Patterns of Change in the Carbon Balance of Organic Soil-Wetlands of the Temperate Zone

Abstract

(1) Organic soil-wetlands, particularly those in the temperate zone, under natural conditions, are net carbon sinks and hence are important links in the global cycling of carbon dioxide and other atmospheric gases. Human alteration of wetlands has brought about shifts in the balance of carbon movement between the wetlands and the atmosphere. Because previous analyses have not fully considered these shifts, disturbance of carbon storage in organic soil-wetlands of the temperate zone has been analysed for the last two centuries and considered in relation to other sources of atmospheric CO2 from the biosphere. (2) Storage before recent disturbance is estimated as 57 to 83 Mt of carbon per year, over two-thirds of this in boreal peatlands. The total storage rate, lower than previous estimates, reflects accumulation rates of carbon of only 0.20 t ha-' yr-1 and less in the boreal zone where 90% of temperate organic soils are found. (3) Widespread drainage of organic soil-wetlands for agriculture has significantly altered the carbon balance. A computer model was used to track the consequent changes in the carbon balance of nine wetland regions. Drainage reduced or eliminated net carbon sinks, converting some wetlands into net carbon sources. Different regions thus can function as smaller carbon sinks, or as sources, depending on the extent of drainage. In either case a shift in carbon balance can be quantified. (4) The net carbon sink in Finland and the U.S.S.R. has been reduced by 21-33%, in Western European wetlands by nearly 50%, and in Central Europe the sink has been completely lost. Overall, by 1900 the temperate zone sink was reduced 28-38% by agricultural drainage alone. (5) By 1980 the total annual shift in carbon balance attributable to agricultural drainage was 63-85 Mt of carbon, 38% in Finland and U.S.S.R. wetlands, and 37% in Europe. Twenty-five percent of the shift occurred in North American wetlands south of the boreal zone. No apparent change occurred in boreal Canada and Alaskan wetlands. (6) Peat combustion for fuel released 32-39 Mt of carbon annually, nearly all in the U.S.S.R. A total of 590-700 Mt of carbon has been released from peat combustion since 1795, compared with a release of 4140-5600 Mt from agricultural drainage. (7) The aggregate shift in the carbon balance of temperate zone wetlands, when added to a far smaller shift from tropical wetlands, equalled 150-185 Mt of carbon in 1980 and 5711-6480 Mt since 1795. Despite occupying an area equivalent to only 2% of the world's tropical forest, the wetlands have experienced an annual shift in carbon balance 15-18% as great. Wetlands thus are seen on an area-specific basis to be concentrated sources of atmospheric CO2 which respond differently from those ecosystems assumed to have no net carbon exchange before disturbance.