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Journal Article

Atmospheric CO2 and soil extracellular enzyme activity: a meta-analysis and CO2 gradient experiment

Authors: Kelley AM, PA Fay, HW Polley, RA Gill, RB Jackson

Rising atmospheric CO2 concentrations can alter carbon and nutrient cycling and microbial processes in terrestrial ecosystems. One of the primary ways microbes interact with soil organic matter is through the production of extracellular enzymes, which break down complex organic molecules and release nutrients into the soil. We conducted a meta-analysis of 34 studies that examined responses in microbial enzyme activity to elevated CO2 and a field study of soil enzyme activity in a tallgrass-prairie ecosystem with sandy loam (with lower organic matter content) and clayey soils (with higher organic matter content) exposed to a continuous gradient of 250 to 500 ppm CO2. Of the ten enzyme groups examined in the meta-analysis, including those degrading starch, b-glucan, cellulose, xylan/hemicellulose, lignin, organic P, and organic N, only the activity one enzyme that degrades the C- and N-containing building blocks of chitin (N-acetyl-glucosaminidase) increased consistently at elevated CO2 by 12.6% (p<0.05), especially in field studies and in woody ecosystems. In our field study, increasing from subambient to elevated CO2 reduced the activity of leucine aminopeptidase by 32% in the black clay soil during the peak of the growing season, while b-1,4-N-acetyl-glucosaminidase increased by 44% near the end of the season, indicating increased N limitation with increasing CO2. In the sandy loam soil, alkaline phosphatase activity increase by 42% with CO2 enrichment at the end of the growing season, implying CO2-induced phosphorus limitation in these soils. Additionally, a 53-83% decrease in the carbon cycling enzymes cellobiohydrolase, a-glucosidase, and xylosidase activity with increased CO2 was found in July. Our field study shows soil type can strongly influence how microbial functioning may change with rising CO2 concentrations. Our meta-analysis revealed that, despite variable enzyme activities with CO2, chitinase activity increased consistently with CO2 across ecosystems. Additionally, our unique field CO2 gradient showed that the microbial responses associated C-, N-, and P-cycling are also likely to change - and may already have changed - with increasing CO2 under some soil types and conditions.

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