Land carbon uptake and residual carbon sink
Previous studies [19,20] showed existence of the residual carbon sink, which is an imbalance between annual average emissions of CO2 and the sum of the annual carbon accumulation in the atmosphere and the annual carbon uptake by the oceans. This imbalance, attributed to processes on land, has slightly increased from 0.3–4.0 PgC/yr in 1980's to 1.6–4.8 PgC/yr in 1990's. Our results suggest that the effect of nitrogen fertilization on land carbon uptake could explain 20–70% of the residual carbon sink (Table 1) depending on our assumptions of the age, proportion, and distribution of re-growing forests. Our estimates for global additional carbon uptake assuming mature forests are lower than comparable estimates from previous study for unmanaged vegetation . The latter study was based on simulations of a model with an annual time step, which ignored seasonal dynamics of carbon – nitrogen interactions, and used only one nitrogen deposition input averaged for the 1990's. It is possible that these simplified assumption in the model and for the nitrogen inputs lead to overestimation of land carbon response to increasing nitrogen loads. Our results suggest twice higher increase in carbon uptake in temperate forests than a study based on 15N tracer field experiments , which suggested increase of 0.25 Pg carbon per year for temperate forests. This discrepancy is probably attributable to the small sample size (only nine forests), which was not very representative of temperate forest ecosystems. It could be also related to under-representation of certain ecosystem processes like pathways of plant nitrogen uptake or reactive nitrogen transformations in soil in our modelling approach which is discussed below.
Land carbon uptake and fossil fuel emissions
Would this additional carbon accumulated on land offset the fossil fuel emissions? During 1980–1999 approximately 110 Pg of carbon has been emitted into the atmosphere from fossil fuel burning, industry, and deforestation (EDGAR-HYDE 1.4, ). Our model predicted that increased atmospheric nitrogen deposition had caused 14, 25, and 45 Pg of additional carbon to accumulate on land during the same time period assuming mature, middle-age, and young forests respectively (compared to a case where we assumed pre-industrial levels of reactive nitrogen deposition). Assuming that age structure of the world forests in 1980–1999 was between ages of the mature and young forests used in model's simulations, increased nitrogen deposition could attenuate rising atmospheric CO2 by something between 13% and 41%. The real number lays somewhere in between, most likely closer to the lower limit, since only small fraction of the world forests in 1980's and 1990's were re-growing. Once the forests mature, their ability to take up more carbon will diminish.
Uncertainties in estimated land carbon uptake
Our budget (Table 1) is subject to some uncertainties related to simplified representation of ecosystem processes as well as land use dynamics in our modelling study. First, our model does not include the mechanism for nitrogen uptake through the stomata of leaves. In closed-canopy forests, forest canopies can intercept atmospheric nitrogen and assimilate retained reactive nitrogen from air. This mechanism was not implemented in the model, because it is not clear how significant the proportion of total incoming inorganic nitrogen intercepted by the canopy is. If this proportion is only 16% as estimated for North American forests , then it would not change our results considerably. However if it reaches 40% or more than 90%  and all intercepted nitrogen is taken up by foliage then a nitrogen-induced carbon sink may be higher than estimated in our study. Second, our model does not include transformations of reactive nitrogen in the soil, which may be locked up in soil or cause production of dissolved organic nitrogen and carbon. Experiments [24,25] suggest that chronic additions of nitrate to terrestrial ecosystems lead to higher leaching of dissolved organic nitrogen and carbon rather than to plant productivity increase. The mechanisms behind formation of these dissolved organic compounds and conditions under which it occurs still have to be understood. Including this feedback may decrease estimates of land carbon uptake in some regions. Third, we assumed that land cover remained constant during 1980–1999. In reality the land cover has been experiencing changes during the last two decades. The estimates of these changes and their locations however are highly uncertain. Forest area was decreasing globally by approximately 2% per decade . In Europe and North America, forest cover increased by approximately 0.14–0.2% per decade [2,26]. In China forest cover was decreasing by 2.3% per decade according to FAO  and increasing by 1.5% per decade according to Fang . Given these uncertainties in forest cover change, we feel that assumption of constant land cover was acceptable in this study.
This study is a frontier research and the results need some confirmation from an independent modelling effort which may include a more detailed treatment of nitrogen cycle and carbon-nitrogen interactions. In addition to the abovementioned processes the next generation of global biogeochemical models may include the following mechanisms: different fates of NHx and NOy within an ecosystem and corresponding effects on ecosystem dynamics; depression of nitrogen mineralization in soils under increasing atmospheric deposition of nitrogen; "N saturation" effects, with N leaching losses approaching N input rates in forests; effect of available nitrogen on carbon allocation within a plant as well as changing plant and soil C:N ratios under chronic N additions. Given complexity of the nitrogen cycle and limitations of any modelling approach, we have to prioritize processes to be included based on their generality and level of understanding.