Short‐term decomposition of litter produced by plants grown in ambient and elevated atmospheric CO₂ concentrations

The effects of elevated atmospheric CO 2 (ambient + 340 μmol mol –1 ) on above‐ground litter decomposition were investigated over a 6‐week period using a field‐based mesocosm system. Soil respiratory activity in mesocosms incubated in ambient and elevated atmospheric CO 2 concentrations were not significantly different ( t ‐test, P > 0.05) indicating that there were no direct effects of elevated atmospheric CO 2 on litter decomposition. A study of the indirect effects of CO 2 on soil respiration showed that soil mesocosms to which naturally senescent plant litter had been added (0.5% w/w) from the C 3 sedge Scirpus olneyi grown in elevated atmospheric CO 2 was reduced by an average of 17% throughout the study when compared to soil mesocosms to which litter from Scirpus olneyi grown in ambient conditions had been added. In contrast, similar experiments using senescent material from the C 4 grass Spartina patens showed no difference in soil respiration rates between mesocosms to which litter from plants grown in elevated or ambient CO 2 conditions had been added. Analysis of the C:N ratio and lignin content of the senescent material showed that, while the C:N ratio and lignin content of the Spartina patens litter did not vary with atmospheric CO 2 conditions, the C:N ratio (but not the lignin content) of the litter from Scirpus olneyi was significantly greater ( t ‐test; P < 0.05) when derived from plants grown under elevated CO 2 (105:1 compared to 86:1 for litter derived from Scirpus olneyi grown under ambient conditions). The results suggest that the increased C:N ratio of the litter from the C 3 plant Scirpus olneyi grown under elevated CO 2 led to the lower rates of biodegradation observed as reduced soil respiration in the mesocosms. Further long‐term experiments are now required to determine the effects of elevated CO 2 on C partitioning in terrestrial ecosystems.