Lolium perenne grasslands may function as a sink for atmospheric carbon dioxide

Model calculations and scenario studies suggest the existence of a considerable positive feedback between temperature and CO2 levels in the atmosphere. Rising temperatures are supposed to increase decomposition of soil organic C leading to increased production of CO2 and this extra CO2 induces a positive feedback by raising the temperature still further. Evidence was found that negative feedback mechanisms also exist: more primary production is allocated to roots as atmospheric CO2 rises and these roots decompose more slowly than roots grown at ambient CO2 levels. Experimental data partly obtained with C-14-techniques were applied in a grassland C model. The model results show that at an atmospheric CO2 concentration of 700 mu L L-1 increased belowground C storage will be more than sufficient to balance the increased decomposition of soil organic C in a ryegrass (Lolium perenne L.) grassland soil. Once a doubling of the present atmospheric CO2 concentration has been reached, C equivalent to 55% of the annual CO2 increase above 1 ha ryegrass can be withdrawn from the atmosphere. This indicates that grassland soils represent a significant sink for rising atmospheric CO2.