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| Titel |
X-ray CT imaging and image-based modelling study of gas exchange in the rice
rhizosphere |
| VerfasserIn |
Marie-Cecile Affholder, Samuel David Keyes, Tiina Roose, James Heppell, Guy Kirk |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250126432
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| Publikation (Nr.) |
 EGU/EGU2016-6151.pdf |
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| Zusammenfassung |
| We used X-ray computer tomography and image-based modelling to investigate
CO2 uptake by rice roots growing in submerged soil, and its consequences for the
chemistry and biology of the rhizosphere. From previous work, three processes
are known to greatly modify the rhizophere of rice and other wetland plants: (1)
oxygenation of the submerged, anoxic soil by O2 transported through the root gas channels
(aerenchyma); (2) oxidation of ferrous iron and resulting accumulation of ferric oxide;
and (3) pH changes due to protons formed in iron oxidation and released from the
roots to balance excess intake of cations over anions. A further process, so far not
much investigated, is the possibility of CO2 uptake by the roots. Large amounts
of CO2 accumulate in submerged soils because CO2 formed in soil respiration
escapes only slowly by diffusion through the water-saturated soil pores. There is
therefore a large CO2 gradient between the soil and the aerenchyma inside the
root, and CO2 may be taken up by the roots and vented to the atmosphere. The
extent of this and its consequences for rhizosphere chemistry and biology are poorly
understood.
We grew rice plants in a submerged, strongly-reduced, Philippine rice soil contained in
10-cm diameter, 20-cm deep Perspex pots. Four-week old rice seedlings, grown in nutrient
culture, were transplanted into the pots at either 1 or 4 plants per pot, planted closely together.
After 3 and 4 weeks, the pots were analysed with an X-ray CT scanner (Custom Nikon/Xtek
Hutch; 80 mm by 56 mm field of view and 40 μm voxel size). Gas bubbles were extracted
from the data by 3D median filtering and roots using a region-growth method. The images
showed prominent and abundant gas bubbles in the soil bulk, but no or very few
bubbles in the soil close to roots. There was a clear relation between the absence of
gas bubbles and the presence of roots, as well as an increasing concentration of
bubbles with depth through the soil. Analysis of the bubbles showed they were
approximately 50% CO2 by volume and 50% CH4. The corresponding concentrations of
dissolved CO2 + HCO3− (NB CO2 is 20-times more soluble than CH4) in the soil
bulk were of the order of 100 mM. We developed a mathematical model of CO2
generation and transport in submerged soil with uptake by and transport through rice
roots, and used it to analyse the images. This showed that the observed depletion of
CO2 around the roots was consistent with realistic values of parameters for the
root gas permeability and rates of CO2 production and diffusion in submerged
soil.
Depletion of CO2 around the roots will have consequences for the chemistry of the rice
rhizosphere and the extent of the root-induced pH changes and other changes listed above. In
continuing work we are investigating the implications for the solubility and root
uptake of soil Zn, deficiency of which is a widespread constraint to rice growth. |
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