Influence of ozone and simulated acidic rain on microorganisms in the rhizosphere of Sorghum.

Seedlings of a sorghum x sudangrass hybrid in pots of non-sterile soil-sand mix were exposed to ozone (O(3)) at 0, 0.15, or 0.30 microl litre(-1) (7 h day(-1), 3 days week(-1)) and simulated rain (SR) adjusted with H(2)SO(4) + HNO(3) to pH 5.5, 4.0, or 2.5 (2 cm in 1.5 h per event; 2 events week(-1)) over 3 weeks in a greenhouse. Ozone suppressed shoot and root growth, but increased acid content (i.e. pH < 5.5) of SR stimulated shoot growth and had inconsistent effects on root growth. Ozone x SR chemistry interactions significantly affected plant growth. Data for 'total' bacterial populations in the rhizosphere (number of colony-forming units per gram of rhizosphere soil) exhibited a curvilinear relationship with O(3) (maximum at 0.15 microl liter(-1)). Increased acid content of SR stimulated numbers of 'total' bacteria but suppressed populations of amylolytic bacteria. Ozone and acid content of SR tended to stimulate numbers of fungal propagules in the rhizosphere, but this effect was not significant. Numbers of rhizosphere bacteria capable of phosphatase activity increased linearly with O(3), but only when SR chemistry was characterised by pH 4.0. Data for other populations of rhizosphere microorganisms did not exhibit significant relationships to O3 x SR chemistry interactions.