Conditions leading to high CO2 (>5 kPa) in waterlogged-flooded soils and possible effects on root growth and metabolism

Aims Soil waterlogging impedes gas exchange with the atmosphere, resulting in low P-O2 and often high P-CO2. Conditions conducive to development of high P-CO2 (5-70 kPa) during soil waterlogging and flooding are discussed. The scant information on responses of roots to high P-CO2 in terms of growth and metabolism is reviewed. Scope P-CO2 at 15-70 kPa has been reported for flooded paddy-field soils; however, even 15 kPa P-CO2 may not always be reached, e.g. when soil pH is above 7. Increases of P-CO2 in soils following waterlogging will develop much more slowly than decreases in P-O2; in soil from rice paddies in pots without plants, maxima in P-CO2 were reached after 2-3 weeks. There are no reliable data on P-CO2 in roots when in waterlogged or flooded soils. In rhizomes and internodes, P-CO2 sometimes reached 10 kPa, 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 P-CO2 is higher in a soil than in roots, P-CO2 in the roots would remain well below the P-CO2 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 P-CO2 at > 5 kPa on growth have nearly all involved sudden increases to 10-100 kPa P-CO2; consequently, the results cannot be extrapolated with certainty to the much more gradual increases of P-CO2 in waterlogged soils. Nevertheless, rice in an anaerobic nutrient solution was tolerant to 50 kPa CO2 being suddenly imposed. By contrast, P-CO2 at 25 kPa 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 10 kPa CO2 would result in an increase of 75-90 mm 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 50 mm, or even lower; succinic dehydrogenase and cytochrome oxidase are inhibited by HCO3- as low as 10 mm. 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 P-CO2 in the roots. Conclusions Measurements are needed on P-CO2 in a range of soil types and in roots of diverse species, during waterlogging and flooding. Species well adapted to high P-CO2 in the root zone, such as rice and other wetland plants, thrive even when P-CO2 is well over 10 kPa; mechanisms of adaptation, or acclimatization, by these species need exploration.