Diversity and Activity of Microbes Oxidizing Methane and Ammonium in Northern Organic Soils under Changing Environmental Conditions

Abstract

Microbial methane and ammonium oxidation in soil has a great importance in the atmospheric gas composition. Peat soils, typical in northern latitudes, are major sources of atmospheric methane as a result of methane producing microbes they harbour. Only a part of the methane produced in anaerobic peat is emitted to atmosphere because of microbial methane oxidation in the uppermost aerobic peat profile. Lowering of water table e.g., for agricultural or forestry purposes, changes the relative activity of methane producing and oxidizing microbes. Drainage generally decreases methane emissions. Nitrogen-rich peat soils are potential sources of nitrous and nitric oxides. Microbes oxidizing ammonium are among the key organisms responsible for the production of nitrogenous oxides. In natural water-logged peat, the production of nitrous and nitric oxides is negligible because activities of nitrifying and denitrifying bacteria are low. Drainage of peat soils for agriculture largely increases decomposition of organic matter and activity of nitrifying and denitrifying bacteria. Among soil ecosystems, organic agricultural soils have the highest nitrous oxide production. Drainage for forestry also induces some nitrous oxide production, especially in the most nitrogen-rich sites. There is a risk that global warming will change the hydrology of northern peatlands and their methane and nitrous oxide dynamics because the microbial processes are closely associated to the hydrological characteristics of peatlands. The global importance of methane and ammonium oxidation in northern organic soils is known but the microbes responsible for these processes are poorly characterized. The isolation of these microbes has proven to be difficult, there are only few isolates. However, PCR amplification and sequencing of DNA extracted from natural and manipulated organic soils suggest a great diversity among these microbial populations. These studies are based on the gene sequences of 16S rDNA, or the genes encoding for the ammonia monooxygenase (AMO) of ammonium-oxidizing bacteria, and the methane monooxygenase (MMO) of methane-oxidizing microbes. The results obtained by molecular biological techniques strongly suggest that in acidic organic soils there are novel organisms responsible for ammonium and methane oxidation. The effect of environmental changes on the diversity of these microbes is unknown. Some recent results indicate that there are high similarities in microbial populations oxidizing ammonium and methane in various northern organic soils although their hydrological and nutritional conditions differ greatly.