Long-term decomposition of grass roots as affected by elevated atmospheric carbon dioxide

Carbon input into the soil and decomposition professes under elevated CO2 are highly relevant for C sequestering in the soil. Plant growth and decomposition of root material under ambient and elevated atmospheric CO2 concentrations were monitored in wind tunnels. Grass roots (Lolium perenne L.) were homogeneously C-14-labeled at 350 and 700 mu L L(-1) CO2 and at two N levels to obtain roots of different qualities. This root material was mixed with fresh loamy sand and transferred to four wind tunnels to observe its decomposition in bare soil and as affected by plant growth (L. perenne) at ambient CO2 and elevated CO2 for two growing seasons. After the second growing season, elevated CO2 had stimulated shoot and root growth by 13 and 92%, respectively. The CO2 and N concentrations at which the grass roots had been grown affected the decomposition rate. After the first growing season, the overall decomposition of 700 roots was 19% loner than that of 350 roots. The C-14-labeled microbial biomass in the soil with 700 roots was higher (44%) compared with 350 roots. After the second growing season, the decomposition of 700 low N roots was 14% lower than that of 350 low N roots, whereas the decomposition of the high N roots was unaffected. The C-14-labeled microbial biomass in the soil with 700 roots was still higher (30%) than with 350 roots. The combination of higher root yields at elevated Co-2 combined with a decrease in root decomposition will lead to a longer residence Lime of C in the soil and probably to a higher C storage.