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 CO{sub 2} levels in the atmosphere. Rising temperatures are supposed to increase decomposition of soil organic C leading to increased production of CO{sub 2} and this extra CO{sub 2} 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 CO{sub 2} rises and these roots decompose more slowly than roots grown at ambient CO{sub 2} levels. Experimental data partly obtained with {sup 14}C-techniques were applied in a grassland C model. The model results show that at an atmospheric CO{sub 2} concentration of 700 {micro}L L{sup {minus}1} increased below ground 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 CO{sub 2} concentration has been reached, C equivalent to 55% of the annual CO{sub 2} increase above 1 ha ryegrass can be withdrawn from the atmosphere. This indicates that grassland soils represent a significant sink for rising atmospheric CO{sub 2}.