ELEVATED CO2 EFFECTS ON CARBON AND NITROGEN CYCLING IN GRASS CLOVER TURVES OF A PSAMMAQUENT SOIL/

Effects of elevated CO2 (525 and 700 mu L L(-1)), and a control (350 m u L L(-1) CO2), on biochemical properties of a Mollic Psammaquent soil in a well-established pasture of C3 and C4 grasses and clover were in vestigated with continuously moist turves in growth chambers over four consecutive seasonal temperature regimes from spring to winter inclus ive. After a further 'spring' period, half of the turves under 350 and 700 mu L L(-1) were subjected to 'summer' drying and were then re-wet ted before a further 'autumn' period; the remaining turves were kept c ontinuously moist throughout these additional three consecutive 'seaso ns'. The continuously moist turves were then pulse-labelled with C-14- CO2 to follow C pathways in the plant/soil system during 35 days. Grow th rates of herbage during the first four 'seasons' averaged 4.6 g m(- 2) day(-1) under 700 mu L L(-1) CO2 and were about 10% higher than und er the other two treatments. Below-ground net productivity at the end of these 'seasons' averaged 465, 800 and 824 g m(-2) in the control, 5 25 and 700 mu L L(-1) treatments, respectively. In continuously moist soil, elevated CO2 had no overall effects on total, extractable or mic robial C and N, or invertase activity, but resulted in increased CO2-C production from soil, and from added herbage during the initial stage s of decomposition over 21 days; rates of root decomposition were unaf fected. CO2 produced h(-1) mg(-1) microbial C was about 10% higher in the 700 mu L L(-1) CO2 treatment than in the other two treatments. Ele vated CO2 had no clearly defined effects on N availability, or on the net N mineralization of added herbage. In the labelling experiment, re latively more C-14 in the plant/soil system occurred below ground unde r elevated CO2, with enhanced turnover of C-14 also being suggested. D rying increased levels of extractable C and organic-N, but decreased m ineral-N concentrations; it had no effect on microbial C, but resulted in lowered microbial N in the control only. In soil that had been pre viously 'summer'-dried, CO2 production was again higher, but net N min eralization was lower, under elevated CO2 than in the control after 'a utumn' pasture growth. Over the trial period of 422 days, elevated CO2 generally appears to have had a greater effect on soil C turnover tha n on soil C pools in this pasture ecosystem.