Specificity of soil functioning and formation on gas-bearing areas

Background, aim, and scope Exploited gas fields and underground gasholders are specific sources of increasing methane concentration. Methane migrates into the soils by diffusion and convection through natural and technogenic cracks in geological structures and influences the function of the soils. Soil cover of gas-bearing area functions as a specific, bilateral, periodically penetrating, geomembrane. Soils shield, transform, and differentiate migrating fluxes of technogenic-allochthonous methane, preventing its emission to the atmosphere. Problems of methane's emission are rather current at the present, as methane is the second in importance after CO₂ greenhouse gas, since its concentration in the atmosphere annually grows by approximately 1%. By global estimations, methane emissions in the gas industry make about 8% of annual receipt to the atmosphere, equal on the average to 500 Тg per a year (Cicerone and Oremland, Global Biogeochem Cy 2:299-327, 1988). But these calculations are based on the account of the technological losses making 3-12% from the mining of natural gas. The contribution of migratory methane fluxes to the atmosphere, as a rule, is not considered. The need for research of soil cover functioning on gas-bearing areas is explained by the fact that processes of methane oxidation, its transformation in soils, and emission to the atmosphere at these objects are now practically not being studied. The aim of our study was to reveal specific processes of soil function and formation on gas-bearing areas by an example of underground gasholder. Materials and methods The material was sampled in 1998-2003 at the territory of underground gasholder located in Albeluvisol's zone in Russia. According to the comparative-geographical method, 51 soil profiles have been studied in similar litologically geomorphological conditions in various geochemical zones: in the industrial zone, in the zone of gas dissipation, and at the regional background. The total square of investigated territory is about 60 km². Six soil profiles were investigated in seasonal dynamics. Samples of soils for physical, chemical, and microbiological analyses were taken from each horizon of soil profiles (202 samples). Samples of soil air for a definition of methane concentration were taken from depths of 20, 40, and 60 cm. Methane emission to the atmosphere was measured near soil's cuts and, in addition, on all area of the investigated territory at knots of squares network through 700-1,000 m, in total at 32-42 points in May, July, and November. Years of investigation have been split by technological and hydrothermal conditions. The periods with the normal and lowered compression of gas in gasholder, dry and warm, and damp and cool years have been allocated. It has influenced the soil function processes and considered an interpretation of the data received. Results The changes of functional parameters of soils at a gas-bearing area influenced by methane fluxes migrating from gas deposits, in comparison with background soils, are revealed. Such functional parameters are methane concentration in the soils, activity of its bacterial oxidation, methane emission to the atmosphere, and oxidation-reduction potential. Spatial and temporary dynamics of these parameters at gas-bearing and background territory are investigated. Discussion Methane interaction with soil's air is in its ascending (descending) and lateral diffusion and convection in soils. Methane fluxes dissipate in porous space of soils forming gas anomalies. The technogenic-allochthonous methane concentration strongly varies in soil's air on gas-bearing area (1-10,500 ppm) and, on average, exceeds the autochthonous, microbiologically produced methane at background territories. Migratory methane is deposited on diffusion and sorption barriers. The capacity of diffusion barrier depends on effective coefficient of diffusion, the attitude of air and general porosity, and granulometric composition and sharply differs in auto-, semi-hydro-, and hydromorphic soils reaching maximum in hydromorphicity and among the soils with identical water content--in heavy soils. The capacity of the sorption barrier is defined by abiotic methane absorption and a specific surface of soils and grows with their increasing intensity in soils to a heavier granulometric composition or into soils with peat and gleyic horizons. The low sorption capacity leads to an increase of methane concentration in the soil's air and decreases its utilization by microorganisms, in which its quantity depends on sorption properties. The central component of functioning that promotes a number of essential transformations in soils on gas-bearing areas is methane interaction with the biotic phase. The periods of methane deposition by diffusion and sorption barriers are used for biological methane oxidation and formation of biogeochemical barriers in soils. The activity of bacterial methane oxidation is characterized by spatial variability and depends on the entrance of methane, defined by granulometric composition, soil moisture, the attitude of air and general porosity, Eh, organic matter content, and salinization. During interaction between technogenic-allochthonous methane and soil on diffusion, sorption, and biogeochemical barriers, its transformation occurs, accompanied by a strengthening of variability of oxidation-reduction potential and formation of pedogenic, bacteriomorphic, and nanodispersic magnetic oxides of iron. Conclusions and perspectives Specificity of soil functioning on a gas-bearing area is in interaction of technogenic-allochtonous methane with solid, liquid, gaseous, and living substance of the soil system. Spatial laws of soils functioning on gas-bearing area in the Albeluvisol's zone are revealed. Distinctions of soil functions depending on litologically geomorphological conditions are shown. The greatest changes of parameters of functioning under the influence of technogenic-allochthonous methane occur in automorphic soils, and it is less in semi-hydromorphic soils. Activity of bacterial methane oxidation in soils, emission, and consumption from the atmosphere and their spatial laws are characterized by the time dynamics depending on hydrothermal and technological conditions of seasons and years. During oxidation in soils of gas-bearing areas, carbon of methane is concentrated on a biogeochemical barrier that is shown in the increase of methylotrophic microorganisms' biomass and leads to a high variability and decrease of Eh and to the formation of magnetic oxides of iron. Recommendations Results of research can be used for carrying out ecological monitoring and an estimation of tightness of objects of the gas industry. Activity of bacterial methane oxidation, Eh, and magnetic oxides of iron can be used as diagnostic parameters of soils on gas-bearing areas.