ELEVATED CO2 INCREASES BELOW-GROUND RESPIRATION IN CALIFORNIA GRASSLANDS

This study was designed to identify potential effects of elevated CO2 on belowground respiration (the sum of root and heterotrophic respirat ion) in field and microcosm ecosystems and on the annual carbon budget . We made three sets of respiration measurements in two CO2 treatments , i.e., (1) monthly in the sandstone grassland and in microcosms from November 1993 to June 1994; (2) at the annual peak of live biomass (Ma rch and April) in the serpentine and sandstone grasslands in 1993 and 1994; and (3) at peak biomass in the microcosms with monocultures of s even species in 1993. To help understand ecosystem carbon cycling, we also made supplementary measurements of belowground respiration monthl y in sandstone and serpentine grasslands located within 500 m of the C O2 experiment site. The seasonal average respiration rate in the sands tone grassland was 2.12 mu mol m(-2) s(-1) in elevated CO2, which was 32% higher than the 1.49 mu mol m(-2) s(-1) measured in ambient CO2 (P = 0.007). Studies of seven individual species in the microcosms indic ated that respiration was positively correlated with plant biomass and increased, on average, by 70% with CO2. Monthly measurements revealed a strong seasonality in belowground respiration, being low (0-0.5 mu mol CO2 m(-2) s(-1) in the two grasslands adjacent to the CO2 site) in the summer dry season and high (2-4 mu mol CO2 m(-1) s(-1) in the san dstone grassland and 2-7 mu mol CO2 m(-1) s(-1) in the microcosms) dur ing the growing season from the onset of fall rains in November to ear ly spring in April and May. Estimated annual carbon effluxes from the soil were 323 and 440 g C m(-2) year(-1) for the sandstone grasslands in ambient and elevated CO2. That CO2-stimulated increase in annual so il carbon efflux is more than twice as big as the increase in abovegro und net primary productivity (NPPa) and approximately 60% of NPPa in t his grassland in the current CO2 environment. The results of this stud y suggest that below-ground respiration can dissipate most of the incr ease in photosynthesis stimulated by elevated CO2.