Smith, K. A., G. P. Robertson, and J. Melillo. 1994. Exchange of trace gases between the terrestrial biosphere and the atmosphere in the midlatitudes. Pages 179-203 in R. G. Prinn, ed. Global Atmospheric-Biospheric Chemistry. Plenum Press, New York, New York.

Most terrestrial ecosystems of the midlatitudes have been subjected to human influence. Large areas of forests and grasslands have been converted to agriculture; conversely, reforestation is also extensive in many regions. These changes in land use, the use of fertilizers on agricultural land, and high precipitation inputs of nitrogen from industrial emissions to forests and other natural ecosystems, all have consequences for trace gas exchange, and thus for the atmospheric concentrations of trace gases that contribute to climate change.

The study of these interactions, on a comprehensive world-wide basis, is the task of the IGAC Activity Trace Gas Exchange in Midlatitude Ecosystems (TRAGEX). Although TRAGEX as a coordinated program is still at the planning stage, there is a substantial current research effort in progress in North America, Western Europe and Australia that has already provided some insight into the most important processes governing trace gas exchange in the midlatitudes. Some of the highlights of this work, and major outstanding questions, are outlined in this paper.

Soil carbon availability, temperature/moisture interactions, pH, and nutrient dynamics have been identified as key variables in methane emission from temperate wetlands, but measurement and modeling processes of transfer to the atmosphere are in their infancy. Inputs of nitrogen and restrictions in aeration of topsoils have been shown to reduce very substantially the soil’s capacity to act as a sink for CH4 by microbial oxidation to CO2. High N inputs promote emissions of N2O, but it has been shown that large effects on fluxes arc caused by variations in soil physical conditions, the chemical form of the N, and soil pH. The very high spatial and temporal variability of N2O and CH4 fluxes have made representative flux estimates very difficult to make, and have stimulated major research efforts in development of improved methods of analysis applicable to areas of 103 to 104 m2 and ultimately to the km 2 scale. The linking of process models with the Global Information System (GIS) for large-scale integration is a focus of current planning activity.

Increased CO2 concentrations and N deposition, and the fact that much of the present forest area of the region is in a mid-successional stage, suggest that there may be a substantial vegetation-related sink for CO2 in the midlatitudes. Changes in soil organic matter in areas of reforestation and in response to changing agricultural practices may also represent an important contemporary sink for atmospheric CO2. Feedbacks associated with CO2 uptake by vegetation, its release during decomposition, and the nitrogen cycle processes that affect plant and microbial growth-including trace gas production-must be better understood to appreciate fully the significance of changing CO2 sink strengths.

High priority must now be given to three areas: (1) the establishment of flux measurement networks in important mid-altitude regions, especially in those such as the former Soviet Union, China, temperate South America, where comparatively little data have been obtained; (2) the development of adequate process-based models; and (3) the scaling-up of the models to predict fluxes over large regions. This should lead to a substantial improvement in the quality of the input to climate models.

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