Soil carbon dioxide venting through rice roots

Abstract

The growth of rice in submerged soils depends on its ability to form continuous gas channels—aerenchyma—through which oxygen (O ₂) diffuses from the shoots to aerate the roots. Less well understood is the extent to which aerenchyma permits venting of respiratory carbon dioxide (CO ₂) in the opposite direction. Large, potentially toxic concentrations of dissolved CO ₂ develop in submerged rice soils. We show using X‐ray computed tomography and image‐based mathematical modelling that CO ₂ venting through rice roots is far greater than thought hitherto. We found rates of venting equivalent to a third of the daily CO ₂ fixation in photosynthesis. Without this venting through the roots, the concentrations of CO ₂ and associated bicarbonate (HCO ₃ ⁻) in root cells would have been well above levels known to be toxic to roots. Removal of CO ₂ and hence carbonic acid (H ₂CO ₃) from the soil was sufficient to increase the pH in the rhizosphere close to the roots by 0.7 units, which is sufficient to solubilize or immobilize various nutrients and toxicants. A sensitivity analysis of the model showed that such changes are expected for a wide range of plant and soil conditions.

Abstract

Large, potentially toxic concentrations of dissolved CO ₂ accumulate in submerged paddy soils because CO ₂ from plant and soil respiration escapes only very slowly. We found, using X‐ray computed tomography and image‐based mathematical modelling, venting of soil CO ₂ through rice roots at rates equivalent to a third of the daily CO ₂ fixation in photosynthesis. Without this venting, the concentrations of CO ₂ and associated bicarbonate in root cells would have been well above levels known to be toxic to roots. Removal of soil CO ₂ and hence carbonic acid will also affect the solubility and hence plant uptake of various nutrients and toxicants in the rhizosphere.