The global climate is expected to change in response to rising concentrations of atmospheric carbon dioxide (CO2), because CO2 in the atmosphere traps heat. The magnitude of this change depends on the rate of CO2 emissions from human activities as well as on carbon uptake by oceans and land. Carbon dioxide fertilization, climate change, nitrogen deposition and land management can enhance carbon uptake on land. A globally significant carbon sink in 1980's-1990's in northern extratropical regions  was inferred from variations in atmospheric CO2 concentrations. Although this sink was attributed mostly to forest ecosystems [2-5], the magnitude and cause of this sink remain uncertain. Population growth, industrial expansion, and political changes lead to exponentially increasing deposition of reactive nitrogen on land and re-growth of forests, which were identified among the major causes of land carbon sink in the Northern Hemisphere.
Nitrogen is a primary limiting nutrient throughout terrestrial ecosystems of mid and high latitudes, and an important limiting nutrient for plant growth throughout subtropical and tropical ecosystems , where phosphorus is a co-limiting or limiting nutrient [7,8]. Additional nitrogen supply through fertilization and atmospheric deposition could reduce or even remove the nitrogen limitation on carbon uptake. Overload of nitrogen deposited on land can lead to decline of ecosystems. Globally the amount of reactive atmospheric nitrogen inputs increased from 41 TgN/yr in 1950 to 103 TgN/yr in 2000 with proportional increase of deposition on land . One third of the global nitrogen inputs entered the land ecosystems and one tenth the forests. Given the high carbon to nitrogen ratios and long lifetimes of carbon in wood, a most significant effect of nitrogen fertilization is expected in forests.
Did land carbon uptake increase in response to the higher nitrogen depositions during the last few decades? Studies addressing this question agreed on the location of major response – the temperate forests located between 25° and 55° north, but disagree on the magnitude of the response. Based on results from a series of 15N-tracer field experiments, Nadelhoffer et al.  argued that increased inputs of combined nitrogen from atmosphere made a minor contribution to land carbon uptake. Their stoichiometric budget suggested that fertilized temperate forests sequestered only 0.25 PgC per year in addition. In contrast, model based estimates [11,12] showed significant increases in land carbon uptake. Townsend et al.  estimated an additional carbon uptake by land in the order of 0.3–1.3 PgC per year, using an ecosystem model and spatially explicit nitrogen depositions from fossil fuels burning (NOy). Using the same vegetation model and various predicted spatial distributions of atmospheric nitrogen deposition (including both NOy and NHx), Holland et al.  showed even higher carbon uptake of 1.5–2.0 Pg per year. Forest regrowth was identified as another major driver of elevated carbon uptake in the Northern Hemisphere in 1980–1999. Changing forest management practices lead to increasing fraction of young forests with higher carbon uptake. Depending on the country these young forests were either planted after harvesting or on abandoned agricultural land. Based on forest inventories a carbon sink of 0.11 PgC/yr in Europe and 0.018 PgC/yr in Japan was primarily attributed to regrowth of young forests [2,3]. In China and the United States both forest regrowth and afforestation lead to carbon sink of 0.03 PgC/yr  and 0.11–0.15 PgC/yr [5,13] respectively. Russian forests have been reported as a highly variable carbon sink (0.06–0.3 Pg C/yr) during those decades . Although the increased uptake of CO2 caused by forest regrowth and afforrestation is relatively undisputed, the relative roles of climate change or land management responsible for this increase has not been defined. In addition, atmospheric nitrogen deposition was exponentially increasing during the same time period. Was nitrogen fertilization leading to a faster forest re-growth and hence to an increased land carbon uptake?
In this study we investigate combined effect of increased nitrogen deposition and forest re-growth on land carbon uptake. We use results of biogeochemical model simulations to show that elevated nitrogen deposition is unlikely to be the major contributor to the increased land carbon sink unless we consider its effects on re-growing forests. The atmospheric nitrogen deposition for 1860–1999 was calculated by the state-of-the-art three dimensional atmospheric chemistry transport model TM3 , which provided spatial distributions of reactive atmospheric nitrogen deposition (NOy and NHx). To assess the effect of enhanced nitrogen deposition on land carbon uptake we used a terrestrial biogeochemical model BIOME-BGC [16-18], which calculates water, carbon, and nitrogen pools dynamics as well as their fluxes. The model considers explicit patterns of nitrogen input and loss from ecosystems (see Methods). To isolate effects of increasing nitrogen deposition we performed model simulations with both increasing CO2 and nitrogen deposition as well as with increasing CO2 and constant nitrogen deposition. We estimate carbon uptake of land ecosystems assuming that temperate forests were at three different growth stages: 'mature', 'middle-aged', and 'young'.