Elevated CO2 effects on decomposition processes in a grazed grassland

The effects of elevated atmospheric CO2 (475 muL L-1) on in situ decomposition of plant litter and animal faecal material were studied over 2 years in a free air CO2 enrichment (FACE) facility. The pasture was grazed by sheep and contained a mixture of C-3 and C-4 grasses, legumes and forbs. There was no effect of elevated CO2 on decomposition within plant species but marked differences between species with faster decomposition in dicots; a group that increased in abundance at elevated CO2. Decomposition of mixed herbage root material occurred at a similar rate to that of leaf litter suggesting that any CO2-induced increase in carbon allocation to roots would not reduce rates of decomposition. Sheep faeces resulting from a 'high-CO2 diet' decomposed significantly slower during summer but not during winter. The overall outcome of these experiments were explored using scenarios that took account of changes in botanical composition, allocation to roots and the presence of herbivores. In the absence of herbivores, elevated CO2 led to a 15% increase in the rate of mass loss and an 18% increase in the rate of nitrogen (N) release. In the presence of herbivores, these effects were partially removed (11% increase in rate of mass loss and 9% decrease in N release rate) because of the recycling occurring through the animals in the form of faeces.