Land and water interface zones

Abstract

This paper reports analyses of C pools and fluxes in land-water interface zones completed at the International Workshop: Terrestrial Biospheric Carbon Fluxes; Quantification of Sinks and Sources of CO₂ (Bad Harzburg, Germany, March 1–5, 1993). The objective was to determine the role of these zones as global sinks of atmospheric CO₂ as part of a larger effort to quantify global C sinks and sources in the past (ca. 1850), the present, and the foreseeable future (ca. 2050). Assuming the world population doubles by the year 2050, storage of atmospheric C in reservoirs will also double, as will river loads of atmospheric C and nutrients. It is estimated that C sinks in temperate and boreal wetlands have decreased by about 50%, from 0.2 to 0.1 Gt C yr⁻¹ since 1850. The total decrease for wetlands may be considerably larger when tropical wetlands are taken into account, however, the area and C density of tropical wetlands are not well known at this time. Changes in cultivation practices and improved sampling of methaneogenesis have caused estimates of CH₄ emissions from ricelands to drop substantially from 150 to 60 Tg yr⁻¹. Even with doubled N and P loads, rivers are unlikely to fertilize more than about 20% of the new primary production in the coastal ocean. The source of C for this new production may not be the atmosphere, however, because the coastal ocean exchanges large quantities of DIC with the open ocean. Until the C fluxes from air-sea exchange of CO₂ and DIC are better quantified, the C-sink potential of the coastal ocean will remain a major uncertainty in the global C cycle. Analysis of model simulations of oceanic C uptake reconfirmed that the open ocean appears to take up about 2.0 Gt C yr⁻¹ from the atmosphere and that model estimates are in better accord now, ±0.5 Gt C yr⁻¹, than ever before. Land use management must consider the unique C sinks in coastal and alluvial wetlands in order to minimize the future negative impacts of agriculture and urban development. Long-term monitoring will be essential to prove the success, or failure, of management practices to sustain wetlands in the future. Relative to the other systems examined at the workshop, the C-sink capacity of the ocean (excluding estuaries) is not likely to be measurably affected in the foreseeable future by the management scenarios considered at the workshop.