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Title: Soil microbial responses to elevated CO2 and O3 in a nitrogen-aggrading agroecosystem

Author
item CHENG, LEI - North Carolina State University
item Booker, Fitzgerald
item Burkey, Kent
item TU, CONG - North Carolina State University
item SHEW, H - North Carolina State University
item RUFTY, THOMAS - North Carolina State University
item Fiscus, Edwin
item HU, SHUIJIN - North Carolina State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2011
Publication Date: 6/22/2011
Citation: Cheng, L., Booker, F.L., Burkey, K.O., Tu, C., Shew, H.D., Rufty, T., Fiscus, E.L., Hu, S. 2011. Soil microbial responses to elevated CO2 and O3 in a nitrogen-aggrading agroecosystem. PLoS One. 6:e21377.

Interpretive Summary: Experimental evidence accumulated over the last several decades has shown that climate change factors such as rising CO2 concentrations in the atmosphere can exert significant impacts on plant growth and subsequent resource availability to soil microorganisms. Soil microbiological processes in turn critically affect ecosystem responses to climate change by influencing decomposition rates and nutrient availability for plants. Air pollutant ozone, a greenhouse gas with demonstrated inhibitory effects on plant growth, has been less well studied but is considered an influence on soil microbial processes. Alterations in soil microbes affect the long-term capacity of terrestrial ecosystems to function as a C sink for mitigating rising atmospheric CO2. However, predicting C sequestration potential is hampered by our limited understanding of the underlying mechanisms by which soil microbes respond to CO2 and ozone. To investigate these mechanisms we conducted a long-term study to examine climate change effects on soil C dynamics in a wheat-soybean agroecosystem. We continuously monitored a suite of soil microbial parameters for four years to ascertain the time course of microbial responses to elevated CO2 and ozone. Results obtained from this study showed that CO2-induced alterations in soil C and N availability exerted interactive controls over soil microbes and microbially-mediated processes in the wheat-soybean system. Ozone suppressed biomass production and N input, but only elevated CO2 significantly affected soil microbial parameters. While soil microbial biomass and activities were little affected by elevated CO2 in the first two years, they significantly responded to CO2 enrichment in the third and fourth years as more N became available, likely though increased atmospheric N2-fixation by soybean plants over the course of the experiment. High N availability positively correlated with high microbial metabolism and organic C turnover. Ozone effects on C and nutrient input were likely insufficient in magnitude to produce detectable changes in the soil microbial parameters measured. These findings suggest that under future CO2 scenarios, high N availability in many agricultural soils may accelerate organic C turnover, constraining the potential of C sequestration in agroecosystems.

Technical Abstract: Despite decades of study, the underlying mechanisms by which soil microbes respond to rising atmospheric CO2 and ozone remain poorly understood. A prevailing hypothesis, which states that changes in C availability induced by elevated CO2 and ozone drive alterations in soil microbes and the processes they regulate, successfully predicts outcomes in some cases, but fails in others. Using a long-term field experiment conducted in a no-till wheat-soybean system, we show that N availability critically influences soil microbial responses to elevated CO2 but not ozone. Elevated CO2 significantly increased above-ground residue mass and residue N inputs to soil by 21% and 17%, respectively, whereas ozone decreased them by 5% and 10%. However, only elevated CO2 significantly affected soil microbial parameters. While it only had marginal effects on microbial respiration in the first two years, elevated CO2 significantly stimulated microbial biomass and decomposition in the third and fourth years when N availability increased, likely due to CO2-enhancement of symbiotic N2 fixation in soybean over the course of the experiment. Ozone effects on C and nutrient input were likely insufficient in magnitude to product detectable changes in the soil microbial parameters measured. These results suggest that high N availability in many agricultural soils may accelerate organic C turnover and limit the potential of C sequestration in agroecosystems under future CO2 scenarios.