THE EFFECT OF ELEVATED CO2 CONCENTRATION AND SOIL-PH ON THE RELATIONSHIP BETWEEN PLANT-GROWTH AND RHIZOSPHERE DENITRIFICATION POTENTIAL

The effect of CO2 concentration on plant growth and the size of the rh izosphere denitrifier population was investigated for ryegrass grown a t 3 different soil pH values (pH 4.3, 5.9 and 7.0). Soil microcosms we re planted with ryegrass and maintained under constant growth conditio ns at either ambient (450ppm) or elevated (720ppm) CO2 concentration. At harvest, the rhizosphere soil was collected and subjected to a pote ntial denitrification assay to provide an estimate of the size of the denitrifier population present. Ryegrass dry matter production varied across the pH range studied and contrary to other studies, elevated CO 2 concentration did not consistently increase growth. Plant growth was reduced by approximate to 35% and 23% at pH 4.3 and pH 5.9, respectiv ely, under elevated CO2 concentration. At pH 7.0, however, plant growt h was increased by approximate to 45% under elevated CO2. Potential de nitrification rates within the rhizosphere followed a similar pattern to plant growth in the different treatments, suggesting that plant gro wth and the size of denitrifier population within the rhizosphere are coupled. This study investigates the relationship between plant growth and rhizosphere denitrification potential, thereby providing an estim ate of the size of the denitrifier population under increased CO2 conc entration and soil pH.