Soil consumption of atmospheric methane: Importance of microbial physiology and diversity

Abstract

Recently, atmospheric CH$\sb4$ concentration has risen dramatically, apparently due to human activities. Since is CH$\sb4$ is involved in several atmospheric processes that regulate Earth's climate, it is important that we understand the factors that control its atmospheric concentration. One such factor is biological CH$\sb4$ consumption in well-drained soils. Although this sink may comprise nearly one-tenth of the annual destruction of atmospheric CH$\sb4$, We know relatively little about it. I conducted a research project to investigate the influences of CH$\sb4$ supply, soil moisture, dissolved salts, and NH$\sb4\sp+$-fertilizer on the activity of soil CH$\sb4$ oxidizers. When starved of CH$\sb4$, two upland taiga soils gradually lost their capacities to oxidize CH$\sb4$, indicating that the process was not merely fortuitous, and that the organisms involved were truly methanotrophic. The relationship between soil moisture and CH$\sb4$ consumption was parabolic, with maximum oxidation occurring at a moisture level that achieved the maximum possible CH$\sb4$ diffusion rate, while minimizing water stress on the methanotrophs. Optimal soil moisture occurred in a relatively narrow range among an array of physically dissimilar soils, providing that moisture content was expressed as a percentage of the water holding capacity fo a particular soil, rather than as absolute water content. In recent years, one of the most intensely investigated controls on soil CH$\sb4$ consumption has been its inhibition by NH$\sb4\sp+$-fertilizer. In addition to NH$\sb4\sp+,$ however, I found that other ions inhibited CH$\sb4$ oxidation. In some soils non-NH$\sb4\sp+$ ions were so toxic that they completely masked the NH$\sb4\sp+$ effect. It is crucial, therefore, to control for salt effects when investigating NH$\sb4\sp+$-inhibition. In both field and laboratory experiments, CH$\sb4$ consumption in a birch soil was sensitive to NH$\sb4\sp+$, whereas a spruce soil was unaffected. In the birch soil, NH$\sb4\sp+$ apparently inhibited methanotroph growth, rather than enzymatic CH$\sb4$ oxidation, whereas methanotrophs in the spruce soil were apparently insensitive to NH$\sb4\sp+$. These results suggest that the primary landscape-level control over the response of soil CH$\sb4$ consumption to NH$\sb4\sp+$-fertilization is the cross-site distribution of physiologically distinct CH$\sb4$ oxidizers.