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Decomposition of Grass Roots as Affected by Elevated CO2
Van Ginkel, J.H., Gorissen, A. and van Veen, J.A.  1996.  Long-term decomposition of grass roots as affected by elevated atmospheric carbon dioxide.  Journal of Environmental Quality 25: 1122-1128.

What was done
Lolium perenne L. cv. Barlet plants were grown from seed in two growth chambers for 71 days under continuous 14CO2-labeling of the atmosphere at CO2 concentrations of 350 and 700 ppm at two different soil nitrogen levels.  At the conclusion of this part of the experiment, the plants were harvested and their roots dried, pulverized and mixed with soil in a number of one-liter pots that were placed within two wind tunnels in an open field, one of which had ambient air of 361 ppm CO2 flowing through it, and one of which had air of 706 ppm CO2 flowing through it.  Several of these containers were then seeded with more Lolium perenne, some were similarly seeded the following year, and some were kept bare for two years.  Plants were harvested at the ends of the first and second years; and the degree of decomposition of the original roots determined.

What was learned
At the conclusion of the experiment, shoot and root growth were found to have been stimulated by 13 and 92%, respectively, in the CO2-enriched treatment.  After the first growing season, the increase in CO2 was found to have enhanced soil microbial biomass by 44%; and after the second growing season, soil microbial biomass was found to be 30% greater in the CO2-enriched treatment.  The decomposition of the high-CO2-grown roots in the high-CO2 wind tunnel was19% lower than that of the low-CO2-grown roots in the low-CO2 wind tunnel at the end of the first year; while it was 14% lower at the end of the second year in the low-nitrogen-grown plants but equivalent in the high-nitrogen-grown plants.  It was also found that the presence of living roots reduced the decomposition rate of dead roots below the dead-root decomposition rate observed in the bare soil treatment.

What it means
The authors conclude that "the combination of higher root yields at elevated CO2 combined with a decrease in root decomposition will lead to a longer residence time of C in the soil and probably to a higher C storage."  These facts, they say, "should be taken into account in models predicting the fate of C under different scenarios for climatic change."  And so they should; for this is a phenomenon that cannot be ignored in this most important enterprise.

Reviewed 1 November 1999