Decomposition of leaf and root tissue of three perennial grass species grown at two levels of atmospheric CO2 and N supply

Leaf and root tissue of Lolium perenne L., Agrostis capillaris L. and Festuca ovina L. grown under ambient (350 μl l-1 CO2) and elevated (700 μl l-1) CO2 in a continuously 14C-labelled atmosphere and at two soil N levels, were incubated at 14°C for 222 days. Decomposition of leaf and root tissue grown in the low N treatment was not affected by elevated [CO2], whereas decomposition in the high N treatment was significantly reduced by 7% after 222 days. Despite the increased C/N ratio (g g-1) of tissue cultivated at elevated [CO2] when compared with the corresponding ambient tissue, there was no significant correlation between initial C/N ratio and 14C respired. This finding suggests that the CO2-induced changes in decomposition rates do not occur via CO2-induced changes in C/N ratios of plant materials. We combined the decomposition data with data on 14C uptake and allocation for the same plants, and give evidence that elevated [CO2] has the potential to increase soil C stores in grassland via increasing C uptake and shifting C allocation towards the roots, with an inherent slower decomposition rate than the leaves. An overall increase of 15% in 14C remaining after 222 days was estimated for the combined tissues, i.e., the whole plants; the leaves made a much smaller contribution to the C remaining (+6%) than the roots (+26%). This shows the importance of clarifying the contribution of roots and leaves with respect to the question whether grassland soils act as a sink or source for atmospheric CO2.