Decomposition of 13C-labelled wheat root systems following growth at different CO2 concentrations

We tested whether the amounts of carbon (C) mineralized from decomposing wheat (Triticum aestivum L. cv. Tonic) roots were related to the quantity (i.e. root dry weight per plant) or the chemical composition of material which had been grown at ambient or elevated CO2 concentrations (350 or 700 μmol CO2 mol−1). Plants were grown in 13C-depleted CO2 to distinguish root-derived C from soil-derived C. Over periods of up to ca. 400 d, root C, soil C and nitrogen (N) mineralization were measured from: (i) root systems left in situ in soil; (ii) soil after removal of visible roots; and (iii) equal amounts of roots added to fresh soil. Root systems in situ showed transiently faster C mineralization rates after growth at elevated [CO2] compared with ambient [CO2]. Ultimately, there were no [CO2]-related differences in the amounts of C or N mineralized from root systems in situ. Specific rates of C loss from extracted roots were not significantly different for roots from the two [CO2] treatments. The potential accuracy of the 13C method was demonstrated and 13C/12C fractionation during root decomposition was negligible. We conclude that when wheat is grown under elevated [CO2], subsequent root decomposition will not necessarily be affected. If it does, it is likely to do so via an effect of [CO2] on the amounts of root material produced per unit of soil rather than on the chemical quality of that material.