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The Impact of Atmospheric CO2 Enrichment on Soil Carbon Beneath a Wheat Crop
Martens, R., Heiduk, K., Pacholski, A. and Weigel, H.-J. 2009. Repeated 14CO2 pulse-labelling reveals an additional net gain of soil carbon during growth of spring wheat under free air carbon dioxide enrichment (FACE). Soil Biology & Biochemistry 41: 2422-2429.

The authors write that "the generally higher above and belowground productivity of C3 plants under elevated CO2 leads to the conclusion that more rhizodepositions (roots and exudates) are transferred into soils, potentially increasing soil carbon content," but they note that most free-air CO2-enrichment (FACE) and outdoor chamber studies have failed to detect significant changes in soil organic carbon (SOC) due to the typically large amount and spatially heterogeneous nature of pre-existing SOC.

What was done
In an attempt to overcome these difficulties, Martens et al. cultivated well watered and fertilized spring wheat (cv. Minaret) within stainless steel cylinders forced into the soil of control and free-air CO2-enriched (to 180 ppm above ambient) FACE plots at the experimental farm of the Federal Research Institute in Braunschweig, Germany, where between stem elongation and beginning of ripening the plants were repeatedly pulse-labelled with 14CO2 and thereafter monitored daily for soil-borne total CO2 and 14CO2 until harvest, after which the distribution of 14C was analyzed in all plant parts, soil, soil mineral fractions and soil microbial biomass.

What was learned
The four researchers report that "in comparison to ambient conditions, 28% more 14CO2 and 12% more total CO2 was evolved from soil under elevated CO2," and that "in the root-free soil 27% more residual 14C was found in the free-air CO2-enriched soil than in the soil from the ambient treatment." In addition, they say that in soil samples from both treatments about 80% of residual 14C was "integrated into the stable, clay bound soil organic matter pool," which suggests, in their words, that "under FACE conditions a considerable contribution was made to the long-term storage of soil carbon in this soil."

What it means
Martens et al. were able to "show for the first time," as they describe it, "that a crop plant grown under FACE conditions deposited significantly more carbon to soil than those grown under ambient CO2 in the field," and that "the additional carbon input under elevated CO2 did not induce an accelerated degradation of pre-existing soil organic matter (no positive priming effect)," thereby demonstrating that "wheat plants grown under elevated CO2 can contribute to an additional net carbon gain in soils," which is especially good news for the biosphere.

Reviewed 17 February 2010