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

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

Average surface area (cm²), volume of root (%) of black oat root systems.

<p>(A) Average surface area of black oat root (cm²). (B) Average volume of black oat root as a percentage of the soil volume. For all graphs data is presented for both the compacted and uncompacted zones over two time periods. Bars denote means, whiskers s.e.</p

Average surface area (cm²), volume of root (%), number of laterals and lateral angle (°) of vetch root systems.

<p>(A) Average surface area of vetch root (cm²) (B) Average volume of vetch root as a percentage of the soil volumes, (C) Average number of lateral roots observed protruding from the taproot into the soil volumes. (D) Average angle of lateral roots protruding from taproot into the soil volumes. For all graphs data is presented for both the compacted and uncompacted zones over two time periods Bars denote means, whiskers pooled SE.</p

Representative 3D renderings of root systems grown in repacked soil columns.

<p>(A) tillage radish after 20 days growth; (B) tillage radish after 58 days growth; (C) vetch after 20 days growth; (D) vetch after 58 days growth; (E) black oat after 20 days growth; (F) black oat after 58 days growth. Red arrows indicate location of change in dry bulk density of soil profile from 1.2 to 1.4 g cm^-3.</p