Submitted to: Ecosphere
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/13/2011
Publication Date: 8/30/2011
Citation: Kelley, A.M., Fay, P.A., Polley, H.W., Gill, R.A., Jackson, R.B. 2011. Atmospheric CO2 and soil extracellular enzyme activity: A meta-analysis and CO2 gradient experiment. Ecosphere. 2(8):art96. Interpretive Summary: The concentration of carbon dioxide (CO2) in the atmosphere has increased by 40% since industrialization and is predicted to reach double the pre-industrial level within the century. Higher CO2 increases the maximum rate at plants can growth, but may indirectly affect plant growth by affecting how soil microorganisms respond to changes in the quantity and quality of organic (plant-derived) compounds in soil. Soil microbes produce extracellular enzymes that break down organic molecules in soil, thereby releasing the nutrient elements that plants require to grow. In order to better understand soil microbial responses to CO2, we conducted two analyses. First, we analyzed responses of microbial enzyme activity to elevated CO2 using a compilation of studies reported in the scientific literature (meta-analysis). Second, we measured soil enzyme activity in both sandy and clay soils with tallgrass-prairie vegetation that had been exposed to a continuous gradient in atmospheric CO2 spanning pre-Industrial to elevated concentrations. Of enzymes in the ten groups examined in the meta-analysis, only an enzyme that degrades the carbon- and nitrogen-containing building blocks of a molecule found primarily in fungi (chitin) changed significantly at elevated CO2 (14.4% average increase). By contrast, both carbon-cycling and element-cycling enzymes responded to CO2 in the field study. Increased CO2 reduced the activity of three carbon-degrading enzymes in the sandy soil, but increased the activity of a phosphorus-releasing enzyme in the sandy soil and chitin-degrading enzyme in the clay soil. Enzyme activities likely responded to changes in either the amount of specific compounds in soil or limitations of nitrogen or phosphorus availability that resulted from an increase in plant growth at elevated CO2. Our results demonstrate that soil type could explain part of the variation in enzyme responses to CO2 that have been observed by others and highlight the importance of considering soil characteristics when predicting CO2 effects on plant and microbial functioning.
Technical Abstract: Rising atmospheric CO2 concentrations may alter carbon and nutrient cycling and microbial processes in terrestrial ecosystems. One of the primary ways that microbes interact with soil organic matter is through the production of extracellular enzymes, which break down large, complex organic molecules and release nutrients into the soil. We combined a meta-analysis of responses in microbial enzyme activity to elevated CO2 with a field study of soil enzyme activity in a tallgrass-prairie ecosystem with sandy loam and clayey soils exposed to a continuous gradient of subambient-to-elevated CO2, ranging from 250 to 500 ppm CO2. Of the ten enzyme groups examined in the meta-analysis, including those that degrade starch, beta-glucan, cellulose, xylan/hemicellulose, lignin, organic P, and organic N, only an enzyme that degrades the C- and N-containing building blocks of chitin (N-acetyl-glucosaminidase) changed significantly at elevated CO2 (14.4% average increase; p < 0.03). In the field study, increasing CO2 reduced the activity of cellobiohydrolase, alpha-glucosidase, and xylosidase activity by between 13.6 – 25.8 nmol g-1 h-1 from subambient to elevated CO2 concentrations in the sandy soil during the peak of the growing season. The increase from subambient to elevated CO2 reduced the activity of leucine aminopeptidase by 130.9 nmol g-1 h-1 in the clay soil during the same period. Near the end of the growing season, CO2 enrichment increased phosphatase activity by 126.6 nmol g-1 h-1 with increasing CO2 along the gradient in the sandy soil, and, as in the meta-analysis, increased activity of N-acetyl-glucosaminidase by 17.0 nmol g-1 h-1 in the clay soil with increasing CO2 along the gradient. Our meta-analysis showed that CO2 effects on enzyme activities vary among ecosystems. Our field study demonstrated that soil type and the timing of sampling may partially explain the diversity of responses of enzyme activity observed in other CO2 experiments.