Carbon mineralization in an organic soil, with and without added grass litter, from a high-CO2 environment at a carbon dioxide spring

Both CO2–C production and the decomposability of grass leaf litter in a gley soil from a naturally occurring CO2 spring were previously shown to be influenced by the atmospheric CO2 concentrations under which the soil and litter were sampled. Here we investigate C mineralization in an organic soil from very high CO2 environments (range 1220–3900 μl l−1) at the same spring, and the effect of added leaf litter on CO2–C production. Carbon mineralization in the organic soil was unusual in two respects: (1) the proportion of labile components was very high, with more than 11% of the initial soil C being metabolized to CO2–C after 56 d at 25 °C; (2) rates of CO2–C production in autumn samples increased on incubation, after an initial decline. Decomposition was initially more rapid in C3 Holcus lanatus (Yorkshire fog) than in C4 Pennisetum clandestinum (kikuyu) litter, but differed little in samples from different atmospheric CO2 concentrations. Overall, the effects of environmental variables on estimates of litter decomposability in the organic soil were similar to, although much less marked than, those in the gley soil. Results suggest that organic components in the organic soil were metabolized at least as readily as those of added litter during the later stages of the 56-d incubation.