IMPACTS OF ROOTS AND MICROBES ON MICROSCALE HYDROLOLGY
Hallett, P.D.1, Feeney, D.S.1,2, Rodger, S.2, Bengough, A.G.1, Gordon, D.C.1,
White, N.A.2, Young, I.M.2
1Plant-Soil Interface Programme, Scottish Crop Research Institute,
Invergowrie, Dundee, DD2 5DA, UK.
2Scottish Informatics Mathematics Biology & Statistics (SIMBIOS) Centre, University of Abertay,
Bell Street, Dundee, DD1 1HG, Scotland, UK.
E-mail: Paul.Hallett@scri.ac.uk
The size and distribution of plant roots and microbes in soil causes very localised and spatially
heterogenous impacts on hydrology. Using a miniaturised infiltrometer, with a tip size as small as 0.2
mm, we have been able to quantify how microscale hydrology can be changed through the production
of biological compounds that either induce hydrophobicity or change the soil pore structure. By
comparing the sorptivity of water, SW with a liquid not influenced by hydrophobicity (i.e. ethanol or
hexane), SE, we obtain physically meaningful data on water transport and describe the potential
impact of hydrophobicity with a repellency index as R = 1.95 SE/SW. R is directly proportional to the
drop in SW caused by repellency. Thorough testing of the accuracy of the infiltrometer using simple
porous media and soil showed that it had a 9.3 % coefficient of variation, similar to the accuracy of
larger infiltrometers.
We hypothesise that the rhizosphere, the zone of soil adjacent to roots, will have greater levels of
repellency in dry soil, because of the presence of root exudates and secondary microbial metabolites.
Barley (Hordeum vulgare) plants, grown in arable soil packed in 22mm deep x 47 mm wide x 300
length plastic tubes within a controlled environment chamber, influenced water repellency by changing
it from an initial non-repellent level (R≈1) to an R of 2.26 ± 0.10. Doubling atmospheric CO2 to 720
ppm caused R to increase further to 2.59 ± 0.09 (P=0.01), suggesting that plant rhizosphere impacts
on hydrology could be accentuated slightly by climate change. Various studies speculate that the root
symbionts, arbuscular mycorrhiza, will influence repellency outside of the rhizosphere through the
production of hydrophobins including Glomalin. To evaluate this impact we grew either a mycorrhizadefective tomato (Lycopersicon esculentum Mill.) mutant, where mycorrhizal colonisation [RMC] is
restricted (Barker et al., 1998) or a wild type tomato cv. 76R1, near isogenic line of RMC, (both from
University of Western Australia) in 150 mm diameter x 300 mm depth cores filled with arable soil
amended with a granular inoculum of AM fungi. The cores were separated in the centre with 35 µm
textile to create two halves, one planted (root section) and the other unplanted (root-free section). The
textile allowed for fungal hyphae to move between the halves, but not plant roots. In dry soil, only the
rooted section of the cores planted with the mycrorrhiza-defective tomato had a water repellency level
that was significantly greater than unplanted control samples. At 1 MPa matric potential, the rooted
section had the greatest repellency, followed by the bulk soil and then the control, with no significant
differences between plants. It therefore appears that the impact of root exudates and secondary
microbial metabolites have a greater impact on hydrology in this experimental system than the
presence of AM fungi. We are currently investigating the biological community in the different sections
of the core, specifically fungal biomass, glomalin content and microbial community structure to see if
this any of these properties can explain the differences in water repellency. Further studies are
investigating the impact of specific fungal species on the development of repellency and the role of
fungi versus bacteria.
Keywords: roots, microbiology, hydrology, microscale infiltrometer, climate change
Barker, S.J., Stummer, B., Gao, L., Dispain, I., O’Conner, P.J., Smith, S.E. 1998. A mutant
Lycopersicon esculentum Mill. With highly reduced VA mycorrhizal colonisation: Isolation and
preliminary characterisation. Plant Journal. 15, 791-797.