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

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. We also conducted a field study of soil enzyme activity in a tallgrass-prairie ecosystem growing in sandy loam (lower organic matter content) and clayey soils (higher organic matter content) exposed to a continuous gradient of 250 to 500 ppm CO2. Of the 10 enzyme groups examined in the meta-analysis, including those degrading starch, beta-glucan, cellulose, xylan/hemicellulose, lignin, organic P, and organic N, only the activity of one enzyme that degrades the C-and N-containing building blocks of chitin (N-acetyl-glucosaminidase) increased consistently at elevated CO2 by an average of 12.6% (p < 0.05), especially in field studies and in woody ecosystems. In our field study, increasing CO2 from subambient to elevated concentrations reduced the activity of leucine aminopeptidase by 32% in the black clay soil during the peak of the growing season, while beta-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 increased by 42% with CO2 enrichment at the end of the growing season, suggesting CO2-induced phosphorus limitation in these soils. Additionally, a 53-83% decrease in the carbon cycling enzymes cellobiohydrolase, alpha-glucosidase, and xylosidase activity with increased CO2 was found in July. Our field study shows that soil type can strongly influence how microbial functioning may change with rising CO2 concentrations and that microbial responses associated with C-, N-, and P-cycling are likely to change-and may already have changed-with increasing CO2 under some soil types and conditions. Our meta-analysis revealed that, despite variable enzyme activities with CO2, chitinase activity increased consistently with CO2 across ecosystems.