title: Soil microbiota in two annual grasslands: responses to elevated atmospheric CO2.
abstract: We measured soil bacteria, fungi, protozoa, nematodes, and biological activity in serpentine and sandstone annual grasslands after 4 years of exposure to elevated atmospheric CO2. Measurements were made during the early part of the season, when plants were in vegetative growth, and later in the season, when plants were approaching their maximum biomass. In general, under ambient CO2, bacterial biomass, total protozoan numbers, and numbers of bactivorous nematodes were similar in the two grasslands. Active and total fungal biomasses were higher on the more productive sandstone grassland compared to the serpentine. However, serpentine soils contained nearly twice the number of fungivorous nematodes compared to the sandstone, perhaps explaining the lower standing crop of fungal biomass in the serpentine and suggesting higher rates of energy flow through the fungal-based soil food web. Furthermore, root biomass in the surface soils of these grasslands is comparable, but the serpentine contains 6 times more phytophagous nematodes compared to the sandstone, indicating greater below-ground grazing pressure on plants in stressful serpentine soils. Elevated CO2increased the biomass of active fungi and the numbers of flagellates in both grasslands during the early part of the season and increased the number of phytophagous nematodes in the serpentine. Elevated CO2had no effect on the total numbers of bactivorous or fungivorous nematodes, but decreased the diversity of the nematode assemblage in the serpentine at both sampling dates. Excepting this reduction in nematode diversity, the effects of elevated CO2disappeared later in the season as plants approached their maximum biomass. Elevated CO2had no effect on total and active bacterial biomass, total fungal biomass, or the total numbers of amoebae and ciliates in either grassland during either sampling period. However, soil metabolic activity was higher in the sandstone grassland in the early season under elevated CO2, and elevated CO2altered the patterns of use of individual carbon substrates in both grasslands at this time. Rates of substrate use were also significantly higher in the sandstone, indicating increased bacterial metabolic activity. These changes in soil microbiota are likely due to an increase in the flux of carbon from roots to soil in elevated CO2, as has been previously reported for these grasslands. Results presented here suggest that some of the carbon distributed below ground in response to elevated CO2affects the soil microbial food web, but that these effects may be more pronounced during the early part of the growing season.
authors.affiliation: Department of Biological Sciences, Box 5640, Northern Arizona University, Flagstaff, AZ 86011-5640, USA e-mail: Bruce.Hungate@nau.edu Tel.: +1-520-5230925, Fax: +1-520-5237500, , , , , , US.
authors.forename: B A
authors.initials: BA
authors.lastname: Hungate
authors.affiliation: Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, USA, , , , , , US.
authors.forename: C H
authors.initials: CH
authors.lastname: Jaeger Iii
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA 94720, USA, , , , , , US.
authors.forename: G
authors.initials: G
authors.lastname: Gamara
authors.affiliation: Institute of Arctic Biology, University of Alaska, Fairbanks, AK 99775, USA, , , , , , US.
authors.forename: F S
authors.initials: FS
authors.lastname: Chapin Iii
authors.affiliation: Carnegie Institution of Washington, Department of Plant Biology, Stanford, CA 94305, USA, , , , , , US.
authors.forename: C B
authors.initials: CB
authors.lastname: Field
keywords: Community structure; Key words Elevated CO2; Nematodes; Protozoa; Soil food web