Methane oxidation in temperate and tropical soils

Rates of methane (CH4) oxidation (-) and emission were measured from a range of soil types and land uses to investigate the soil and environmental variables which affected the land-atmosphere exchange of CH4. The influence of soil characteristics, land use, inputs of nitrogen (N) and sulphur (S), and temporal variations in soil water content and temperature on CH4 flux were examined in the field using a static chamber technique, and in the laboratory using soil cores. The flux of CH4 was measured from a range of sites in the UK, Cameroon and Borneo, including temperate and tropical forest, agricultural land, moorland and blanket bog. In mineral soils, rates of CH4 oxidation ranged from a maximum uptake of -27.2 ng m 2 s in an undisturbed forest in Cameroon, to a small net emission of 0.8 ng m 2 s' from an agricultural field in Scotland. Rates of CH4 oxidation, both in temperate and tropical climates, were found to be inversely correlated with soil bulk density, indicating that the rate of gaseous diffusion of CH4 to methanotrophs was the primary factor controlling rates of CH4 oxidation. Soil bulk density may therefore be used to predict rates of CH4 oxidation: CH4 oxidation rate (ng m 2 s') = 25.1 x bulk density (g cm 3) 33.6 (r2 = 0.786, p<O.00l). Seasonal variability in CH4 flux was observed and was determined by both soil water content and temperature. The influence of each variable was related to the soil type. Land use strongly affected CH4 flux with rates of CH4 oxidation from disturbed sites (logged forest or agricultural land) between 10 and 90 % smaller relative to undisturbed sites. Compaction (and the associated affects on the soil structure) and fertilisation (and resultant changes on N turnover) appeared to be the main factors responsible for the inhibition. Inputs of N between 40 and 260 kg N ha y, through pollutant deposition and fertilisation respectively, were observed to inhibit CH4 oxidation rates by up to 65%. Laboratory experiments showed that rates of CH4 oxidation were inhibited by up to 87% following application of various N compounds (40 kg N ha y), however, it was observed that the application of an equivalent amount of NaCl resulted in a similar inhibition. In peats, soil water content controlled the magnitude and direction of flux within sites by affecting the degree of anaerobicity of the peat and hence the depth of the CH4 oxidising layer. Other factors such as peat depth and substrate quality influenced inter-site variability. Water content determined seasonal variability in CH4 flux. However, soil temperature was also shown to have a strong effect on CH4 emission rates from peat monoliths when the water table was at a constant height (Ea's between 50 and 75 Id Vascular transport was found to be important in the transport of CH4 from zones of CH4 production to the atmosphere. The largest CH4 emission rate at 780.4 ng m 2 s was observed through the vascular plant Menvanthes tr? foliata in pool areas of the blanket bogs in Caithness. S application (100 kg S ha' y') to peat monoliths, which were net emitters of CH4, inhibited emission rates by up to 50% as a result of competitive inhibition between methanogens and sulphate-reducing microorganisms. The contribution of CH4 produced by termites to the CH4 budget was investigated in undisturbed and disturbed forests in Cameroon and Borneo. In Cameroon a positive correlation was observed between termite biomass in the soil and the flux of CH4 from or to the forest floor, with the direction of flux changing from a net sink to a source for CH4 at a termite biomass greater than between 3.8 and 18.0 g m 2, depending on the time of year and level of disturbance. Large rates of CH4 emission up to 2000 ng s.1 mound were observed from individual termite mounds. CH4 budgets were calculated based on surveys of the mounds and CH4 flux measurements from the forest floor and from the mounds. All forest sites were net sinks for CH4 at between -3.05 and -6.15 kg ha'y 1 . However, termite CH4 emissions and deforestation significantly mediated the soil sink strength by up to 28 and 35% respectively. In summary, rates of CH4 oxidation from mineral soils were low and covered a small range relative to CH4 emission rates from peat. Rates of CH4 oxidation were significantly inhibited by anthropogenic disturbance such as deforestation, conversion to agriculture, and inputs of N. Spatial variability was controlled by the bulk density of the soil, both in temperate and tropical climates, demonstrating the importance of the ga seous diffusion status of the soil and the secondary role of temperature in regulating oxidation rates. Seasonal variability affected both CH4 emission and oxidation rates. The relative influence of soil water content and temperature on the CH4 flux varied between sites and was dependent on the soil type.