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Effects of Elevated CO2 on Below-Ground Carbon Allocation in Three Grasses
Cotrufo, M.F. and Gorissen, A.  1997.  Elevated CO2 enhances below-ground C allocation in three perennial grass species at different levels of N availability.  New Phytologist 137: 421-431.

What was done
The authors grew three grass species (one per pot from seed) - Lolium perenne L. cv. Barlet, Agrostis capillaris L. cv. Bardot and Festuca ovina L. cv. Barok - in 0.65-liter pots under two different levels of soil nitrogen in growth chambers maintained at 350 and 700 ppm CO2 in continuously 14C-labelled atmospheres.  Half the plants were harvested after 32 days, and the other half were harvested after 55 days.  The shoots, roots and soils were all analyzed for total carbon (C), as well as 14C content.

What was learned
Elevated CO2 increased the whole-plant dry weights of all three species by an average of 20% and, in general, increased root growth more than shoot growth.  It also increased soil microbial biomass by slightly more than 15%.  Of the 14C that was fixed, 90% remained in the plant, while 10% was transferred to the soil.  There, 7% was found in the bulk soil, while 3% was found in the rhizoshpere soil close to the plant roots, split about equally between soil microbial biomass and soil residue.

What it means
The authors state that what they observed "is consistent with the general finding that the soil microbial biomass is C-limited, and supports the hypothesis that a greater C input below-ground, as a result of increased size of the root system, at high CO2 elicits an increase in soil microbial biomass."  They also concluded that "elevated CO2 will induce an increase in relative C allocation to below-ground sinks independent of nitrogen level."  Finally, they conclude that their findings for all three grasses confirm the hypothesis that "elevated CO2 could result in greater soil C stores due to increased C-input into soils," as well as "a higher residence time for C in soils, thus counteracting increased decomposition under higher temperatures."  Hence, they state that "we may well expect that this increased C input below-ground will be sustained in the longer term."  And this phenomenon would clearly be expected to reduce the level to which atmospheric CO2 concentrations would rise in its absence.

Reviewed 1 November 1999