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Carbon and pH as Controls of Microbial and Microarthropod Communities in a Northern Hardwood Forest

This study examines the effects of carbon availability and pH on soil microbial and microarthropod communities in a northern hardwood forest. The experiment was conducted at the Hubbard Brook Experimental Forest in New Hampshire from 2000-2001. Results showed that calcium addition increased soil pH and respiration, but had no effect on microbial biomass. There was a non-significant decreasing trend in microarthropods in the summer of 2000, continuing to 2001.

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Carbon and pH as Controls of Microbial and Microarthropod Communities in a Northern Hardwood Forest

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  1. Carbon and pH as controls of microbial and microarthropod communities in a northern hardwood forest “The experiment that couldn’t fail…” - M. Fisk and P. Groffman Will Kessler Department of Natural Resources Cornell University

  2. Question(s): • Does carbon availability • Does pH control soil microarthropod populations indirectly through microbial communities in the northern hardwood forest?

  3. Microarthropods Microbes Forest floor substrate • Microbes:soil bacteria and saprophytic soil fungi • Microarthropods: small invertebrates occupying trophic levels above microbes in below ground food-webs. • Primarily mites (Acari) and certain insects.

  4. C addition Ca addition C & Ca addition Control Experimental design Conducted at the Hubbard Brook Experimental Forest (HBEF) NH, summers 2000-2001. • In this study, I manipulated:  •  Forest floor carbon availabilityGlucose/Cellulose addition • Forest floor pH, Calcium (CaSiO3) addition 4 Treatment Plots at each of 5 sites

  5. Addition timeline: Summer 2000 Ca+ C+ C+ C+ C+ C+ 5-Jun 14-Jun 28-Jun 13-Jul 23-Jul Ca+ : Calcium addition performed; 667 g Ca m-2 C+: Carbon addition performed; totaling 120 g C m-2 for summer Background figures at Hubbard Brook: • Microbial biomass: 28 g C m-2 • Annual C input to organic soil: 250 g C m-2 (litter, roots, exudates)

  6. Calcium:added as ground wollastonite powder, sifted over each plot like flour. Carbon: mixed in 3L water solution and sprayed over plots with a backpack sprayer. Control plots received equivalent amounts of water throughout study.

  7. Data recorded: WhatWhen • Soil pH 2000-2001 • Microbial respiration (field) 2000 • Microbial respiration (lab) 2000 • Microbial biomass C 2000 • Microbial biomass N 2000 • Microarthropod abundance 2000-2001

  8. High hopes… • Expected increased pH in Ca addition plots • Expected to see positive response in microbial respiration as C availability increased • Curious about microbial biomass with respect to C availability • Curious about abundance of microarthropods with respect to both C availability and pH

  9. Bummer! No significant response in C plots: Respiration (e.g. ) Biomass C Biomass N Microarthropods However; lack of response to C addition allowed a statistical regrouping of data into Ca and no-Ca blocks…

  10. Soil pH response to Ca addition

  11. Microbial respiration response to Ca addition

  12. Microbial biomass C

  13. Trend in Microarthropods? Saw weak evidence (non-significant) of a decline in microarthropods in Ca addition plots

  14. Additional evidence across season: microarthropod decline Similar dates in consecutive years appear to have decreasing microarthropods per g dry soil

  15. Summary • No observed C addition effect (microbial respiration, biomass, or microarthropods). • Ca addition resulted in an increase in soil pH and respiration, but had no effect on microbial biomass. • Microarthropods showed a non-significant decreasing trend in the summer of 2000, continuing to 2001.

  16. Linking pH to microbial activity(3 scenarios) • Increased pH could translate to lower stress for microbes, meaning more activity, growth, and mortality. Net signal is then increased CO2, but not biomass. • Increased pH, could mean greater stress for an acidic-adapted community. Thus, CO2 signal is stress induced, and no biomass is added. • Increased pH could favor bacteria over fungi in the forest floor. Bacteria are less efficient than fungi, and would thus produce more CO2 per unit biomass.

  17. Interpretation • Raised pH in Ca addition plots may have shifted the microbial community, resulting in a decrease in the fungal to bacterial ratio (F:B). • In addition to increased respiration, a lower F:B in Ca plots could explain the negative trend in microarthropods (primarily fungivorous).

  18. B B F F Interpretation (ctd.) Trophic consequence for microarthropods? Microarthropods Micro- arthropods pH increase +1

  19. Thanks! • Melany Fisk • Tim Fahey

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