Carbon dioxide and methane formation in forest mineral and peat soils during aerobic and anaerobic incubations

Mineral and peat forest soils, sampled at three depths, were incubated aerobically and anaerobically at 16 degrees C for 26 wk and were monitored for CH4 and CO2 formation. Mean CO2 production rates were similar under aerobic and anaerobic conditions in mineral subsoils with very low organic matter content, whereas in organic soils the rate was about 10 times greater under aerobic than anaerobic conditions. The correlations coefficient (r) between mean aerobic and mean anaerobic production rates was 0.98, for the entire data set. Mineral and organic samples incubated anaerobically both showed a decline in CO2 production rate with increasing time of incubation. The decline was more strongly related to incubation time than to the absolute concentrations of CO2 in the vials. The rates after 19 wk were approx. 10% of the initial rates. This inhibition was largely offset by flushing the vials with N2 gas, indicating the volatile nature of the inhibiting substances. Although CO2 production rates increased immediately upon flushing, there were still weak indications of a residual inhibiting effect. That, together with the rapid return of inhibition, suggested that non-volatile substances were also involved. One implication of these results is that the activity in anaerobic field soils very much depends on the removal of the inhibiting agents, e.g. by aeration during periodic draining of soil layers and by the movement of water through the soil. Methane formation occurred in most mor layers and peats. A well humified wood-Cyperaceae peat was, however, a notable exception. Methane production rates increased during the experimental period. The increasing CH4 production rates, and the final (26 wk) measured rates, were approximately proportional to the frequency of anaerobic conditions in the original field situation.