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Title: Arbuscular mycorrhizal fungi increase organic carbon decomposition under elevated carbon dioxide

Author
item CHENG, LEI - North Carolina State University
item Booker, Fitzgerald
item TU, CONG - North Carolina State University
item Burkey, Kent
item ZHOU, L - North Carolina State University
item SHEW, DAVID - North Carolina State University
item RUFTY, THOMAS - North Carolina State University
item HU, SHUIJIN - North Carolina State University

Submitted to: Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2012
Publication Date: 8/31/2012
Citation: Cheng, L., Booker, F.L., Tu, C., Burkey, K.O., Zhou, L., Shew, D., Rufty, T., Hu, S. 2012. Arbuscular mycorrhizal fungi increase organic carbon decomposition under elevated carbon dioxide. Science. 337:1084-1087.

Interpretive Summary: Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with roots of most plants. The fungi receive organic sustenance from host plants while contributing mineral nutrients to them. AMF have a major role in global carbon cycling because they consume up to 20% of plant assimilates and deposit slowly decomposing organic compounds in the soil, thus promoting soil carbon sequestration. Atmospheric carbon dioxide (CO2) enrichment often stimulates AMF growth, suggesting that soil carbon storage via enhanced AMF activity will also be increased in the future. This hypothesis, however, does not consider the effect of AMF on decomposition of soil organic matter under elevated CO2. Here we present evidence from four experiments demonstrating that CO2-enhancement of AMF significantly increases carbon losses from soil. Combining dual stable isotope labeling and hyphae in-growth techniques, we show that CO2-enhancement of AMF increased decomposition of both soil- and residue-derived carbon in microcosms as well as in the field. In the presence of AMF, elevated CO2 also promoted plant nitrogen uptake from isotopically-labelled decomposing residues, reduced soil ammonium, but had no impacts or even increased soil nitrate. Together, our findings challenge the current paradigm of mycorrhizal protection of organic carbon in soil and raise questions about the current prediction of ecosystem carbon balance under future climate change scenarios.

Technical Abstract: A major goal of climate change research is to understand whether and how terrestrial ecosystems can sequester more carbon to mitigate rising atmospheric carbon dioxide (CO2) levels. The stimulation of arbuscular mycorrhizal fungi (AMF) by elevated atmospheric CO2 has been assumed to be a major mechanism facilitating soil carbon sequestration by increasing carbon inputs to soil and by protecting organic carbon from decomposition. This hypothesis, however, has not been vigorously tested because the net effect of CO2-stimulation of AMF on soil organic carbon dynamics remains to be directly evaluated. Emerging evidence that AMF may also enhance residue decomposition further contributes to the uncertainty. Here we present evidence from four independent but complementary experiments demonstrating that CO2-enhancement of AMF significantly increases carbon losses from soil. Combining dual stable isotope labeling and hyphae in-growth techniques, we show that CO2-enhancement of AMF increased decomposition of both soil- and residue-derived carbon in microcosms as well as in the field. In the presence of AMF, elevated CO2 also promoted plant nitrogen uptake from 15N-labelled decomposing residues, reduced soil ammonium, but had no impacts or even increased soil nitrate. Together, our findings challenge the current paradigm of mycorrhizal protection of organic carbon in soil and raise questions about the current prediction of ecosystem carbon balance under future climate change scenarios.