Conditions Leading to High CO2 (>5kPa) in Waterlogged-Flooded Soils and Possible Effects on Root Growth and Metabolism

Aims Soil waterlogging impedes gas exchange with the atmosphere, resulting in low PO2 and often high PCO2. Conditions conducive to development of high PCO2 (5-70kPa) during soil waterlogging and flooding are discussed. The scant information on responses of roots to high PCO2 in terms of growth and metabolism is reviewed.Scope PCO2 at 15-70kPa has been reported for flooded paddy-field soils; however, even 15kPa PCO2 may not always be reached, e.g. when soil pH is above 7. Increases of PCO2 in soils following waterlogging will develop much more slowly than decreases in PO2; in soil from rice paddies in pots without plants, maxima in PCO2 were reached after 2-3 weeks. There are no reliable data on PCO2 in roots when in waterlogged or flooded soils. In rhizomes and internodes, PCO2 sometimes reached 10kPa, inferring even higher partial pressures in the roots, as a CO2 diffusion gradient will exist from the roots to the rhizomes and shoots. Preliminary modelling predicts that when PCO2 is higher in a soil than in roots, PCO2 in the roots would remain well below the PCO2 in the soil, particularly when there is ventilation via a well-developed gas-space continuum from the roots to the atmosphere. The few available results on the effects of PCO2 at > 5kPa on growth have nearly all involved sudden increases to 10-100kPa PCO2; consequently, the results cannot be extrapolated with certainty to the much more gradual increases of PCO2 in waterlogged soils. Nevertheless, rice in an anaerobic nutrient solution was tolerant to 50kPa CO2 being suddenly imposed. By contrast, PCO2 at 25kPa retarded germination of some maize genotypes by 50%. With regard to metabolism, assuming that the usual pH of the cytoplasm of 7·5 was maintained, every increase of 10kPa CO2 would result in an increase of 75-90mm HCO3 in the cytoplasm. pH maintenance would depend on the biochemical and biophysical pH stats (i.e. regulatory systems). Furthermore, there are indications that metabolism is adversely affected when HCO3 in the cytoplasm rises above 50mm, or even lower; succinic dehydrogenase and cytochrome oxidase are inhibited by HCO3 as low as 10mm. Such effects could be mitigated by a decrease in the set point for the pH of the cytoplasm, thus lowering levels of HCO3 at the prevailing PCO2 in the roots.Conclusions Measurements are needed on PCO2 in a range of soil types and in roots of diverse species, during waterlogging and flooding. Species well adapted to high PCO2 in the root zone, such as rice and other wetland plants, thrive even when PCO2 is well over 10kPa; mechanisms of adaptation, or acclimatization, by these species need exploration.