Responses of Soil Biota to Elevated Co2 in a Chaparral Ecosystem

Atmospheric CO2 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 CO2 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 CO2. Standing crops of fungi and bacteria rarely changed with CO2. 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 CO2 up to 550 ppm CO2. 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 CO2 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 CO2 is due to increasing turnover and to increasing N deficiency. Assessing C sequestration and other impacts of elevated CO2 on ecosystems requires a comprehensive, interactive, and dynamic evaluation of soil organismal responses.