Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2006
Publication Date: 8/1/2006
Citation: Kandeler, E., Mosier, A., Morgan, J.A., Milchunas, D., King, J., Rudolph, S., Tscherko, D. 2006. Response of soil microbial biomass and enzyme activities to the transient elevation of carbon dioxide in a semi-arid dryland. Soil Biology and Biochemistry. 38(8):2448-2460. Interpretive Summary: It is widely accepted that the release of greenhouse gases into the atmosphere will have profound impacts on the earth’s climate, including global warming, altered precipitation patterns, and increased storm intensities. But greenhouse gases may have direct effects on plant ecology that are in addition to the more widely-publicized effects due to climate change. In particular, plants respond directly to the greenhouse gas carbon dioxide (CO2) since CO2 is a substrate for photosynthesis. Increases in this gas alone can enhance terrestrial photosynthesis, thereby increasing the amount of ecosystem C which eventually finds its way to belowground pools of C, which in turn can affect nutrient cycling, ecosystem performance, and eventually feed-back on soil available nutrients, thereby further modulating plant responses. Considerable research has been conducted understanding direct plant responses to CO2, but less is known about the consequences of these atmospheric changes on soil nutrient cycling, which ultimately may determine how plants respond to global change. This experiment investigates how doubling the CO2 concentration over a native Colorado prairie effects soil biology, and the secondary effects of those changes on the cycling of soil C. The results indicate that the additional C which enters grasslands as a result of rising atmospheric CO2 ends up primarily in a fast-cycling C pool, rather than residing in more long-term soil storage C. The relatively fast cycling of new C suggests an adaptive capacity for grasslands to atmospheric CO2 enrichment, but also that they may not serve as strong C sinks in the future to help mitigate the problem of rising atmospheric greenhouse gases.
Technical Abstract: Although elevation of CO2 has been reported to impact soil microbial functions, little information is available on the spatial and temporal variation of this effect. The objective of the study was to determine the microbial response of a northern Colorado shortgrass steppe to a five-year CO2 elevation of the atmosphere as well as the reversibility of the microbial response during a period of several months after shutting off the CO2 amendment. The experiment was comprised of nine experimental plots: three chambered plots maintained at present CO2 levels of 360 parts per million (ambient treatment), three chambered plots maintained at 720 parts per million CO2 (elevated treatment) and three unchambered plots of equal ground area used as controls to monitor the chamber effect. Elevation of CO2 induced mainly an increase of enzyme activities (protease, xylanase, invertase, alkaline phosphatase, arlysulfatase) in the upper 5 cm of the soil and did not change microbial biomass in the soil profile. Since rhizodeposition and newly formed roots enlarged the pool of easily available substrates mainly in the upper soil layers, enzyme regulation (production and activity) rather than shifts in microbial abundance was the driving factor for higher enzyme activities in the upper soil. Repeated soil sampling during the third to fifth year of the experiment revealed an enhancement of enzyme activities which varied in the range of 20-80 percent. Discriminant analyses (DA) including all microbiological properties showed that an increasing percentage of the variance within the data set could be explained by the first axis during the time course. Although microbial biomass did not show any response to the elevation of CO2 during the main experiment, a significant increase of soil microbial N was detected as a post effect probably due to lower nutrient (nitrogen) competition between microorganisms and plants in this N-limited ecosystem. Whereas most enzyme activities showed a significant post CO2 effect in spring following the conclusion of CO2 enrichment the previous autumn, selective depletion of substrates might be the cause for non significant treatment effects of most enzyme activities in summer and autumn of the following year. Therefore, additional below ground carbon input entered mainly the fast cycling carbon pool and did not contribute very much to the long-term storage of carbon.