RESPONSES OF SOIL BIOTA TO ELEVATED CO 2 IN A CHAPARRAL ECOSYSTEM

Atmospheric CO 2 is rapidly increasing without an integrative understanding of the responses of soil organisms. We sampled soils in a chaparral ecosystem at 18 intervals over a 3‐yr period in replicated field chambers ranging from 250 to 750 ppm CO 2 at 100 ppm increments. We assessed three distinct soil energy channels: mycorrhizal fungi, saprotrophic fungi‐mite/collembola, and bacteria‐protozoa/nematode. C allocation belowground increased with elevated CO 2 . Standing crops of fungi and bacteria rarely changed with CO 2 . Mass of bacteria‐feeding nematodes increased during wet periods, but the effects on soil bacteria were not detectable. However, grazing of fungi by mites increased with increasing CO 2 up to 550 ppm CO 2 . Above this threshold, allocation of C to the fungal channel declined. Direct measures of mycorrhizal fungi (percentage infection, arbuscular mycorrhizal [AM] fungal hyphal length) showed no changes with CO 2 enrichment, but indirect measures (macroaggregates with newly fixed C) increased suggesting increasing allocation of C through this channel. We postulate that the lack of change in standing crop in microbes to elevated CO 2 is due to increasing turnover and to increasing N deficiency. Assessing C sequestration and other impacts of elevated CO 2 on ecosystems requires a comprehensive, interactive, and dynamic evaluation of soil organismal responses.