Decomposability of C3 and C4 grass litter sampled under different concentrations of atmospheric carbon dioxide at a natural CO2 spring

Elevated concentrations of atmospheric CO2 can influence the relative proportions, biomass and chemical composition of plant species in an ecosystem and, thereby, the input of litter nutrients to soil. Plant growth under elevated CO2 appears to have no consistent effect on rates of litter decomposition; decomposition can, however, differ in C3 and C4 plant material from the same CO2 environment. We here describe the decomposability of leaf litter of two grass species - the C3 Holcus lanatus L. (Yorkshire fog) and C4 Pennisetum clandestinum Hochst. (kikuyu) - from an unfertilized, ungrazed grassland at a cold CO2 spring in Northland, New Zealand. Decomposability was measured by net CO2-C production from litter incubated for 56 days at 25 degreesC in a gley soil from the site; net mineral-N production from litter was also determined. Both litter and soils were sampled under `low' and `high' concentrations of atmospheric CO2. Decomposition of H. lanatus litter was greater than that of P. clandestinum litter throughout the 56-day incubation. Decomposition tended to be greater in `high-CO2' than in `low-CO2' H. lanatus litter, but lower in `high-CO2' than `low-CO2' P. clandestinum litter; differences were, however, non-significant after 28 days. Overall, litter decomposition was greater in the `low-CO2' than `high-CO2' soil. Differences in decomposition rates were related negatively to litter N concentrations and positively to C:N ratios, but were not predictable from lignin:total N ratios. Net mineral-N production from litter decomposition did not differ significantly in `high-CO2' and `low-CO2' samples incubated in `low-CO2' soil; in `high-CO2' soil some net immobilization was observed. Overall, results indicate the likely complexity of litter decomposition in the field but, nevertheless, strongly suggest that rates of decomposition will not necessarily decline in a `high-CO2' environment.