Boreal wetlands are thought to be a large source of atmospheric methane, but this idea is based on very few measurements. Thus, a regional survey in the low boreal forest region of central Ontario, Canada, consisting of 24 sites over 12 wetlands and three beaver ponds was conducted to determine the temporal and spatial trends in emissions and the net annual methane (CH4) flux. Conifer swamps represented nearly 50 percent of the wetland coverage, but emit a small amount of CH4 (seasonal means less than 8 mg/sq m per d). The significant emitters of CH4, in order from highest to lowest seasonal means, were beaver ponds (30-90 mg/sq m per d), thicket swamps (0.1-88 mg/sq m per d), and bogs (6-21 mg/sq m per d). Mixed swamps, marshes, and fens emitted very little CH4 (less than 3 mg/sq m per d). Moisture saturation was the key determinant of high emissions, and, when satisfied, differences in emissions could be explained by peat and sediment temperatures. On the basis of the areal extent of wetlands from peatland inventories, it is calculated that the low boreal region of Canada contributes approximately 0.15 Tg CH4/yr to the atmosphere. This is an order of magnitude lower than the flux would be using the estimate of Aselmann and Crutzen (1989) for the same boreal region.

Measurements of carbon-14 in small samples of methane from major biogenic sources, from biomass burning, and in clean air samples from both the Northern and Southern hemispheres reveal that methane from ruminants contains contemporary carbon, whereas that from wetlands, peat bogs, rice fields, and tundra, is somewhat depleted in carbon-14. Atmospheric (C-14)H4 seems to have increased from 1986 to 1987, and levels at the end of 1987 were 123.3 + or - 0.8 percent modern carbon in the Northern Hemisphere and 120.0 + or - 0.7 percent modern carbon in the Southern Hemisphere.

Measured rates of soil respiration from terrestrial and wetland ecosystems are reviewed to define the annual global CO2 flux from soils, identify uncertainties in the global flux estimates, and to investigate the influences of temperature, precipitation, and vegetation on soil respiration rates. The annual global CO2 flux from soils is estimated to average 68 +/- 4 PgC/yr, based on extrapolations from biome land areas. On a global scale, soil-respiration rates are positively correlated with mean annual air temperatures and mean annual precipitation. There is a chosen correlation between mean annual net primary productivity of different vegetation biomes and their mean annual soil respiration rates. Estimates of soil C turnover rates range from 500 years in tundra and peaty wetlands to 10 years in tropical savannas. The impacts of human activities on soil-respiration rates are poorly documented, and vary among sites. Of particular importance are potential changes in temperatures and precipitation. Based on a review of in situ measurements, the Q10 value for total soil respiration has a median value of 2.4. Increased soil respiration with global warming is likely to provide a positive feedback to the greenhouse effect.