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

Abstract. European ecosystems are thought to take up large amounts of carbon, but neither the rate nor the contributions of the underlying processes are well known. In the second half of the 20th century, carbon dioxide concentrations have risen by more that 100 ppm, atmospheric nitrogen deposition has more than doubled, and European mean temperatures were increasing by 0.02 °C yr⁻¹. The extents of forest and grasslands have increased with the respective rates of 5800 km² yr⁻¹ and 1100 km² yr⁻¹ as agricultural land has been abandoned at a rate of 7000 km² yr⁻¹. In this study, we analyze the responses of European land ecosystems to the aforementioned environmental changes using results from four process-based ecosystem models: BIOME-BGC, JULES, ORCHIDEE, and O-CN. The models suggest that European ecosystems sequester carbon at a rate of 56 TgC yr⁻¹ (mean of four models for 1951–2000) with strong interannual variability (±88 TgC yr⁻¹, average across models) and substantial inter-model uncertainty (±39 TgC yr⁻¹). Decadal budgets suggest that there has been a continuous increase in the mean net carbon storage of ecosystems from 85 TgC yr⁻¹ in 1980s to 108 TgC yr⁻¹ in 1990s, and to 114 TgC yr⁻¹ in 2000–2007. The physiological effect of rising CO₂ in combination with nitrogen deposition and forest re-growth have been identified as the important explanatory factors for this net carbon storage. Changes in the growth of woody vegetation are suggested as an important contributor to the European carbon sink. Simulated ecosystem responses were more consistent for the two models accounting for terrestrial carbon-nitrogen dynamics than for the two models which only accounted for carbon cycling and the effects of land cover change. Studies of the interactions of carbon-nitrogen dynamics with land use changes are needed to further improve the quantitative understanding of the driving forces of the European land carbon balance.