In this study we analysed the combined effect of increasing nitrogen deposition and forest re-growth on the global land uptake of carbon. We conclude 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. Moreover carbon uptake associated with forest re-growth and amplified by high nitrogen deposition will decrease as soon as forests reach maturity. To refine our estimates of land carbon uptake we would need to know the locations of re-growing forests and their age. We also may need to revise the model's representation of carbon-nitrogen interactions in ecosystems once we better understand different pathways of plant nitrogen uptake, role of nitrogen availability in the allocation of carbon, and formation of dissolved organic nitrogen in soil.
Although fossil fuel burning, intensive agriculture, and forest management increase CO2 emissions to the atmosphere, they can also stimulate land carbon uptake, which partially offsets increasing CO2 from fossil fuel emissions. These secondary effects such as increased nitrogen deposition and re-growth of forests should not be considered as purely beneficial. High nitrogen deposition can lead also to nitrogen saturation which, depending on ecosystem type, can cause forest decline [27,28] or dramatic increases in export of dissolved organic carbon and nitrogen from forest ecosystems to streams, rivers, lakes, and coastal systems . High NOx emissions lead to ozone production , which can have damaging effects on plant growth.