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

Carbon input into the soil and decomposition processes 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 14C-labeled at 350 and 700 mL 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% lower than that of 350 roots. The 14C-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 14C- 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 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. | Carbon input into the soil and decomposition processes under elevated CO2 are highly relevant for C sequestering in the soil. In the study reported here, plants were grown and homogeneously 14C-labeled in growth chambers at two different CO2 concentrations and two N supply regimes. After labeling, the roots were harvested, mixed into fresh soil and put into soil containers. Several soil containers were planted while others were kept bare. The soil containers were placed outside in wind tunnels containing ambient CO2 on elevated CO2. The decomposition rates of the added 14C-labeled roots and the formation of the soil microbial biomass 14C in the soil were monitored for two growing seasons.