A LABORATORY STUDY OF FACTORS INFLUENCING METHANE UPTAKE BY SOILS

Concern that increasing atmospheric concentrations of CH4 will contribute to global climatic change has stimulated research to assess the potential of soils to serve as a sink for atmospheric CH4. Laboratory studies were performed to identify factors influencing CH4 uptake by soils collected from a variety of ecosystems. Freshly collected samples from wetland and forest systems were all capable of depleting atmospheric concentrations of CH4, whereas none of the samples collected from cultivated fields were capable of CH4-Uptake when incubated under near tropospheric concentrations of CH4. However, exposure of cultivated soils to high concentrations of CH4 (more than 1 000-mu-l l-1) for several days induced uptake. When active soils were exposed to elevated concentrations, their initial rates of CH4-Uptake also increased. Incubation of two active soils for periods of more than 12 h under atmospheres containing CH4 concentrations in excess of their K(m) values (51 and 192-mu-mol CH4) caused an exponential increase in the rate of CH4-uptake, whereas incubation under low CH4 concentrations caused a rapid decline in activity. Little difference in CH4-Uptake was observed when H2O content was adjusted between 25% and 75% of pore volume, but air-drying completely inhibited CH4-uptake. Saturation of soil decreased the average rate of CH4-uptake by 56%, presumably because of reduced gaseous diffusion. Optimal CH4 consumption occurred in samples incubated at temperatures between 20 and 30-degrees-C and was greatly reduced at 40-degrees-C. Amending soils with clover residues reduced CH4-uptake by an average of 43%. Also, addition of 7-mu-mol NH4+ g-1 inhibited CH4-uptake, whereas additions of 7-mu-mol NO3- g-1 had no effect. The inhibitory effects of NH4+ appear irreversible and persist after nitrification of added NH4+. Because of the association of methanotrophic activity with CH4 supply, recently drained or intermittently flooded soils are apt to display the greatest capacity for CH4-uptake.