Elevated atmospheric CO2 affects decomposition of Festuca vivipara (L) Sm litter and roots in experiments simulating environmental change in two contrasting arctic ecosystems

Mass loss, together with nitrogen and carbon loss, from above-ground material and roots of Festuca vivipara were followed for 13 months in a high Arctic polar semidesert and a low Arctic tree-line dwarf shrub heath. Festuca vivipara for the study was obtained from plants cultivated at two different CO2 concentrations (350 and 500 mu L L(-1)) in controlled environment chambers in the UK. Each of the four resource types (shoots or roots from plants grown in elevated or ambient CO2 concentrations) was subsequently placed in an experiment simulating aspects of environmental change in each Arctic ecosystem. Air, litter and soil temperatures were increased using open-topped polythene tents at both sites, and a 58% increase in summer precipitation was simulated at the high Arctic site. Mass loss was greatest at the low Arctic site, and from the shoot material, rather than the roots. Shoots grown under an elevated CO2 concentration decomposed more slowly at the high Arctic site, and more quickly at the low Arctic one, than shoots grown at ambient CO2. After 13 months, greater amounts of C and N remained in above-ground litter from plants grown under elevated, rather than ambient, CO2 at the polar semi-desert site, although lower amounts of C remained in elevated CO2 litter at the low Arctic ecosystem. In the high Arctic, roots grown in the 500 mu L L(-1) CO2 concentration decomposed significantly more slowly than below-ground material derived from the ambient CO2 chambers. Elevated CO2 concentrations significantly increased the initial C:N ratio, % soluble carbohydrates and alpha-cellulose content, and significantly decreased the initial N content, of the above-ground material compared to that derived from the ambient treatment. Initially, the C:N ratio and percentage N were similar in both sets of roots derived from the two different CO2 treatments, but soluble carbohydrate and or-cellulose concentrations were higher, and percentage lignin lower, in the elevated CO2 treatments. The tent treatments significantly retarded shoot decomposition in both ecosystems, probably because of lower litter bag moisture contents, although the additional precipitation treatment had no effect on mass loss from the above-ground material. The results suggest that neither additional summer precipitation (up to 58%), nor soil temperature increase of 1 degrees C, which may occur by the end of the next century as an effect of a predicted 4 degrees C rise in air temperature, had an appreciable effect on root decomposition in the short term in a high Arctic soil. However, at the low Arctic site, greater root decomposition, and a lower pool of root N remaining, were observed where soil temperature was increased by 2 degrees C in response to a 4 degrees C rise in air temperature. These results suggest that decomposition below-ground in this ecosystem would increase as an effect of predicted climate change. These data also show that there is a difference in the initial results of decomposition processes between the two Arctic ecosystems in response to simulated environmental change.