacted soil conditions. To do this three-dimensional visualisations of the root architecture were generated via X-ray Computed Tomography, using a number of different automated and semi-automated root-segmentation techniques. The impact of plant root growth on soil structure was then analysed at both rhizosphere and soil column scales, over timescales of several weeks. In high soil compaction treatments changes in pore space were localised to the rhizosphere zone up to 420 µm distance from the root surface. Across the species investigated the porosity was always greatest closest to the root surface and decreased with distance form the roots surface. This trend was also observed when plants were grown in different soil textures. Soil texture was observed to significantly affect plant growth, with notably less root and shoot development by plants grown in the clay soils in comparison to the clay loam and sand treatments. The generation of root mass was species-dependent and linked to the penetration resistance of the soil. These parameters subsequently affected average root diameters of the plants, which had a direct effect on pore space creation in the rhizosphere. Roots with different morphological traits were observed to respond contrastingly to a compacted soil layer, with taprooted species undergoing greatest architectural change in comparison to fibrous root systems. The work indicates that changes to soil structure via the growth of plant roots occurs primarily in root:soil contact zone, and is a temporally dynamic process. Tap-rooted plants appear to be particularly effective in terms of being able to grow in highly compacted soils. These findings have implications for the potential remediation of compacted soils, as they indicate that plants demonstrate an inherent ability to instigate soil structural genesis, but this differs between species

Interactions between plant roots and the structure of compacted soils

Compaction is a form of soil physical degradation. It occurs through both human-induced and natural mechanisms such as the passage of heavy farm machinery in agricultural settings as well as the impact of animal trampling and glaciation in natural ecosystems. It results in the rearrangement of soil particles, resulting in an increase in bulk density and subsequent loss of porosity and pore connectivity. These changes in the soil matrix make it difficult for plant roots to penetrate deep into the soil profile as the connected macropore network which provides the path of least resistance has been destroyed. Currently, there are limited ways to remediate the impact of soil compaction, mechanical methods that exist only provide temporary relief and often exacerbate the problem at depth. The use of plant roots as a 'biotillage' tool could offer a natural means to help remediate soils subject to compaction. The objective of this study was to visualise and quantify the root architectures of selected cover crop and wild plant species grown in a variety of compacted soil conditions. To do this three-dimensional visualisations of the root architecture were generated via X-ray Computed Tomography, using a number of different automated and semi-automated root-segmentation techniques. The impact of plant root growth on soil structure was then analysed at both rhizosphere and soil column scales, over timescales of several weeks. In high soil compaction treatments changes in pore space were localised to the rhizosphere zone up to 420 µm distance from the root surface. Across the species investigated the porosity was always greatest closest to the root surface and decreased with distance form the roots surface. This trend was also observed when plants were grown in different soil textures. Soil texture was observed to significantly affect plant growth, with notably less root and shoot development by plants grown in the clay soils in comparison to the clay loam and sand treatments. The generation of root mass was species-dependent and linked to the penetration resistance of the soil. These parameters subsequently affected average root diameters of the plants, which had a direct effect on pore space creation in the rhizosphere. Roots with different morphological traits were observed to respond contrastingly to a compacted soil layer, with taprooted species undergoing greatest architectural change in comparison to fibrous root systems. The work indicates that changes to soil structure via the growth of plant roots occurs primarily in root:soil contact zone, and is a temporally dynamic process. Tap-rooted plants appear to be particularly effective in terms of being able to grow in highly compacted soils. These findings have implications for the potential remediation of compacted soils, as they indicate that plants demonstrate an inherent ability to instigate soil structural genesis, but this differs between species

Visual representation of convex hull volumes for representative root systems, red arrows indicate change in dry bulk density of soil profile.

<p>(A) Tillage radish after 20 days growth. (B) Vetch after 20 days growth. (C) Black oat after 20 days growth. (D) Tillage radish after 58 days growth. (E) Vetch after 58 days growth. (F) Black oat after 58 days growth.</p

Location maps of roots in 2D cross-section representative slices of segmented root systems shown in Fig 1.

<p>(A-D) Tillage radish: (A-B)) Day 20; (C-D) Day 58; (E-H) Vetch: (E-F) Day 20; (G-H) Day 58; (I-L) Black oat: (I-J) Day 20; (K-L) Day 58. Orange circles denote position of root within column and do not relate to root diameter (actual sizes would be indiscernible in many cases at this scale of imaging; diameter of core, here black circles, is 75 mm). For all species 3 cm depth falls within the uncompacted zone and 10 cm depth is within the uncompacted zone.</p

Average root systems convex hull (cm³) and solidity value of full soil column.

<p>(A) Convex hull (cm³) of tillage radish, vetch and black oat root systems. Data represents full root systems, derived from X-ray CT. Bars denote means, whiskers pooled SE. (B) Solidity values of tillage radish, vetch and black oat root systems. Data represents full root systems, derived from X-ray CT data. Due to extreme range of values, tillage radish the data was not subject to ANOVA. Bars denote means, whiskers standard error.</p

Average surface area of root (cm²) and root volume as% of full soil column.

<p>(A) Surface area of root (cm²) within full soil column for tillage radish, vetch and black oat root systems, derived from X-ray CT data. Bars denote means, whiskers pooled SE. (B) Volume of root as a percentage of the uncompacted and compacted soil zones for tillage radish, vetch and black oat root systems, derived from X-ray CT data. Due to the disproportionately large increase in total root volume for tillage radish the data was not subject to ANOVA. Bars denote means, whiskers individual standard deviation.</p

Soil porosity of uncompacted and compacted layers with tillage radish, vetch and black oat roots systems growing at three time points.

<p>Average percentage of pore space from representative samples within both the uncompacted and compacted soil volumes, bars denote means, whiskers pooled s.e.</p