IMPACT OF ELEVATED ATMOSPHERIC CO2 CONCENTRATION ON SOIL MICROBIAL BIOMASS AND ACTIVITY IN A COMPLEX, WEEDY FIELD MODEL ECOSYSTEM

Although soil organisms play an essential role in the cycling of eleme nts in terrestrial ecosystems, little is known of the impact of increa sing atmospheric CO2 concentrations on soil microbial processes. We de termined microbial biomass and activity in the soil of multitrophic mo del ecosystems housed in the Ecotron (NERC Centre for Population Biolo gy, Ascot, UK) under two atmospheric CO2 concentrations (ambient vs. a mbient + 200 ppm). The model communities consist of four annual plant species which naturally co-occur in weedy fields and disturbed ground throughout southern England, together with their herbivores, parasitoi ds and soil biota. At the end of two experimental runs lasting 9 and 4 .5 months, respectively, root dry weight and quality showed contradict ory responses to elevated CO2 concentrations, probably as a consequenc e of the different time-periods (and hence number of plant generations ) in the two experiments. Despite significant root responses no differ ences in microbial biomass could be detected. Effects of CO2 concentra tion on microbial activity were also negligible. Specific enzymes (pro tease and xylanase) showed a significant decrease in activity in one o f the experimental runs. This could be related to the higher C:N ratio of root tissue. We compare the results with data from the literature and conclude that the response of complex communities cannot be predic ted on the basis of oversimplified experimental set-ups.