Interactions between nitrogen deposition, land cover conversion, and climate change determine the contemporary carbon balance of Europe

摘要

European ecosystems are thought to take up large amounts of carbon, butneither the rate nor the contributions of the underlying processes are wellknown. In the second half of the 20th century, carbon dioxideconcentrations have risen by more that 100 ppm, atmospheric nitrogendeposition has more than doubled, and European mean temperatures wereincreasing by 0.02 °C yr?1. The extents of forest and grasslands haveincreased with the respective rates of 5800 km2 yr?1 and 1100 km2 yr?1 as agricultural land has been abandoned at a rate of7000 km2 yr?1. In this study, we analyze the responses of European landecosystems to the aforementioned environmental changes using results fromfour process-based ecosystem models: BIOME-BGC, JULES, ORCHIDEE, and O-CN.The models suggest that European ecosystems sequester carbon at a rate of56 TgC yr?1 (mean of four models for 1951–2000) with strong interannualvariability (±88 TgC yr?1, average across models) and substantialinter-model uncertainty (±39 TgC yr?1). Decadal budgets suggest thatthere has been a continuous increase in the mean net carbon storage ofecosystems from 85 TgC yr?1 in 1980s to 108 TgC yr?1 in 1990s, and to114 TgC yr?1 in 2000–2007. The physiological effect of rising CO₂ incombination with nitrogen deposition and forest re-growth have beenidentified as the important explanatory factors for this net carbon storage.Changes in the growth of woody vegetation are suggested as an importantcontributor to the European carbon sink. Simulated ecosystem responses weremore consistent for the two models accounting for terrestrialcarbon-nitrogen dynamics than for the two models which only accounted forcarbon cycling and the effects of land cover change. Studies of theinteractions of carbon-nitrogen dynamics with land use changes are needed tofurther improve the quantitative understanding of the driving forces of theEuropean land carbon balance.