Jannette MacDonald

Methane emission rates from a northern wetland; response to temperature, water table and transport

Static chamber measurements of CH4 flux were made from a range of micro-environments in an area of blanket bog in Northern Scotland. CH4 flux covered a wide range, the largest rate of CH4 emission, at 175.6 μmol m−2 h−1, was observed in pool areas through the vascular plant Menyanthes trifoliata. Investigations into the response of net CH4 emission rates to temperature and water table were carried out under semi-natural conditions on 45 large peat monoliths, maintained in open-top chambers, over a three-year period. The mean rate of CH4 emission at 10°C was an order of magnitude larger from pool monoliths (surface water table) at 78.0 μmol m−2 h−1, than from hummock monoliths (water table 15 cm below surface) at 8.4 μmol m−2 h−1. Rates of CH4 emission showed a positive linear response to increasing temperature from pool and lawn monoliths with activation energies of 74.3 and 79.5 kJ mol−1 and Q10 values of 3.0 and 3.3, respectively. When conditions of temperature, water table, light and humidity were controlled pool cores showed an exponential increase in CH4 emission rates between 5 and 30°C.

Soil environmental variables affecting the flux of methane from a range of forest, moorland and agricultural Soils

Measurements of the net methane exchange over a range of forest, moorland, and agricultural soils in Scotland were made during the period April to June 1994 and 1995. Fluxes of CH4 ranged from oxidation −12.3 to an emission of 6.8 ng m−2 s−1. The balance between CH4 oxidation and emission depended on the physical conditions of the soil, primarily soil moisture. The largest oxidation rates were found in the mineral forest soils, and CH4 emission was observed in several peat soils. The smallest oxidation rate was observed in an agricultural soil. The relationship between CH4 flux and soil moisture observed in peats (FluxCH4 = 0.023 × %H2O (dry weight) − 7.44, p > 0.05) was such that CH4 oxidation was observed at soil moistures less than 325%( ± 80%). CH4 emission was found at soil moistures exceeding this value. A large range of CH4 oxidation rates were observed over a small soil moisture range in the mineral soils. CH4 oxidation in mineral soils was negatively correlated with soil bulk density (FluxCH4 = −37.35 × bulk density (g cm−3) + 48.83, p > 0.05). Increased nitrogen loading of the soil due to N fixation, atmospheric deposition of N, and fertilisation, were consistently associated with decreases in the soil sink for CH4, typically in the range 50 to 80%, on a range of soil types and land uses.

Global Impact of Termites on the Carbon Cycle and Atmospheric Trace Gases

Termites have high biomass in many tropical ecosystems and emit the greenhouse gases CO2 and CH4. They are also recognized as ecosystem engineers, mediating decomposition and other aspects of soil function. Therefore, termites may be significant contributors to biogeochemical cycles, notably those of carbon and methane. We review methods of assessing carbon fluxes through termite populations and argue that direct measurements of net CO2 and CH4 emissions from termites in natural settings (in their nests or in the soil) are the best data for scaling-up calculations, if accompanied by accurate estimates of biomass and assemblage feeding-group composition. Actual determinations of gas fluxes from termites, and the attendant computation of regional and global budgets made over the past two decades are reviewed. For CO2, it is concluded that termites contribute up to 2% of the natural efflux from terrestrial sources, a large contribution for a single animal taxon, but small in the global context. For CH4, we note that calculations are still hampered by uncertainties over termite biomass distribution and a general failure to consider local and landscape-level oxidation by methylotrophic microorganisms as a factor mitigating net fluxes. Nevertheless the balance of evidence, including new data on local oxidation, suggests that annual contributions by termites are almost certainly less than 20 Tg, and probably less than 10 Tg (ca. 4% and 2% of global totals from all sources, respectively). Climate changes and land use intensification may cause minor modifications of the overall distribution of termites, but a more serious impact on soil stability and function could result from changes in the balance of feeding groups. The response of termites to changes in the quality and quantity of plant litters is uncertain, but direct effects from elevated atmospheric CO2 are unlikely. Global changes will broadly favour wood- and litter-feeding termites over soil-feeders, but with regional differences and complications arising from patterns of landscape fragmentation and historical factors.